   Generated by Sphinx 7.2.6.

The Logtalk Handbook
********************

     The Logtalk Handbook v3.80.0, Jun 04, 2024

     Paulo Moura

     Copyright © 1998-2024, Paulo Moura

1 User Manual
*************

1.1 Declarative object-oriented programming
===========================================

Logtalk is a 'declarative object-oriented logic programming language'.
This means that Logtalk shares key concepts with other object-oriented
programming languages but abstracts and reinterprets these concepts in
the context of declarative logic programming.

The key concepts in 'declarative' object-oriented programming are
'encapsulation' and 'reuse patterns'.  Notably, the concept of 'mutable
state', which is an 'imperative' concept, is not a significant concept
in 'declarative' object-oriented programming.  Declarative
object-oriented programming concepts can be materialized in both logic
and functional languages.  In this section, we focus only in declarative
object-oriented logic programming.

The first key generalization of object-oriented programming concepts is
the concept of object itself.  What an object encapsulates depends on
the 'base programming paradigm' where we apply object-oriented
programming concepts.  When these concepts are applied to an imperative
language, where mutable state and destructive assignment are central,
objects naturally encapsulate and abstract mutable state, providing
disciplined access and modification.  When these concepts are applied to
a declarative logic language such as Prolog, objects naturally
encapsulate predicates.  Therefore, an object can be seen as a 'theory',
expressed by a set of related predicates.  Theories are usually static
and thus Logtalk objects are static by default.  This contrasts with
imperative object-oriented languages where usually classes are static
and objects are dynamic.  This view of an object as a set of predicates
also forgo a distinction between 'data' and 'procedures' that is central
to imperative object-oriented languages but moot in declarative, see
homoiconic logic languages.

The second key generalization concerns the relation between objects and
other entities such as protocols (interfaces) and ancestor objects.  The
idea is that entity relations define 'reuse patterns' and the 'roles'
played by the participating entities.  A common reuse pattern is
'inheritance'.  In this case, an entity inherits, and thus reuses,
resources from an ancestor entity.  In a reuse pattern, each
participating entity plays a specific 'role'.  The same entity, however,
can play multiple roles depending on its relations with other entities.
For example, an object can play the role of a class for its instances,
the role of a subclass for its superclasses, and the role of an instance
for its metaclass.  Another common reuse pattern is 'protocol
implementation'.  In this case, an object implementing a protocol reuses
its predicate declarations by providing an implementation for those
predicates and exposing those predicates to its clients.  An essential
consequence of this generalization is that protocols, objects, and
categories are first-class entities while e.g.  'prototype', 'parent',
'class', 'instance', 'metaclass', 'subclass', 'superclass', or
'ancestor' are just 'roles' that an object can play.  When sending a
message to an object, the corresponding predicate 'declaration' and
predicate 'definition' lookup procedures (reuse patterns) depend on the
role or roles that the object plays (see the Inheritance.
section for details).  Another consequence of this generalization is
that a language can provide multiple reuse patterns instead of selecting
a set of patterns and supporting this set as a design choice that
excludes other reuse patterns.  For example, most imperative
object-oriented languages are either class-based or prototype-based.  In
contrast, Logtalk supports both classes and prototypes by providing the
corresponding reuse patterns using objects as first-class entities
capable of playing multiple roles.

1.2 Main features
=================

Several years ago, I decided that the best way to learn object-oriented
programming was to build my own object-oriented language.  Prolog being
always my favorite language, I chose to extend it with object-oriented
capabilities.  Strong motivation also come from my frustration with
Prolog shortcomings for writing large applications.  Eventually this
work has led to the Logtalk programming language as its know today.  The
first system to use the name Logtalk appeared in February 1995.  At that
time, Logtalk was mainly an experiment in computational reflection with
a rudimentary runtime and no compiler.  Based on feedback by users and
on the author subsequent work, the name was retained and Logtalk as
created as a full programming language focusing on using object-oriented
concepts for code encapsulation and reuse.  Development started on
January 1998 with the first public alpha version released in July 1998.
The first stable release (2.0) was published in February 1999.
Development of the third generation of Logtalk started in 2012 with the
first public alpha version in August 2012 and the first stable release
(3.0.0) in January 2015.

Logtalk provides the following features:

1.2.1 Integration of logic and object-oriented programming
----------------------------------------------------------

     Logtalk tries to bring together the main advantages of these two
     programming paradigms.  On one hand, the object orientation allows
     us to work with the same set of entities in the successive phases
     of application development, giving us a way of organizing and
     encapsulating the knowledge of each entity within a given domain.
     On the other hand, logic programming allows us to represent, in a
     declarative way, the knowledge we have of each entity.  Together,
     these two advantages allow us to minimize the distance between an
     application and its problem domain, turning the writing and
     maintenance of programming easier and more productive.

     From a pragmatic perspective, Logtalk objects provide Prolog with
     the possibility of defining several namespaces, instead of the
     traditional Prolog single database, addressing some of the needs of
     large software projects.

1.2.2 Integration of event-driven and object-oriented programming
-----------------------------------------------------------------

     Event-driven programming enables the building of reactive systems,
     where computing which takes place at each moment is a result of the
     observation of occurring events.  This integration complements
     object-oriented programming, in which each computing is initiated
     by the explicit sending of a message to an object.  The user
     dynamically defines what events are to be observed and establishes
     monitors for these events.  This is specially useful when
     representing relationships between objects that imply constraints
     in the state of participating objects [Rumbaugh87],
     [Rumbaugh88], [Fornarino_et_al_89], [Razek92].
     Other common uses are reflective applications like code
     debugging or profiling [Maes87].  Predicates can be
     implicitly called when a spied event occurs, allowing programming
     solutions which minimize object coupling.  In addition, events
     provide support for behavioral reflection and can be used to
     implement the concepts of 'pointcut' and 'advice' found on
     Aspect-Oriented Programming.

1.2.3 Support for component-based programming
---------------------------------------------

     Predicates can be encapsulated inside 'categories' which can be
     imported by any object, without any code duplication and
     irrespective of object hierarchies.  A category is a first-class
     encapsulation entity, at the same level as objects and protocols,
     which can be used as a component when building new objects.  Thus,
     objects may be defined through composition of categories, which act
     as fine-grained units of code reuse.  Categories may also extend
     existing objects.  Categories can be used to implement 'mixins' and
     'aspects'.  Categories allows for code reuse between non-related
     objects, independent of hierarchy relations, in the same vein as
     protocols allow for interface reuse.

1.2.4 Support for both prototype and class-based systems
--------------------------------------------------------

     Almost any (if not all) object-oriented languages available today
     are either class-based or prototype-based [Lieberman86],
     with a strong predominance of class-based languages.  Logtalk
     provides support for both hierarchy types.  That is, we can have
     both prototype and class hierarchies in the same application.
     Prototypes solve a problem of class-based systems where we
     sometimes have to define a class that will have only one instance
     in order to reuse a piece of code.  Classes solves a dual problem
     in prototype based systems where it is not possible to encapsulate
     some code to be reused by other objects but not by the
     encapsulating object.  Stand-alone objects, that is, objects that
     do not belong to any hierarchy, are a convenient solution to
     encapsulate code that will be reused by several unrelated objects.

1.2.5 Support for multiple object hierarchies
---------------------------------------------

     Languages like Smalltalk-80 [Goldberg83], Objective-C
     [Cox86] and Java [Joy_et_al_00] define a
     single hierarchy rooted in a class usually named ‘Object’.  This
     makes it easy to ensure that all objects share a common behavior
     but also tends to result in lengthy hierarchies where it is
     difficult to express objects which represent exceptions to default
     behavior.  In Logtalk we can have multiple, independent, object
     hierarchies.  Some of them can be prototype-based while others can
     be class-based.  Furthermore, stand-alone objects provide a simple
     way to encapsulate utility predicates that do not need or fit in an
     object hierarchy.

1.2.6 Separation between interface and implementation
-----------------------------------------------------

     This is an expected (should we say standard ?)  feature of almost
     any modern programming language.  Logtalk provides support for
     separating interface from implementation in a flexible way:
     predicate directives can be contained in an object, a category or a
     protocol (first-order entities in Logtalk) or can be spread in both
     objects, categories and protocols.

1.2.7 Private, protected and public inheritance
-----------------------------------------------

     Logtalk supports private, protected and public inheritance in a
     similar way to C++ See [Stroustrup86], enabling us to
     restrict the scope of inherited, imported or implemented predicates
     (by default inheritance is public).

1.2.8 Private, protected and public object predicates
-----------------------------------------------------

     Logtalk supports data hiding by implementing private, protected and
     public object predicates in a way similar to C++ See
     [Stroustrup86].  Private predicates can only be called from the
     container object.  Protected predicates can be called by the
     container object or by the container descendants.  Public
     predicates can be called from any object.

1.2.9 Parametric objects
------------------------

     Object names can be compound terms (instead of atoms), providing a
     way to parameterize object predicates.  Parametric objects are
     implemented in a similar way to ‘L&O’ [McCabe92], ‘OL(P)’
     [Fromherz93] or ‘SICStus Objects’ [SICStus95].
     (however, access to parameter values is done via a built-in method
     instead of making the parameters scope global over the whole
     object).  Parametric objects allows us to treat any predicate
     clause as defining an 'instantiation' of a parametric object.
     Thus, a parametric object allows us to encapsulate and associate
     any number of predicates with a compound term.

1.2.10 High level multi-threading programming support
-----------------------------------------------------

     High level multi-threading programming is available when running
     Logtalk with selected backend Prolog compilers, allowing objects to
     support both synchronous and asynchronous messages.  Logtalk allows
     programmers to take advantage of modern multi-processor and
     multi-core computers without bothering with the details of creating
     and destroying threads, implement thread communication, or
     synchronizing threads.

1.2.11 Smooth learning curve
----------------------------

     Logtalk has a smooth learning curve, by adopting standard Prolog
     syntax and by enabling an incremental learning and use of most of
     its features.

1.2.12 Compatibility with most Prolog systems and the ISO standard
------------------------------------------------------------------

     The Logtalk system has been designed to be compatible with most
     Prolog compilers and, in particular, with the ISO Prolog standard
     [ISO95].  It runs in almost any computer system with a
     modern Prolog compiler.

1.2.13 Performance
------------------

     The current Logtalk implementation works as a trans-compiler:
     Logtalk source files are first compiled to Prolog source files,
     which are then compiled by the chosen Prolog compiler.  Therefore,
     Logtalk performance necessarily depends on the backend Prolog compiler.
     The Logtalk compiler preserves the programmers
     choices when writing efficient code that takes advantage of tail
     recursion and first-argument indexing.

     As an object-oriented language, Logtalk can use both static binding
     and dynamic binding for matching messages
     and methods.  Furthermore, Logtalk entities (objects, protocols,
     and categories) are independently compiled, allowing for a very
     flexible programming development.  Entities can be edited,
     compiled, and loaded at runtime, without necessarily implying
     recompilation of all related entities.

     When dynamic binding is used, the Logtalk runtime engine implements
     caching of message lookups. (including messages to 'self'
     and 'super' calls), ensuring a performance level close to what
     could be achieved when using static binding.

     For more detailed information on performance, see its dedicated
     section.

1.2.14 Logtalk scope
--------------------

Logtalk, being a superset of Prolog, shares with it the same preferred
areas of application but also extends them with those areas where
object-oriented features provide an advantage compared to plain Prolog.
Among these areas we have:

'Logic and object-oriented programming teaching and researching'

     Logtalk smooth learning curve, combined with support for both
     prototype and class-based programming, protocols, components or
     aspects via category-based composition, and other advanced
     object-oriented features allow a smooth introduction to
     object-oriented programming to people with a background in Prolog
     programming.  The distribution of Logtalk source code using an
     open-source license provides a framework for people to learn and
     then modify to try out new ideas on object-oriented programming
     research.  In addition, the Logtalk distribution includes plenty of
     programming examples that can be used in the classroom for teaching
     logic and object-oriented programming concepts.

'Structured knowledge representations and knowledge-based systems'

     Logtalk objects, coupled with event-driven programming features,
     enable easy implementation of frame-like systems and similar
     structured knowledge representations.

'Blackboard systems, agent-based systems, and systems with complex object relationships'

     Logtalk support for event-driven programming can provide a basis
     for the dynamic and reactive nature of blackboard type
     applications.

'Highly portable applications'

     Logtalk is compatible with most modern Prolog systems that support
     official and de facto standards.  Used as a way to provide Prolog
     with namespaces, it avoids the porting problems of most Prolog
     module systems.  Platform, operating system, or compiler specific
     code can be isolated from the rest of the code by encapsulating it
     in objects with well-defined interfaces.

'Alternative to a Prolog module system'

     Logtalk can be used as an alternative to a Prolog compiler module
     system.  Most Prolog applications that use modules can be converted
     into Logtalk applications, improving portability across Prolog
     systems and taking advantage of the stronger encapsulation and
     reuse framework provided by Logtalk object-oriented features.

'Integration with other programming languages'

     Logtalk support for most key object-oriented features helps users
     integrating Prolog with object-oriented languages like C++, Java,
     or Smalltalk by facilitating a high-level mapping between the two
     languages.

1.3 Nomenclature
================

Depending on your logic programming and object-oriented programming
background (or lack of it), you may find Logtalk nomenclature either
familiar or at odds with the terms used in other languages.  In
addition, being a superset of Prolog, terms such as 'predicate' and
'method' are often used interchangeably.  Logtalk inherits most of its
nomenclature from Prolog and Smalltalk.

Note that the same terms can have different meanings in different
languages.  A good example is 'class'.  The support for meta-classes in
e.g.  Smalltalk translates to a concept of class that is different in
key aspects from the concept of class in e.g.  Java or C++.  Other terms
that can have different meanings are 'delegation' and 'forwarding'.
There are also cases where the same concept is found under different
names in some languages (e.g.  'self' and 'this') but that can also mean
different concepts in Logtalk and other languages.  Always be aware of
these differences and be cautious with assumptions carried from other
programming languages.

In this section, we map nomenclatures from Prolog and popular OOP
languages such as Smalltalk, C++, Java, and Python to the Logtalk
nomenclature.  The Logtalk distribution includes several examples of how
to implement common concepts found in other languages, complementing the
information in this section.  This Handbook also features a Prolog interoperability section
and an extensive glossary.
providing the exact meaning of the names commonly used in Logtalk
programming.

1.3.1 Prolog nomenclature
-------------------------

Being a superset of Prolog, Logtalk inherits its nomenclature.  But
Logtalk also aims to fix several Prolog shortcomings, thus introducing
new concepts and refining existing Prolog concepts.  Logtalk
object-oriented nature also introduces names and concepts that are not
common when discussing logic programming semantics.  We mention here the
most relevant ones, notably those where semantics or common practice
differ.  Further details can be found elsewhere in this Handbook.

'arbitrary goals as directives'

     Although not ISO Prolog Core standard compliant, several Prolog
     systems accept using arbitrary goal as directives.  This is not
     supported in Logtalk source files.  Always use an
     initialization/1 directive to wrap those goals.  This ensure
     that any initialization goals, which often have side-effects, are
     only called if the source file is successfully compiled and loaded.

'calling a predicate'

     Sending a message to an object is similar to 'calling a
     goal' with the difference that the actual predicate that is called
     is determined not just by the message 'term' but also by the object
     receiving the message and possibly its ancestors.  This is also
     different from calling a Prolog module predicate: a message may
     result e.g.  in calling a predicate inherited by the
     object but calling a module predicate requires the predicate to
     exist in (or be reexported by) the module.

'closed world assumption semantics'

     Logtalk provides clear closed world assumption semantics: messages
     or calls for declared but undefined predicates fail.  Messages or
     calls for unknown (i.e.  not declared) predicates throw an error.
     Crucially, this semantics apply to both 'static' and 'dynamic'
     predicates.  But in Prolog workarounds are required to have a
     static predicate being known by the runtime without it being also
     defined (so that calling it would fail instead of throwing a
     predicate existence error).

'compiling and loading source files'

     Logtalk provides its own built-in predicates for compiling and loading
     source files.  It also provides convenient
     top-level interpreter shorthands for these and other
     frequent operations.  In general, the traditional Prolog built-in
     predicates and top-level interpreter shorthands cannot be used to
     load Logtalk source files.

'debugging'

     In most (if not all) Prolog systems, debugging support is a
     built-in feature made available using a set of built-in predicates
     like ‘trace/0’ and ‘spy/1’.  But in Logtalk the default debugger
     is a regular application, implemented using a public
     reflection API.  This means that the debugger must be
     explicitly loaded (either automatically from a settings file
     at startup or from the top-level).  It also means that the
     debugger can be easily extended or replaced by an alternative
     application.

'directive operators'

     Some Prolog systems declare directive names as operators (e.g.
     ‘dynamic’, ‘multifile’, …).  This is not required by the ISO Prolog
     Core standard.  It’s a practice that should be avoided as it makes
     code non-portable.

'encapsulation'

     Logtalk enforces encapsulation of object predicates, generating a
     permission error when a predicate is not within the scope of the
     caller.  In contrast, most Prolog module systems allow any module
     predicate to be called by using explicit qualification, even if not
     exported.  Worse, some Prolog systems also allow defining clauses
     for a module predicate outside the module, without declaring the
     predicate as multifile, by simply writing clauses with explicit
     module-qualified heads.

'entity loading'

     When using Prolog modules, ‘use_module/1-2’ (or equivalent)
     directives both load the module files and declare that the
     (implicitly or explicitly) imported predicates can be used with
     implicit module qualification.  But Logtalk separates entity
     (object, protocol, category, or module) predicate 'usage'
     declarations (via uses/1 and uses/2 or its
     own use_module/1 and use_module/2 directives)
     from 'loading' goals (using the logtalk_load/1 and
     logtalk_load/2 predicates), called using an explicit and
     disciplined approach from loader files.

'flags scope'

     The set_logtalk_flag/2. 'directive' is always local to
     the entity or source file that contains it.  Only calls to the
     set_logtalk_flag/2. 'predicate' set the global default
     value for a flag.  This distinction is lacking in Prolog (where
     directives usually have a global scope) and Prolog modules (where
     some flags are local to modules in some systems and global in other
     systems).

'meta-predicate call semantics'

     Logtalk provides consistent meta-predicate call
     semantics: meta-arguments are always called in the meta-predicate
     calling context.  This contrasts with Prolog module meta-predicates
     where the semantics of implicitly qualified calls is different from
     explicitly qualified calls.

'operators scope'

     Operators declared inside an entity (object, protocol, or category)
     are local to the entity.  But operators defined in a source file
     but outside and entity are global for compatibility with existing
     Prolog code.

'predicates scope'

     In plain Prolog, all predicates are visible.  In a Prolog module, a
     predicate can be exported or local.  In Logtalk, a predicate can be
     public, protected, private, or
     local.

'predicate declaration'

     Logtalk provides a clear distinction between declaring a
     predicate and defining a predicate.  This is a
     fundamental requirement for the concept of protocol. (aka
     interface) in Logtalk: we must be able to 'declare' a predicate
     without necessarily 'defining' it.  This clear distinction is
     missing in Prolog and Prolog modules.  Notably, it’s a compiler
     error for a module to try to export a predicate that it does not
     define.

'predicate loading conflicts'

     Logtalk does not use predicate import/export semantics.  Thus,
     there are never conflicts when loading entities (objects,
     protocols, or categories) that declare the same public predicates.
     But attempting to load two Prolog modules that export the same
     predicate results in a conflict, usually a compilation error (this
     is specially problematic when the ‘use_module/1’ directive is used;
     e.g.  adding a new exported predicate can break applications that
     use the module but not the new predicate).

1.3.2 Smalltalk nomenclature
----------------------------

The Logtalk name originates from a combination of the Prolog and
Smalltalk names.  Smalltalk had a significant influence in the design of
Logtalk and thus inherits some of its ideas and nomenclature.  The
following list relates the most commonly used Smalltalk terms with their
Logtalk counterparts.

'abstract class'

     Similar to Smalltalk, an abstract class is just a class not meant
     to be instantiated by not understanding a message to create
     instances.

'assignment statement'

     Logtalk, as a superset of Prolog, uses 'logic variables' and
     'unification' and thus provides no equivalent to the Smalltalk
     assignment statement.

'block'

     Logtalk supports lambda expressions and meta-predicates, which can
     be used to provide similar functionality to Smalltalk blocks.

'class'

     In Logtalk, 'class' is a just a 'role' that an object can play.
     This is similar to Smalltalk where classes are also objects.

'class method'

     Class methods in Logtalk are simply instance methods declared and
     defined in the class metaclass.

'class variable'

     Logtalk objects, which can play the roles of class and instance,
     encapsulate predicates, not state.  Class variables, which in
     Smalltalk are really shared instance variables, can be emulated in
     a class by defining a predicate locally instead of defining it in
     the class instances.

'inheritance'

     While Smalltalk only supports single inheritance, Logtalk supports
     single inheritance, multiple inheritance, and multiple
     instantiation.

'instance'

     While in Smalltalk every object is an 'instance' of same class,
     objects in Logtalk can play different roles, including the role of
     a prototype where the concepts of instance and class don’t apply.
     Moreover, instances can be either created dynamically or defined
     statically.

'instance method'

     Instance methods in Logtalk are simply predicates declared and
     defined in a class and thus inherited by the class instances.

'instance variable'

     Logtalk being a 'declarative' language, objects encapsulate a set
     of predicates instead of encapsulating 'state'.  But different
     objects may provide different definitions of the same predicates.
     Mutable internal state as in Smalltalk can be emulated by using
     dynamic predicates.

'message'

     Similar to Smalltalk, a 'message' is a request for an operation,
     which is interpreted in Logtalk as a logic query, asking for the
     construction of a proof that something is true.

'message selector'

     Logtalk uses the predicate template (i.e.  the predicate callable
     term with all its arguments unbound) as message selector.  The
     actual type of the message arguments is not considered.  Like
     Smalltalk, Logtalk uses 'single dispatch' on the message receiver.

'metaclass'

     Metaclasses are optional in Logtalk (except for a root class) and
     can be shared by several classes.  When metaclasses are used,
     infinite regression is simply avoided by making a class an instance
     of itself.

'method'

     Same as in Smalltalk, a 'method' is the actual code (i.e.
     predicate definition) that is run to answer a message.  Logtalk
     uses the words 'method' and 'predicate' interchangeably.

'method categories'

     There is no support in Logtalk for partitioning the methods of an
     object in different categories.  The Logtalk concept of 'category'
     (a first-class entity) was, however, partially inspired by
     Smalltalk method categories.

'object'

     Unlike Smalltalk, where 'everything' is an object, Logtalk language
     constructs includes both 'terms' (as in Prolog representing e.g.
     numbers and structures) and three first-class entities: objects,
     protocols, and categories.

'pool variables*'

     Logtalk, as a superset of Prolog, uses 'predicates' with no
     distinction between 'variables' and 'methods'.  Categories can be
     used to share a set of predicate definitions between any number of
     objects.

'protocol'

     In Smalltalk, an object 'protocol' is the set of messages it
     understands.  The same concept applies in Logtalk.  But Logtalk
     also supports protocols as first-class entities where a protocol
     can be implemented by multiple objects and an object can implement
     multiple protocols.

'self'

     Logtalk uses the same definition of 'self' found in Smalltalk: the
     object that received the message being processed.  Note, however,
     that 'self' is not a keyword in Logtalk but implicit in the
     (;;)/1 message to 'self' control construct.

'subclass'

     Same definition in Logtalk.

'super'

     As in Smalltalk, the idea of 'super' is to allow calling an
     inherited predicate (that is usually being redefined).  Note,
     however, that 'super' is not a keyword in Logtalk, which provides
     instead a (^^)/1. 'super' call control construct.

'superclass'

     Same definition in Logtalk.  But while in Smalltalk a class can
     only have a single superclass, Logtalk support for multiple
     inheritance allows a class to have multiple superclasses.

1.3.3 C++ nomenclature
----------------------

There are several C++ glossaries available on the Internet.  The list
that follows relates the most commonly used C++ terms with their Logtalk
equivalents.

'abstract class'

     Logtalk uses an 'operational' definition of abstract class: any
     class that does not inherit a method for creating new instances can
     be considered an abstract class.  Moreover, Logtalk supports
     interfaces/protocols, which are often a better way to provide
     the functionality of C++ abstract classes.

'base class'

     Logtalk uses the term superclass with the same meaning.

'data member'

     Logtalk uses predicates for representing both behavior
     and data.

'constructor function'

     There are no special methods for creating new objects in Logtalk.
     Instead, Logtalk provides a built-in predicate, see
     create_object/4, which can be used as a building block to
     define more sophisticated object creation predicates.

'derived class'

     Logtalk uses the term subclass with the same meaning.

'destructor function'

     There are no special methods for deleting new objects in Logtalk.
     Instead, Logtalk provides a built-in predicate, see
     abolish_object/1, which is often used to define more
     sophisticated object deletion predicates.

'friend function'

     Not supported in Logtalk.  Nevertheless, see the User Manual
     section on meta-predicates.

'instance'

     In Logtalk, an instance can be either created dynamically at
     runtime or defined statically in a source file in the same way as
     classes.

'member'

     Logtalk uses the term predicate.

'member function'

     Logtalk uses predicates for representing both behavior
     and data.

'namespace'

     Logtalk does not support multiple identifier namespaces.  All
     Logtalk entity identifiers share the same namespace (Logtalk
     entities are objects, categories, and protocols).

'nested class'

     Logtalk does not support nested classes.

'static member'

     Logtalk does not support a ‘static’ keyword.  But the equivalent to
     static members can be declared in a class metaclass.

'template'

     Logtalk supports parametric objects, which allows you to
     get the similar functionality of templates at runtime.

'this'

     Logtalk uses the built-in context method self/1 for
     retrieving the instance that received the message being processed.
     Logtalk also provides a this/1 method but for returning
     the class containing the method being executed.  Why the name
     clashes?  Well, the notion of self was inherited from
     Smalltalk, which predates C++.

'virtual member function'

     There is no ‘virtual’ keyword in Logtalk.  Any inherited or
     imported predicate can be redefined (either overridden or
     specialized).  Logtalk can use static binding or
     dynamic binding for locating both method declarations and
     method definitions.  Moreover, methods that are declared but not
     defined simply fail when called (as per closed-world assumption).

1.3.4 Java nomenclature
-----------------------

There are several Java glossaries available on the Internet.  The list
that follows relates the most commonly used Java terms with their
Logtalk equivalents.

'abstract class'

     Logtalk uses an 'operational' definition of abstract class: any
     class that does not inherit a method for creating new instances is
     an abstract class.  I.e.  there is no ‘abstract’ keyword in
     Logtalk.

'abstract method'

     In Logtalk, you may simply declare a method (see predicate)
     in a class without defining it, leaving its definition to some
     descendant subclass.

'assertion'

     There is no ‘assertion’ keyword in Logtalk.  Assertions are
     supported using Logtalk compilation hooks and developer tools.

'class'

     Logtalk objects can play the role of classes, instances, or
     protocols (depending on their relations with other objects).

'extends'

     There is no ‘extends’ keyword in Logtalk.  Class inheritance is
     indicated using 'specialization relations'.  Moreover, the 'extends
     relation' is used in Logtalk to indicate protocol, category, or
     prototype extension.

'interface'

     Logtalk uses the term protocol with similar meaning.
     But note that Logtalk objects and categories declared as
     implementing a protocol are not required to provide definitions for
     the declared predicates (see closed-world assumption).

'callback method'

     Logtalk supports event-driven programming, the most
     common usage context of callback methods.  Callback methods can
     also be implemented using meta-predicates.

'constructor'

     There are no special methods for creating new objects in Logtalk.
     Instead, Logtalk provides a built-in predicate, see
     create_object/4, which is often used to define more
     sophisticated object creation predicates.

'final'

     There is no ‘final’ keyword in Logtalk.  Predicates can always be
     redeclared and redefined in subclasses (and instances!).

'inner class'

     Inner classes are not supported in Logtalk.

'instance'

     In Logtalk, an instance can be either created dynamically at
     runtime or defined statically in a source file in the same way as
     classes.

'method'

     Logtalk uses the term predicate interchangeably with the
     term 'method'.

'method call'

     Logtalk usually uses the expression 'message sending' for method
     calls, true to its Smalltalk heritage.

'method signature'

     Logtalk selects the method/predicate to execute in order to answer
     a method call based only on the method name and number of
     arguments.  Logtalk (and Prolog) are not typed languages in the
     same sense as Java.

'package'

     There is no concept of packages in Logtalk.  All Logtalk entities
     (objects, protocols, categories) share a single namespace.  But
     Logtalk does support a concept of library that allows
     grouping of entities whose source files share a common path prefix.

'reflection'

     Logtalk features a 'white box' API supporting 'structural'
     reflection about entity contents, a 'black box' API
     supporting 'behavioral' reflection about object protocols,
     and an events. API for reasoning about messages
     exchanged at runtime.

'static'

     There is no ‘static’ keyword in Logtalk.  See the entries below on
     'static method' and 'static variable'.

'static method'

     Static methods may be implemented in Logtalk by using a
     metaclass for the class and defining the static methods in the
     metaclass.  I.e.  static methods are simply instance methods of the
     class metaclass.

'static variable'

     Static variables are 'shared instance variables' and can simply be
     both declared and defined in a class.  The built-in database
     methods can be used to implement destructive updates if necessary
     by accessing and updated a single clause of a dynamic predicate
     stored in the class.

'super'

     Instead of a ‘super’ keyword, Logtalk provides a super operator and
     control construct, see (^^)/1, for calling overridden
     methods.

'synchronized'

     Logtalk supports multi-threading programming in selected
     Prolog compilers, including a synchronized/1 predicate
     directive.  Logtalk allows you to synchronize a predicate or a set
     of predicates using per-predicate or per-predicate-set 'mutexes'.

'this'

     Logtalk uses the built-in context method self/1 for
     retrieving the instance that received the message being processed.
     Logtalk also provides a this/1 method but for returning
     the class containing the method being executed.  Why the name
     clashes?  Well, the notion of self was inherited from
     Smalltalk, which predates C++.

1.3.5 Python nomenclature
-------------------------

The list that follows relates the commonly used Python concepts with
their Logtalk equivalents.

'abstract class'

     Logtalk uses a different definition of abstract class: a class that
     does not inherit a method for creating new instances.  Notably, the
     presence of 'abstract methods' (i.e.  predicates that are declared
     but not defined) does not make a class abstract.

'abstract method'

     Logtalk uses the term 'predicate' interchangeably with 'method'.
     Predicates can be declared without being also defined in an object
     (or category).

'class'

     Logtalk objects can play the role of classes, instances, or
     protocols (depending on their relations with other objects).

'dictionary'

     There is no native, built-in associative data type.  But the
     library provides several implementations of a dictionary protocol.

'function'

     The closest equivalent is a predicate defined in ‘user’, a
     pseudo-object for predicates not defined in regular objects, and
     thus callable from anywhere without requiring a scope directive.

'function object'

     Predicates calls (goals) can be passed or returned from other
     predicates and unified with other terms (e.g.  variables).

'import path'

     Logtalk uses the term 'library' to refer to a directory of source
     files and supports defining aliases (symbolic names) to library
     paths to abstract the actual locations.

'lambda'

     Logtalk natively supports lambda expressions.

'list'

     Lists are compound terms with native syntax support.

'list comprehensions'

     There is no native, built-in support for list comprehensions.  But
     the standard ‘findall/3’ predicate can be used to construct a list
     by calling a goal that generates the list elements.

'loader'

     Logtalk uses the term 'loader' to refer to source files whose main
     or sole purpose is to load other source files.

'loop'

     There are no native loop control constructs based on a counter.
     But the library provides implementations of several loop
     predicates.

'metaclass'

     Logtalk objects play the role of metaclasses when instantiated by
     objects that play the role of classes.

'method'

     Logtalk uses the terms 'method' and 'predicate' interchangeably.
     Predicates can be defined in objects (and categories).  The value
     of 'self' is implicit unlike in Python where it is the first
     parameter of any method.

'method resolution order'

     Logtalk uses a depth-first algorithm to lookup method (predicate)
     declarations and definitions.  It’s possible to use predicate
     'aliases' to access predicate declarations and definitions other
     than the first ones found by the lookup algorithm.

'object'

     Objects are first-class entities that can play multiple roles,
     including prototype, class, instance, and metaclass.

'package'

     Logtalk uses the term 'library' to refer to a directory of source
     files defining objects, categories, and protocols.

'set'

     There is no native, built-in set type.  But the library provides
     set implementations.

'string'

     The interpretation of text between double-quotes depends on the
     ‘double_quotes’ flag.  Depending on this flag, double-quoted text
     can be interpreted as a list of characters, a list of character
     codes, or an atom.  Some backend Prolog compilers allow
     double-quoted text to be interpreted as a string in the Python
     sense.

'tuple'

     Compound terms can be used to represent tuples of any complexity.

'variable'

     Logtalk works with 'logical variables', which are close to the
     mathematical concept of variables and distinct from variables in
     imperative or imperative-based OOP languages where they are
     symbolic names for memory locations.  Logical variables can be
     'unified' with any term, including other variables.

'while loop'

     The built-in ‘forall/2’ predicate implements a 'generate-and-test'
     loop.

1.4 Messages
============

Messages allows us to ask an object to prove a goal and must always
match a declared predicate within the scope of the 'sender' object.
Note that sending a message is fundamentally different from calling a
predicate.  When calling a predicate, the caller decides implicitly
which predicate definition will be executed.  When sending a message, it
is the receiving object, not the sender, that decides which predicate
definition (if any) will be called to answer the message.  The predicate
definition that is actually used to answer a message depends on the
relations between the object and its imported categories and ancestor
objects (if any).  See the Inheritance section for details on
the predicate declaration and predicate definition lookup procedures.

When a message corresponds to a meta-predicate, the
meta-arguments are always called in the context of the object (or
category) sending the message.

Logtalk uses nomenclature similar to in other object-oriented
programming languages such as Smalltalk.  Therefore, the terms 'query'
and 'message' are used interchangeably when referring to a declared
predicate that is part of an object interface.  Likewise, the terms
'predicate' and 'method' are used interchangeably when referring to the
predicate definition (inside an object or category) that is called to
answer a message.

1.4.1 Operators used in message sending
---------------------------------------

Logtalk declares the following operators for the message sending control
constructs:

     :- op(600, xfy, ::).
     :- op(600,  fy, ::).
     :- op(600,  fy, ^^).

It is assumed that these operators remain active (once the Logtalk
compiler and runtime files are loaded) until the end of the Prolog
session (this is the usual behavior of most Prolog compilers).  Note
that these operator definitions are compatible with the predefined
operators in the Prolog ISO standard.

1.4.2 Sending a message to an object
------------------------------------

Sending a message to an object is accomplished by using the
(;;)/2 control construct:

     ..., Object::Message, ...

The message must match a public predicate declared for the receiving
object.  The message may also correspond to a protected or private
predicate if the 'sender' matches the predicate scope container.  If the
predicate is declared but not defined, the message simply fails (as per
the closed-world assumption).

1.4.3 Delegating a message to an object
---------------------------------------

It is also possible to send a message to an object while preserving the
original 'sender' by using the []/1 delegation control
construct:

     ..., [Object::Message], ....

This control construct can only be used within objects and categories
(in the top-level interpreter, the 'sender' is always the pseudo-object
‘user’ so using this control construct would be equivalent to use the
‘(::)/2’ message sending control construct).

1.4.4 Sending a message to 'self'
---------------------------------

While defining a predicate, we sometimes need to send a message to
'self', i.e., to the same object that has received the original message.
This is done in Logtalk through the (;;)/1 control construct:

     ..., ::Message, ....

The message must match either a public or protected predicate declared
for the receiving object or a private predicate within the scope of the
'sender' otherwise an error will be thrown.  If the message is sent from
inside a category or if we are using private inheritance, then the
message may also match a private predicate.  Again, if the predicate is
declared but not defined, the message simply fails (as per the
closed-world assumption).

1.4.5 Broadcasting
------------------

In the Logtalk context, 'broadcasting' is interpreted as the sending of
several messages to the same object.  This can be achieved by using the
message sending control construct described above.  However, for
convenience, Logtalk implements an extended syntax for message sending
that may improve program readability in some cases.  This extended
syntax uses the ‘(,)/2’, ‘(;)/2’, and ‘(->)/2’ control constructs (plus
the ‘(*->)/2’ soft-cut control construct when provided by the backend
Prolog compiler).  For example, if we wish to send several messages to
the same object, we can write:

     | ?- Object::(Message1, Message2, ...).

This is semantically equivalent to:

     | ?- Object::Message1, Object::Message2, ... .

This extended syntax may also be used with the ‘(::)/1’ message sending
control construct.

1.4.6 Calling imported and inherited predicates
-----------------------------------------------

When redefining a predicate, sometimes we need to call the inherited
definition in the new code.  This functionality, introduced by the
Smalltalk language through the ‘super’ primitive, is available in
Logtalk using the (^^)/1 control construct:

     ..., ^^Predicate, ....

Most of the time we will use this control construct by instantiating the
pattern:

     Predicate :-
         ...,            % do something
         ^^Predicate,    % call inherited definition
         ... .           % do something more

This control construct is generalized in Logtalk where it may be used to
call any imported or inherited predicate definition.  This control
construct may be used within objects and categories.  When combined with
static binding, this control construct allows imported and
inherited predicates to be called with the same performance of local
predicates.  As with the message sending control constructs, the
‘(^^)/1’ call simply fails when the predicate is declared but not
defined (as per the closed-world assumption).

1.4.7 Message sending and event generation
------------------------------------------

Assuming the events flag is set to ‘allow’ for the object (or
category) sending a message using the (;;)/2 control
construct, two events are generated, one before and one after the
message execution.  Messages that are sent using the (;;)/1.
(message to 'self') control construct or the (^^)/1 super
mechanism described above do not generate any events.  The rationale
behind this distinction is that messages to 'self' and 'super' calls are
only used internally in the definition of methods or to execute
additional messages with the same target object (represented by 'self').
In other words, events are only generated when using an object’s public
interface; they cannot be used to break object encapsulation.

If we need to generate events for a public message sent to 'self', then
we just need to write something like:

     Predicate :-
         ...,
         % get self reference
         self(Self),
         % send a message to self using (::)/2
         Self::Message,
         ... .

If we also need the sender of the message to be other than the object
containing the predicate definition, we can write:

     Predicate :-
         ...,
         % send a message to self using (::)/2
         % sender will be the pseudo-object user
         self(Self),
         {Self::Message},
         ... .

When events are not used, is possible to turn off event generation
globally or on a per entity basis by using the ‘events’ compiler flag to
optimize message sending performance (see the Event-driven programming
section for more details).

1.4.8 Sending a message from a module
-------------------------------------

Messages can be sent to object from within a Prolog module.  Depending
on the backend Prolog system and on the optimize flag being
turned on, the messages will use static binding when possible.  This
optimization requires the object to be compiled and loaded before the
module.  Note that the module can be ‘user’.  This is usually the case
when sending the message from the top-level interpreter.  Thus, the same
conditions apply in this case.

     Warning: If you want to benchmark the performance of a message
     sending goal at the top-level interpreter, be careful to check
     first if the goal is pre-compiled to use static binding, otherwise
     you will also be benchmarking the Logtalk compiler itself.

1.4.9 Message sending performance
---------------------------------

For a detailed discussion on message sending performance, see the
Performance section.

1.5 Objects
===========

The main goal of Logtalk objects is the encapsulation and reuse of
predicates.  Instead of a single database containing all your code,
Logtalk objects provide separated namespaces or databases allowing the
partitioning of code in more manageable parts.  Logtalk is a
'declarative programming language' and does not aim to bring some sort
of new dynamic state change concept to Logic Programming or Prolog.

Logtalk, defines two built-in objects, user(1) and logtalk(2), which are
described at the end of this section.

   ---------- Footnotes ----------

   (1) ../../docs/user_0.html#user-0

   (2) ../../docs/logtalk_0.html#logtalk-0

1.5.1 Objects, prototypes, classes, and instances
-------------------------------------------------

There are only three kinds of encapsulation entities in Logtalk:
'objects', 'protocols', and 'categories'.  Logtalk uses the term
'object' in a broad sense.  The terms 'prototype', 'parent', 'class',
'subclass', 'superclass', 'metaclass', and 'instance' always designate
an object.  Different names are used to emphasize the 'role' played by
an object in a particular context.  I.e.  we use a term other than
object when we want to make the relationship with other objects
explicit.  For example, an object with an 'instantiation' relation with
other object plays the role of an 'instance', while the instantiated
object plays the role of a 'class'; an object with a 'specialization'
relation with other object plays the role of a 'subclass', while the
specialized object plays the role of a 'superclass'; an object with an
'extension' relation with other object plays the role of a 'prototype',
the same for the extended object.  A 'stand-alone' object, i.e.  an
object with no relations with other objects, is always interpreted as a
prototype.  In Logtalk, entity relations essentially define 'patterns'
of code reuse.  An entity is compiled accordingly to the roles it plays.

Logtalk allows you to work from standalone objects to any kind of
hierarchy, either class-based or prototype-based.  You may use single or
multiple inheritance, use or forgo metaclasses, implement reflective
designs, use parametric objects, and take advantage of protocols and
categories (think components).

1.5.1.1 Prototypes
..................

Prototypes are either self-defined objects or objects defined as
extensions to other prototypes with whom they share common properties.
Prototypes are ideal for representing one-of-a-kind objects.  Prototypes
usually represent concrete objects in the application domain.  When
linking prototypes using 'extension' relations, Logtalk uses the term
'prototype hierarchies' although most authors prefer to use the term
'hierarchy' only with class generalization/specialization relations.  In
the context of logic programming, prototypes are often the ideal
replacement for modules.

1.5.1.2 Classes
...............

Classes are used to represent abstractions of common properties of sets
of objects.  Classes often provide an ideal structuring solution when
you want to express hierarchies of abstractions or work with many
similar objects.  Classes are used indirectly through 'instantiation'.
Contrary to most object-oriented programming languages, instances can be
created both dynamically at runtime or defined in a source file like
other objects.  Using classes in requires defining at least one
metaclass, as explained below.

1.5.2 Defining a new object
---------------------------

We can define a new object in the same way we write Prolog code: by
using a text editor.  Logtalk source files may contain one or more
objects, categories, or protocols.  If you prefer to define each entity
in its own source file, it is recommended that the file be named after
the object.  By default, all Logtalk source files use the extension
‘.lgt’ but this is optional and can be set in the adapter files.
Intermediate Prolog source files (generated by the Logtalk compiler)
have, by default, a ‘_lgt’ suffix and a ‘.pl’ extension.  Again, this
can be set to match the needs of a particular Prolog compiler in the
corresponding adapter file.  For instance, we may define an object named
‘vehicle’ and save it in a ‘vehicle.lgt’ source file which will be
compiled to a ‘vehicle_lgt.pl’ Prolog file (depending on the
backend compiler, the names of the intermediate Prolog files may
include a directory hash and a process identifier to prevent file name
clashes when embedding Logtalk applications or running parallel Logtalk
processes).

Object names can be atoms or compound terms (when defining parametric
objects, see below).  Objects, categories, and protocols share the same
name space: we cannot have an object with the same name as a protocol or
a category.

Object code (directives and predicates) is textually encapsulated by
using two Logtalk directives: object/1-5 and
end_object/0.  The most simple object will be one that is
self-contained, not depending on any other Logtalk entity:

     :- object(Object).
         ...
     :- end_object.

If an object implements one or more protocols then the opening directive
will be:

     :- object(Object,
         implements([Protocol1, Protocol2, ...])).
         ...
     :- end_object.

An object can import one or more categories:

     :- object(Object,
         imports([Category1, Category2, ...])).
         ...
     :- end_object.

If an object both implements protocols and imports categories then we
will write:

     :- object(Object,
         implements([Protocol1, Protocol2, ...]),
         imports([Category1, Category2, ...])).
         ...
     :- end_object.

In object-oriented programming objects are usually organized in
hierarchies that enable interface and code sharing by inheritance.  In
Logtalk, we can construct prototype-based hierarchies by writing:

     :- object(Prototype,
         extends(Parent)).
         ...
     :- end_object.

We can also have class-based hierarchies by defining instantiation and
specialization relations between objects.  To define an object as a
class instance we will write:

     :- object(Object,
         instantiates(Class)).
         ...
     :- end_object.

A class may specialize another class, its superclass:

     :- object(Class,
         specializes(Superclass)).
         ...
     :- end_object.

If we are defining a reflexive system where every class is also an
instance, we will probably be using the following pattern:

     :- object(Class,
         instantiates(Metaclass),
         specializes(Superclass)).
         ...
     :- end_object.

In short, an object can be a 'stand-alone' object or be part of an
object hierarchy.  The hierarchy can be prototype-based (defined by
extending other objects) or class-based (with instantiation and
specialization relations).  An object may also implement one or more
protocols or import one or more categories.

A 'stand-alone' object (i.e.  an object with no extension,
instantiation, or specialization relations with other objects) always
plays the role of a prototype, that is, a self-describing object.  If we
want to use classes and instances, then we will need to specify at least
one instantiation or specialization relation.  The best way to do this
is to define a set of objects that provide the basis of a reflective
system [Cointe87], [Moura94].  For example:

     % avoid the inevitable unknown entity warnings as in a
     % reflective system there will always be references to
     % an entity that will be defined after the reference

     :- set_logtalk_flag(unknown_entities, silent).

     % default root of the inheritance graph
     % providing predicates common to all objects

     :- object(object,
         instantiates(class)).
         ...
     :- end_object.

     % default metaclass for all classes providing
     % predicates common to all instantiable classes

     :- object(class,
         instantiates(class),
         specializes(abstract_class)).
         ...
     :- end_object.

     % default metaclass for all abstract classes
     % providing predicates common to all classes

     :- object(abstract_class,
         instantiates(class),
         specializes(object)).
         ...
     :- end_object.

Note that with these instantiation and specialization relations,
‘object’, ‘class’, and ‘abstract_class’ are, at the same time, classes
and instances of some class.  In addition, each object inherits its own
predicates and the predicates of the other two objects without any
inheritance loop.

When a full-blown reflective system solution is not needed, the above
scheme can be simplified by making an object an instance of itself, i.e.
by making a class its own metaclass.  For example:

     :- object(class,
         instantiates(class)).
         ...
     :- end_object.

We can use, in the same application, both prototype and class-based
hierarchies (and freely exchange messages between all objects).  We
cannot however mix the two types of hierarchies by, e.g., specializing
an object that extends another object in this current Logtalk version.

Logtalk also supports public, protected, and private inheritance.  See
the inheritance section for details.

1.5.3 Parametric objects
------------------------

Parametric objects have a compound term as identifier where all the
arguments of the compound term are variables.  These variables can be
bound when sending a message or become bound when a message to the
object succeeds, thus acting as 'object parameters'.  The object
predicates can be coded to depend on those parameters, which are logical
variables shared by all object predicates.  When an object state is set
at object creation and never changed, parameters provide a better
solution than using the object’s database via asserts.  Parametric
objects can also be used to associate a set of predicates to terms that
share a common functor and arity.

1.5.3.1 Accessing object parameters
...................................

Object parameters can be accessed using parameter variables.
or built-in execution context methods.  Parameter variables is the
recommended solution to access object parameters.  Although they
introduce a concept of entity global variables, their unique syntax
(‘_ParameterName_’) avoids conflicts, makes them easily recognizable,
and distinguishes them from other named anonymous variables.  For
example:

     :- object(foo(_Bar_, _Baz_, ...)).

         ...
         bar(_Bar_).

         baz :-
             baz(_Baz_),
             ... .

Note that using parameter variables doesn’t change the fact that entity
parameters are logical variables.  Parameter variables simplify code
maintenance by allowing parameters to be added, reordered, or removed
without having to specify or update parameter indexes.

Logtalk provides also a parameter/2 built-in local method to
access individual parameters:

     :- object(foo(_Bar, _Baz, ...)).

         ...
         bar(Bar) :-
             parameter(1, Bar).

         baz :-
             parameter(2, Baz),
             baz(Baz),
             ... .

An alternative solution is to use the built-in local method
this/1, which allows to access all parameters with a single call.
For example:

     :- object(foo(_Bar, _Baz, ...)).

         ...
         baz :-
             this(foo(_, Baz, ...)),
             baz(Baz),
             ... .

Both solutions are equally efficient as calls to the methods ‘this/1’
and ‘parameter/2’ are usually compiled inline into a clause head
unification.  The drawback of this second solution is that we must check
all calls of ‘this/1’ if we change the object name.  Note that we can’t
use these method with the message sending operators (see (;;)/2,
(;;)/1, or (^^)/1).

When storing a parametric object in its own source file, the convention
is to name the file after the object, with the object arity appended.
For instance, when defining an object named ‘sort(Type)’, we may save it
in a ‘sort_1.lgt’ text file.  This way it is easy to avoid file name
clashes when saving Logtalk entities that have the same functor but
different arity.

1.5.3.2 Parametric object proxies
.................................

Compound terms with the same functor and with the same number of
arguments as a parametric object identifier may act as 'proxies' to a
parametric object.  Proxies may be stored on the database as Prolog
facts and be used to represent different instantiations of a parametric
object identifier.  For example:

     :- object(circle(_Id_, _Radius_, _Color_)).

         :- public(area/1).
         ...

     :- end_object.

     % parametric object proxies:
     circle('#1', 1.23, blue).
     circle('#2', 3.71, yellow).
     circle('#3', 0.39, green).
     circle('#4', 5.74, black).
     circle('#5', 8.32, cyan).

Logtalk provides a convenient notation for accessing proxies represented
as Prolog facts when sending a message:

     ..., {Proxy}::Message, ...

For example, using the ‘circle/3’ parametric object above, we can
compute a list with the areas of all circles using the following goal:

     | ?- findall(Area, {circle(_, _, _)}::area(Area), Areas).

     Areas = [4.75291, 43.2412, 0.477836, 103.508, 217.468].

In this context, the proxy argument is proved as a plain Prolog goal.
If successful, the message is sent to the corresponding parametric
object.  Typically, the proof allows retrieving of parameter
instantiations.  This construct can either be used with a proxy argument
that is sufficiently instantiated in order to unify with a single Prolog
fact or with a proxy argument that unifies with several facts on
backtracking.

1.5.4 Finding defined objects
-----------------------------

We can find, by backtracking, all defined objects by calling the
current_object/1 built-in predicate with a unbound argument:

     | ?- current_object(Object).
     Object = logtalk ;
     Object = user ;
     ...

This predicate can also be used to test if an object is defined by
calling it with a valid object identifier (an atom or a compound term).

1.5.5 Creating a new object in runtime
--------------------------------------

An object can be dynamically created at runtime by using the
create_object/4 built-in predicate:

     | ?- create_object(Object, Relations, Directives, Clauses).

The first argument should be either a variable or the name of the new
object (a Prolog atom or compound term, which must not match any
existing entity name).  The remaining three arguments correspond to the
relations described in the opening object directive and to the object
code contents (directives and clauses).

For example, the call:

     | ?- create_object(
              foo,
              [extends(bar)],
              [public(foo/1)],
              [foo(1), foo(2)]
          ).

is equivalent to compiling and loading the object:

     :- object(foo,
         extends(bar)).

         :- dynamic.

         :- public(foo/1).
         foo(1).
         foo(2).

     :- end_object.

If we need to create a lot of (dynamic) objects at runtime, then is best
to define a metaclass or a prototype with a predicate that will call
this built-in predicate to make new objects.  This predicate may provide
automatic object name generation, name checking, and accept object
initialization options.

1.5.6 Abolishing an existing object
-----------------------------------

Dynamic objects can be abolished using the abolish_object/1.
built-in predicate:

     | ?- abolish_object(Object).

The argument must be an identifier of a defined dynamic object,
otherwise an error will be thrown.

1.5.7 Object directives
-----------------------

Object directives are used to set initialization goals, define object
properties, to document an object dependencies on other Logtalk
entities, and to load the contents of files into an object.

1.5.7.1 Object initialization
.............................

We can define a goal to be executed as soon as an object is (compiled
and) loaded to memory with the initialization/1 directive:

     :- initialization(Goal).

The argument can be any valid Logtalk goal.  For example, a call to a
local predicate:

     :- object(foo).

         :- initialization(init).
         :- private(init/0).

         init :-
             ... .

         ...

     :- end_object.

Or a message to another object:

     :- object(assembler).

         :- initialization(control::start).
         ...

     :- end_object.

Another common initialization goal is a message to 'self' in order to
call an inherited or imported predicate.  For example, assuming that we
have a ‘monitor’ category defining a ‘reset/0’ predicate, we could
write:

     :- object(profiler,
         imports(monitor)).

         :- initialization(::reset).
         ...

     :- end_object.

Note, however, that descendant objects do not inherit initialization
directives.  In this context, 'self' denotes the object that contains
the directive.  Also note that object initialization does not
necessarily mean setting an object dynamic state.

1.5.7.2 Dynamic objects
.......................

Similar to Prolog predicates, an object can be either static or dynamic.
An object created during the execution of a program is always dynamic.
An object defined in a file can be either dynamic or static.  Dynamic
objects are declared by using the dynamic/0 directive in the
object source code:

     :- dynamic.

The directive must precede any predicate directives or clauses.  Please
be aware that using dynamic code results in a performance hit when
compared to static code.  We should only use dynamic objects when these
need to be abolished during program execution.  In addition, note that
we can declare and define dynamic predicates within a static object.

1.5.7.3 Object documentation
............................

An object can be documented with arbitrary user-defined information by
using the info/1 entity directive.  See the
Documenting section for details.

1.5.7.4 Loading files into an object
....................................

The include/1 directive can be used to load the contents of a
file into an object.  A typical usage scenario is to load a plain Prolog
file into an object thus providing a simple way to encapsulate its
contents.  For example, assume a ‘cities.pl’ file defining facts for a
‘city/4’ predicate.  We could define a wrapper for this database by
writing:

     :- object(cities).

         :- public(city/4).

         :- include(dbs('cities.pl')).

     :- end_object.

The ‘include/1’ directive can also be used when creating an object
dynamically.  For example:

     | ?- create_object(cities, [], [public(city/4), include(dbs('cities.pl'))], []).

1.5.7.5 Declaring object aliases
................................

The uses/1 directive can be used to declare object aliases.
The typical uses of this directive include shortening long object names,
working consistently with specific parameterizations of parametric
objects, and simplifying experimenting with different object
implementations of the same protocol when using explicit message
sending.

1.5.8 Object relationships
--------------------------

Logtalk provides six sets of built-in predicates that enable us to query
the system about the possible relationships that an object may have with
other entities.

The instantiates_class/2-3 built-in predicates can be used to
query all instantiation relations:

     | ?- instantiates_class(Instance, Class).

or, if we also want to know the instantiation scope:

     | ?- instantiates_class(Instance, Class, Scope).

Specialization relations can be found by using the
specializes_class/2-3 built-in predicates:

     | ?- specializes_class(Class, Superclass).

or, if we also want to know the specialization scope:

     | ?- specializes_class(Class, Superclass, Scope).

For prototypes, we can query extension relations using with the
extends_object/2-3 built-in predicates:

     | ?- extends_object(Object, Parent).

or, if we also want to know the extension scope:

     | ?- extends_object(Object, Parent, Scope).

In order to find which objects import which categories we can use the
imports_category/2-3 built-in predicates:

     | ?- imports_category(Object, Category).

or, if we also want to know the importation scope:

     | ?- imports_category(Object, Category, Scope).

To find which objects implements which protocols we can use the
implements_protocol/2-3 and conforms_to_protocol/2-3.
built-in predicates:

     | ?- implements_protocol(Object, Protocol, Scope).

or, if we also want to consider inherited protocols:

     | ?- conforms_to_protocol(Object, Protocol, Scope).

Note that, if we use a unbound first argument, we will need to use the
current_object/1 built-in predicate to ensure that the entity
returned is an object and not a category.

To find which objects are explicitly complemented by categories we can
use the complements_object/2 built-in predicate:

     | ?- complements_object(Category, Object).

Note that more than one category may explicitly complement a single
object and a single category can complement several objects.

1.5.9 Object properties
-----------------------

We can find the properties of defined objects by calling the built-in
predicate object_property/2.:

     | ?- object_property(Object, Property).

The following object properties are supported:

‘static’

     The object is static

‘dynamic’

     The object is dynamic (and thus can be abolished in runtime by
     calling the abolish_object/1 built-in predicate)

‘built_in’

     The object is a built-in object (and thus always available)

‘threaded’

     The object supports/makes multi-threading calls

‘file(Path)’

     Absolute path of the source file defining the object (if
     applicable)

‘file(Basename, Directory)’

     Basename and directory of the source file defining the object (if
     applicable); ‘Directory’ always ends with a ‘/’

‘lines(BeginLine, EndLine)’

     Source file begin and end lines of the object definition (if
     applicable)

‘directive(BeginLine, EndLine)’

     Source file begin and end lines of the object opening directive (if
     applicable)

‘context_switching_calls’

     The object supports context switching calls (i.e.  can be used with
     the (<<)/2 debugging control construct)

‘dynamic_declarations’

     The object supports dynamic declarations of predicates

‘events’

     Messages sent from the object generate events

‘source_data’

     Source data available for the object

‘complements(Permission)’

     The object supports complementing categories with the specified
     permission (‘allow’ or ‘restrict’)

‘complements’

     The object supports complementing categories

‘public(Resources)’

     List of public predicates and operators declared by the object

‘protected(Resources)’

     List of protected predicates and operators declared by the object

‘private(Resources)’

     List of private predicates and operators declared by the object

‘declares(Predicate, Properties)’

     List of properties for a predicate declared by the
     object

‘defines(Predicate, Properties)’

     List of properties for a predicate defined by the object

‘includes(Predicate, Entity, Properties)’

     List of properties for an object multifile predicate
     that are defined in the specified entity (the properties include
     ‘number_of_clauses(Number)’, ‘number_of_rules(Number)’,
     ‘lines(Start,End)’, and ‘line_count(Start)’ with ‘Start’ being the
     begin line of the first multifile predicate clause)

‘provides(Predicate, Entity, Properties)’

     List of properties for other entity multifile predicate
     that are defined in the object (the properties include
     ‘number_of_clauses(Number)’, ‘number_of_rules(Number)’, and
     ‘lines(Start,End)’, and ‘line_count(Start)’ with ‘Start’ being the
     begin line of the first multifile predicate clause)

‘alias(Entity, Properties)’

     List of properties for an entity alias.
     declared by the object (the properties include ‘object’ in case of
     an object alias, ‘module’ in case of a module alias,
     ‘for(Original)’, ‘lines(Start,End)’, and ‘line_count(Start)’ with
     ‘Start’ being the begin line of the ‘uses/1’ or ‘use_module/1’
     directive)

‘alias(Predicate, Properties)’

     List of properties for a predicate alias.
     declared by the object (the properties include ‘predicate’,
     ‘for(Original)’, ‘from(Entity)’, ‘non_terminal(NonTerminal)’,
     ‘lines(Start,End)’, and ‘line_count(Start)’ with ‘Start’ being the
     begin line of the alias directive)

‘calls(Call, Properties)’

     List of properties for predicate calls made by the
     object (‘Call’ is either a predicate indicator or a control
     construct such as ‘(::)/1-2’ or ‘(^^)/1’ with a predicate indicator
     as argument; note that ‘Call’ may not be ground in case of a call
     to a control construct where its argument is only know at runtime;
     the properties include ‘caller(Caller)’, ‘alias(Alias)’,
     ‘non_terminal(NonTerminal)’, ‘lines(Start,End)’,
     ‘line_count(Start)’ with ‘Caller’, ‘Alias’, and ‘NonTerminal’ being
     predicate indicators and ‘Start’ being the begin line of the
     predicate clause or directive making the call)

‘updates(Predicate, Properties)’

     List of properties for dynamic predicate updates (and
     also access using the ‘clause/2’ predicate) made by the object
     (‘Predicate’ is either a predicate indicator or a control construct
     such as ‘(::)/1-2’ or ‘(:)/2’ with a predicate indicator as
     argument; note that ‘Predicate’ may not be ground in case of a
     control construct argument only know at runtime; the properties
     include ‘updater(Updater)’, ‘alias(Alias)’,
     ‘non_terminal(NonTerminal)’, ‘lines(Start,End)’, and
     ‘line_count(Start)’ with ‘Updater’ being a (possibly multifile)
     predicate indicator, ‘Alias’ and ‘NonTerminal’ being predicate
     indicators, and ‘Start’ being the begin line of the predicate
     clause or directive updating the predicate)

‘number_of_clauses(Number)’

     Total number of predicate clauses defined in the object at
     compilation time (includes both user-defined clauses and auxiliary
     clauses generated by the compiler or by the expansion hooks
     but does not include clauses for multifile predicates defined
     for other entities or clauses for the object own multifile
     predicates contributed by other entities)

‘number_of_rules(Number)’

     Total number of predicate rules defined in the object at
     compilation time (includes both user-defined rules and auxiliary
     rules generated by the compiler or by the expansion hooks
     but does not include rules for multifile predicates defined for
     other entities or rules for the object own multifile predicates
     contributed by other entities)

‘number_of_user_clauses(Number)’

     Total number of user-defined predicate clauses defined in the
     object at compilation time (does not include clauses for multifile
     predicates defined for other entities or clauses for the object own
     multifile predicates contributed by other entities)

‘number_of_user_rules(Number)’

     Total number of user-defined predicate rules defined in the object
     at compilation time (does not include rules for multifile
     predicates defined for other entities or rules for the object own
     multifile predicates contributed by other entities)

‘debugging’

     The object is compiled in debug mode

‘module’

     The object resulted from the compilation of a Prolog module

When a predicate is called from an ‘initialization/1’ directive, the
argument of the ‘caller/1’ property is ‘:-/1’.

Some properties such as line numbers are only available when the object
is defined in a source file compiled with the source_data.
flag turned on.  Moreover, line numbers are only supported in
backend Prolog compilers that provide access to the start line of a
read term.  When such support is not available, the value ‘-1’ is
returned for the start and end lines.

The properties that return the number of clauses (rules) report the
clauses (rules) 'textually defined in the object' for both multifile and
non-multifile predicates.  Thus, these numbers exclude clauses (rules)
for multifile predicates 'contributed' by other entities.

1.5.10 Built-in objects
-----------------------

Logtalk defines some built-in objects that are always available for any
application.

1.5.10.1 The built-in pseudo-object ‘user’
..........................................

The built-in user(1) pseudo-object virtually contains all user predicate
definitions not encapsulated in a Logtalk entity (or a Prolog module for
backends supporting a module system).  These predicates are assumed to
be implicitly declared public.  Messages sent from this pseudo-object,
which includes messages sent from the top-level interpreter, generate
events when the default value of the events flag is set to
‘allow’.  Defining complementing categories for this pseudo-object is
not supported.

With some of the backend Prolog compilers that support a
module system, it is possible to load (the) Logtalk (compiler/runtime)
into a module other than the pseudo-module ‘user’.  In this case, the
Logtalk pseudo-object ‘user’ virtually contains all user predicate
definitions defined in the module where Logtalk was loaded.

   ---------- Footnotes ----------

   (1) ../../docs/user_0.html#user-0

1.5.10.2 The built-in object ‘logtalk’
......................................

The built-in logtalk(1) object provides message printing.
predicates, see question asking predicates, see debug and trace event
predicates, predicates for accessing the internal database of
loaded files and their properties, and also a set of low-level utility
predicates normally used when defining hook objects.  Consult its API
documentation for details.

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0

1.6 Protocols
=============

Protocols enable the separation between interface and implementation:
several objects can implement the same protocol and an object can
implement several protocols.  Protocols may contain only predicate
declarations.  In some languages the term 'interface' is used with
similar meaning.  Logtalk allows predicate declarations of any scope
within protocols, contrary to some languages that only allow public
declarations.

Logtalk defines three built-in protocols, monitoring(1), expanding(2),
and forwarding(3), which are described at the end of this section.

   ---------- Footnotes ----------

   (1) ../../docs/monitoring_0.html#monitoring-0

   (2) ../../docs/expanding_0.html#expanding-0

   (3) ../../docs/forwarding_0.html#forwarding-0

1.6.1 Defining a new protocol
-----------------------------

We can define a new protocol in the same way we write Prolog code: by
using a text editor.  Logtalk source files may contain one or more
objects, categories, or protocols.  If you prefer to define each entity
in its own source file, it is recommended that the file be named after
the protocol.  By default, all Logtalk source files use the extension
‘.lgt’ but this is optional and can be set in the adapter files.
Intermediate Prolog source files (generated by the Logtalk compiler)
have, by default, a ‘_lgt’ suffix and a ‘.pl’ extension.  Again, this
can be set to match the needs of a particular Prolog compiler in the
corresponding adapter file.  For example, we may define a protocol named
‘listp’ and save it in a ‘listp.lgt’ source file that will be compiled
to a ‘listp_lgt.pl’ Prolog file (depending on the backend compiler, the
names of the intermediate Prolog files may include a directory hash and
a process identifier to prevent file name clashes when embedding Logtalk
applications or running parallel Logtalk processes).

Protocol names must be atoms.  Objects, categories and protocols share
the same namespace: we cannot have a protocol with the same name as an
object or a category.

Protocol directives are textually encapsulated by using two Logtalk
directives: protocol/1-2 and end_protocol/0.  The
most simple protocol will be one that is self-contained, not depending
on any other Logtalk entity:

     :- protocol(Protocol).
         ...
     :- end_protocol.

If a protocol extends one or more protocols, then the opening directive
will be:

     :- protocol(Protocol,
         extends([Protocol1, Protocol2, ...])).
         ...
     :- end_protocol.

In order to maximize protocol reuse, all predicates specified in a
protocol should relate to the same functionality.  Therefore, the only
recommended use of protocol extension is when you need both a minimal
protocol and an extended version of the same protocol with additional,
convenient predicates.

1.6.2 Finding defined protocols
-------------------------------

We can find, by backtracking, all defined protocols by using the
current_protocol/1 built-in predicate with a unbound argument:

     | ?- current_protocol(Protocol).

This predicate can also be used to test if a protocol is defined by
calling it with a valid protocol identifier (an atom).

1.6.3 Creating a new protocol in runtime
----------------------------------------

We can create a new (dynamic) protocol at runtime by calling the Logtalk
built-in predicate create_protocol/3.:

     | ?- create_protocol(Protocol, Relations, Directives).

The first argument should be either a variable or the name of the new
protocol (a Prolog atom, which must not match an existing entity name).
The remaining two arguments correspond to the relations described in the
opening protocol directive and to the protocol directives.

For instance, the call:

     | ?- create_protocol(ppp, [extends(qqq)], [public([foo/1, bar/1])]).

is equivalent to compiling and loading the protocol:

     :- protocol(ppp,
         extends(qqq)).

         :- dynamic.

         :- public([foo/1, bar/1]).

     :- end_protocol.

If we need to create a lot of (dynamic) protocols at runtime, then is
best to define a metaclass or a prototype with a predicate that will
call this built-in predicate in order to provide more sophisticated
behavior.

1.6.4 Abolishing an existing protocol
-------------------------------------

Dynamic protocols can be abolished using the abolish_protocol/1
built-in predicate:

     | ?- abolish_protocol(Protocol).

The argument must be an identifier of a defined dynamic protocol,
otherwise an error will be thrown.

1.6.5 Protocol directives
-------------------------

Protocol directives are used to define protocol properties and
documentation.

1.6.5.1 Dynamic protocols
.........................

As usually happens with Prolog code, a protocol can be either static or
dynamic.  A protocol created during the execution of a program is always
dynamic.  A protocol defined in a file can be either dynamic or static.
Dynamic protocols are declared by using the dynamic/0.
directive in the protocol source code:

     :- dynamic.

The directive must precede any predicate directives.  Please be aware
that using dynamic code results in a performance hit when compared to
static code.  We should only use dynamic protocols when these need to be
abolished during program execution.

1.6.5.2 Protocol documentation
..............................

A protocol can be documented with arbitrary user-defined information by
using the info/1 entity directive.  See the
Documenting section for details.

1.6.5.3 Loading files into a protocol
.....................................

The include/1 directive can be used to load the contents of a
file into a protocol.  See the Objects section for an example
of using this directive.

1.6.6 Protocol relationships
----------------------------

Logtalk provides two sets of built-in predicates that enable us to query
the system about the possible relationships that a protocol have with
other entities.

The extends_protocol/2-3 built-in predicates return all pairs
of protocols so that the first one extends the second:

     | ?- extends_protocol(Protocol1, Protocol2).

or, if we also want to know the extension scope:

     | ?- extends_protocol(Protocol1, Protocol2, Scope).

To find which objects or categories implement which protocols we can
call the implements_protocol/2-3 built-in predicates:

     | ?- implements_protocol(ObjectOrCategory, Protocol).

or, if we also want to know the implementation scope:

     | ?- implements_protocol(ObjectOrCategory, Protocol, Scope).

Note that, if we use a non-instantiated variable for the first argument,
we will need to use the current_object/1 or
current_category/1 built-in predicates to identify the kind of
entity returned.

1.6.7 Protocol properties
-------------------------

We can find the properties of defined protocols by calling the
protocol_property/2 built-in predicate:

     | ?- protocol_property(Protocol, Property).

A protocol may have the property ‘static’, ‘dynamic’, or ‘built_in’.
Dynamic protocols can be abolished in runtime by calling the
abolish_protocol/1 built-in predicate.  Depending on the
backend Prolog compiler, a protocol may have additional properties
related to the source file where it is defined.

The following protocol properties are supported:

‘static’

     The protocol is static

‘dynamic’

     The protocol is dynamic (and thus can be abolished in runtime by
     calling the abolish_category/1 built-in predicate)

‘built_in’

     The protocol is a built-in protocol (and thus always available)

‘source_data’

     Source data available for the protocol

‘file(Path)’

     Absolute path of the source file defining the protocol (if
     applicable)

‘file(Basename, Directory)’

     Basename and directory of the source file defining the protocol (if
     applicable); ‘Directory’ always ends with a ‘/’

‘lines(BeginLine, EndLine)’

     Source file begin and end lines of the protocol definition (if
     applicable)

‘directive(BeginLine, EndLine)’

     Source file begin and end lines of the protocol opening directive
     (if applicable)

‘public(Resources)’

     List of public predicates and operators declared by the protocol

‘protected(Resources)’

     List of protected predicates and operators declared by the protocol

‘private(Resources)’

     List of private predicates and operators declared by the protocol

‘declares(Predicate, Properties)’

     List of properties for a predicate declared by the
     protocol

‘alias(Predicate, Properties)’

     List of properties for a predicate alias.
     declared by the protocol (the properties include ‘for(Original)’,
     ‘from(Entity)’, ‘non_terminal(NonTerminal)’, and ‘line_count(Line)’
     with ‘Line’ being the begin line of the alias directive)

Some of the properties such as line numbers are only available when the
protocol is defined in a source file compiled with the
source_data flag turned on.

1.6.8 Implementing protocols
----------------------------

Any number of objects or categories can implement a protocol.  The
syntax is very simple:

     :- object(Object,
         implements(Protocol)).
         ...
     :- end_object.

or, in the case of a category:

     :- category(Object,
         implements(Protocol)).
         ...
     :- end_category.

To make all public predicates declared via an implemented protocol
protected or to make all public and protected predicates private we
prefix the protocol’s name with the corresponding keyword.  For
instance:

     :- object(Object,
         implements(private::Protocol)).
         ...
     :- end_object.

or:

     :- object(Object,
         implements(protected::Protocol)).
         ...
     :- end_object.

Omitting the scope keyword is equivalent to writing:

     :- object(Object,
         implements(public::Protocol)).
         ...
     :- end_object.

The same rules applies to protocols implemented by categories.

1.6.9 Built-in protocols
------------------------

Logtalk defines a set of built-in protocols that are always available
for any application.

1.6.9.1 The built-in protocol ‘expanding’
.........................................

The built-in expanding(1) protocol declares the term_expansion/2
and goal_expansion/2 predicates.  See the description of
the hook compiler flag for more details.

   ---------- Footnotes ----------

   (1) ../../docs/expanding_0.html#expanding-0

1.6.9.2 The built-in protocol ‘monitoring’
..........................................

The built-in monitoring(1) protocol declares the before/3.
and after/3 public event handler predicates.  See the
Event-driven programming section for more details.

   ---------- Footnotes ----------

   (1) ../../docs/monitoring_0.html#monitoring-0

1.6.9.3 The built-in protocol ‘forwarding’
..........................................

The built-in forwarding(1) protocol declares the forward/1.
user-defined message forwarding handler, which is automatically called
(if defined) by the runtime for any message that the receiving object
does not understand.  See also the []/1 control construct.

   ---------- Footnotes ----------

   (1) ../../docs/forwarding_0.html#forwarding-0

1.7 Categories
==============

Categories are 'fine-grained units of code reuse' and can be regarded as
a dual concept of protocols.  Categories provide a way to encapsulate a
set of related predicate declarations and definitions that do not
represent a complete object and that only make sense when composed with
other predicates.  Categories may also be used to break a complex object
in functional units.  A category can be imported by several objects
(without code duplication), including objects participating in prototype
or class-based hierarchies.  This concept of categories shares some
ideas with Smalltalk-80 functional categories [Goldberg83],
Flavors mix-ins [Moon86]. (without necessarily implying
multi-inheritance), and Objective-C categories [Cox86].
Categories may also 'complement' existing objects, thus providing a
hot patching mechanism inspired by the Objective-C
categories functionality.

Logtalk defines a built-in category, core_messages(1), which is
described at the end of this section.

   ---------- Footnotes ----------

   (1) ../../docs/core_messages_0.html#core-messages-0

1.7.1 Defining a new category
-----------------------------

We can define a new category in the same way we write Prolog code: by
using a text editor.  Logtalk source files may contain one or more
objects, categories, or protocols.  If you prefer to define each entity
in its own source file, it is recommended that the file be named after
the category.  By default, all Logtalk source files use the extension
‘.lgt’ but this is optional and can be set in the adapter files.
Intermediate Prolog source files (generated by the Logtalk compiler)
have, by default, a ‘_lgt’ suffix and a ‘.pl’ extension.  Again, this
can be set to match the needs of a particular Prolog compiler in the
corresponding adapter file.  For example, we may define a category named
‘documenting’ and save it in a ‘documenting.lgt’ source file that will
be compiled to a ‘documenting_lgt.pl’ Prolog file (depending on the
backend compiler, the names of the intermediate Prolog files
may include a directory hash and a process identifier to prevent file
name clashes when embedding Logtalk applications or running parallel
Logtalk processes).

Category names can be atoms or compound terms (when defining parametric
categories).  Objects, categories, and protocols share the same name
space: we cannot have a category with the same name as an object or a
protocol.

Category code (directives and predicates) is textually encapsulated by
using two Logtalk directives: category/1-4 and
end_category/0.  The most simple category will be one that is
self-contained, not depending on any other Logtalk entity:

     :- category(Category).
         ...
     :- end_category.

If a category implements one or more protocols then the opening
directive will be:

     :- category(Category,
         implements([Protocol1, Protocol2, ...])).
         ...
     :- end_category.

A category may be defined as a composition of other categories by
writing:

     :- category(Category,
         extends([Category1, Category2, ...])).
         ...
     :- end_category.

This feature should only be used when extending a category without
breaking its functional cohesion (for example, when a modified version
of a category is needed for importing on several unrelated objects).
The preferred way of composing several categories is by importing them
into an object.  When a category overrides a predicate defined in an
extended category, the overridden definition can still be called by
using the (^^)/1 control construct.

Categories cannot inherit from objects.  In addition, categories cannot
define clauses for dynamic predicates.  This restriction applies because
a category can be imported by several objects and because we cannot use
the database handling built-in methods with categories (messages can
only be sent to objects).  A consequence of this restriction is that a
category cannot declare a predicate (or non-terminal) as both multifile
and dynamic.  However, categories may contain declarations for dynamic
predicates and they can contain predicates which handle dynamic
predicates.  For example:

     :- category(attributes).

         :- public(attribute/2).
         :- public(set_attribute/2).
         :- public(del_attribute/2).

         :- private(attribute_/2).
         :- dynamic(attribute_/2).

         attribute(Attribute, Value) :-
             % called in the context of "self"
             ::attribute_(Attribute, Value).

         set_attribute(Attribute, Value) :-
             % retract old clauses in "self"
             ::retractall(attribute_(Attribute, _)),
             % assert new clause in "self"
             ::assertz(attribute_(Attribute, Value)).

         del_attribute(Attribute, Value) :-
             % retract clause in "self"
             ::retract(attribute_(Attribute, Value)).

     :- end_category.

Each object importing this category will have its own ‘attribute_/2’
private, dynamic predicate.  The predicates ‘attribute/2’,
‘set_attribute/2’, and ‘del_attribute/2’ always access and modify the
dynamic predicate contained in the object receiving the corresponding
messages (i.e.  'self').  But it’s also possible to define predicates
that handle dynamic predicates in the context of 'this' instead of
'self'.  For example:

     :- category(attributes).

         :- public(attribute/2).
         :- public(set_attribute/2).
         :- public(del_attribute/2).

         :- private(attribute_/2).
         :- dynamic(attribute_/2).

         attribute(Attribute, Value) :-
             % call in the context of "this"
             attribute_(Attribute, Value).

         set_attribute(Attribute, Value) :-
             % retract old clauses in "this"
             retractall(attribute_(Attribute, _)),
             % asserts clause in "this"
             assertz(attribute_(Attribute, Value)).

         del_attribute(Attribute, Value) :-
             % retract clause in "this"
             retract(attribute_(Attribute, Value)).

     :- end_category.

When defining a category that declares and handles dynamic predicates,
working in the context of 'this' ties those dynamic predicates to the
object importing the category while working in the context of 'self'
allows each object inheriting from the object that imports the category
to have its own set of clauses for those dynamic predicates.

1.7.2 Hot patching
------------------

A category may also explicitly complement one or more existing objects,
thus providing hot patching functionality inspired by
Objective-C categories:

     :- category(Category,
         complements([Object1, Object2, ....])).
         ...
     :- end_category.

This allows us to add missing directives (e.g.  to define aliases
for complemented object predicates), replace broken predicate
definitions, add new predicates, and add protocols and categories to
existing objects without requiring access or modifications to their
source code.  Common scenarios are adding logging or debugging
predicates to a set of objects.  Complemented objects need to be
compiled with the complements compiler flag set ‘allow’ (to
allow both patching and adding functionality) or ‘restrict’ (to allow
only adding new functionality).  A complementing category takes
preference over a previously loaded complementing category for the same
object thus allowing patching a previous patch if necessary.

When replacing a predicate definition, it is possible to call the
overriden definition in the object from the new definition in the
category by using the (@)/1 control construct.  This
construct is only meaningful when used within categories and requires a
compile time bound goal argument, which is called in this.
(i.e.  in the context of the complemented object or the object importing
a category).  As an example, consider the following object:

     :- object(bird).

         :- set_logtalk_flag(complements, allow).

         :- public(make_sound/0).
         make_sound :-
             write('Chirp, chirp!'), nl.

     :- end_object.

We can use the ‘(@)/1’ control construct e.g.  wrap the original
‘make_sound/0’ predicate definition by writing:

     :- category(logging,
         complements(bird)).

         make_sound :-
             write('Started making sound...'), nl,
             @make_sound,
             write('... finished making sound.'), nl.

     :- end_category.

After loading the object and the category, calling the ‘make_sound/0’
predicate will result in the following output:

     | ?- bird::make_sound.

     Started making sound...
     Chirp, chirp!
     ... finished making sound.
     yes

Note that super calls from predicates defined in
complementing categories lookup inherited definitions as if the calls
were made from the complemented object instead of the category
ancestors.  This allows more comprehensive object patching.  But it also
means that, if you want to patch an object so that it imports a category
that extends another category and uses super calls to access the
extended category predicates, you will need to define a (possibly empty)
complementing category that extends the category that you want to add.

An unfortunate consequence of allowing an object to be patched at
runtime using a complementing category is that it disables the use of
static binding optimizations for messages sent to the
complemented object as it can always be later patched, thus rendering
the static binding optimizations invalid.

Another important caveat is that, while a complementing category can
replace a predicate definition, local callers of the replaced predicate
will still call the non-patched version of the predicate.  This is a
consequence of the lack of a portable solution at the backend Prolog compiler
level for replacing static predicate definitions.

1.7.3 Finding defined categories
--------------------------------

We can find, by backtracking, all defined categories by using the
current_category/1 built-in predicate with a unbound argument:

     | ?- current_category(Category).

This predicate can also be used to test if a category is defined by
calling it with a valid category identifier (an atom or a compound
term).

1.7.4 Creating a new category in runtime
----------------------------------------

A category can be dynamically created at runtime by using the
create_category/4 built-in predicate:

     | ?- create_category(Category, Relations, Directives, Clauses).

The first argument should be either a variable or the name of the new
category (a Prolog atom, which must not match with an existing entity
name).  The remaining three arguments correspond to the relations
described in the opening category directive and to the category code
contents (directives and clauses).

For example, the call:

     | ?- create_category(
              ccc,
              [implements(ppp)],
              [private(bar/1)],
              [(foo(X):-bar(X)), bar(1), bar(2)]
          ).

is equivalent to compiling and loading the category:

     :- category(ccc,
         implements(ppp)).

         :- dynamic.

         :- private(bar/1).

         foo(X) :-
             bar(X).

         bar(1).
         bar(2).

     :- end_category.

If we need to create a lot of (dynamic) categories at runtime, then is
best to define a metaclass or a prototype with a predicate that will
call this built-in predicate in order to provide more sophisticated
behavior.

1.7.5 Abolishing an existing category
-------------------------------------

Dynamic categories can be abolished using the abolish_category/1
built-in predicate:

     | ?- abolish_category(Category).

The argument must be an identifier of a defined dynamic category,
otherwise an error will be thrown.

1.7.6 Category directives
-------------------------

Category directives are used to define category properties, to document
a category dependencies on other Logtalk entities, and to load the
contents of files into a category.

1.7.6.1 Dynamic categories
..........................

As usually happens with Prolog code, a category can be either static or
dynamic.  A category created during the execution of a program is always
dynamic.  A category defined in a file can be either dynamic or static.
Dynamic categories are declared by using the dynamic/0.
directive in the category source code:

     :- dynamic.

The directive must precede any predicate directives or clauses.  Please
be aware that using dynamic code results in a performance hit when
compared to static code.  We should only use dynamic categories when
these need to be abolished during program execution.

1.7.6.2 Category documentation
..............................

A category can be documented with arbitrary user-defined information by
using the info/1 entity directive.  See the
Documenting section for details.

1.7.6.3 Loading files into a category
.....................................

The include/1 directive can be used to load the contents of a
file into a category.  See the Objects section for an example
of using this directive.

1.7.6.4 Declaring object aliases
................................

The uses/1 directive can be used to declare object aliases.
The typical uses of this directive is to shorten long object names and
to simplify experimenting with different object implementations of the
same protocol when using explicit message sending.

1.7.7 Category relationships
----------------------------

Logtalk provides two sets of built-in predicates that enable us to query
the system about the possible relationships that a category can have
with other entities.

The built-in predicates implements_protocol/2-3 and
conforms_to_protocol/2-3 allows us to find which categories
implements which protocols:

     | ?- implements_protocol(Category, Protocol, Scope).

or, if we also want to consider inherited protocols:

     | ?- conforms_to_protocol(Category, Protocol, Scope).

Note that, if we use a unbound first argument, we will need to use the
current_category/1 built-in predicate to ensure that the
returned entity is a category and not an object.

To find which objects import which categories we can use the
imports_category/2-3 built-in predicates:

     | ?- imports_category(Object, Category).

or, if we also want to know the importation scope:

     | ?- imports_category(Object, Category, Scope).

Note that a category may be imported by several objects.

To find which categories extend other categories we can use the
extends_category/2-3 built-in predicates:

     | ?- extends_category(Category1, Category2).

or, if we also want to know the extension scope:

     | ?- extends_category(Category1, Category2, Scope).

Note that a category may be extended by several categories.

To find which categories explicitly complement existing objects we can
use the complements_object/2 built-in predicate:

     | ?- complements_object(Category, Object).

Note that a category may explicitly complement several objects.

1.7.8 Category properties
-------------------------

We can find the properties of defined categories by calling the built-in
predicate category_property/2.:

     | ?- category_property(Category, Property).

The following category properties are supported:

‘static’

     The category is static

‘dynamic’

     The category is dynamic (and thus can be abolished in runtime by
     calling the abolish_category/1 built-in predicate)

‘built_in’

     The category is a built-in category (and thus always available)

‘file(Path)’

     Absolute path of the source file defining the category (if
     applicable)

‘file(Basename, Directory)’

     Basename and directory of the source file defining the category (if
     applicable); ‘Directory’ always ends with a ‘/’

‘lines(BeginLine, EndLine)’

     Source file begin and end lines of the category definition (if
     applicable)

‘directive(BeginLine, EndLine)’

     Source file begin and end lines of the category opening directive
     (if applicable)

‘events’

     Messages sent from the category generate events

‘source_data’

     Source data available for the category

‘public(Resources)’

     List of public predicates and operators declared by the category

‘protected(Resources)’

     List of protected predicates and operators declared by the category

‘private(Resources)’

     List of private predicates and operators declared by the category

‘declares(Predicate, Properties)’

     List of properties for a predicate declared by the
     category

‘defines(Predicate, Properties)’

     List of properties for a predicate defined by the
     category

‘includes(Predicate, Entity, Properties)’

     List of properties for an object multifile predicate
     that are defined in the specified entity (the properties include
     ‘number_of_clauses(Number)’, ‘number_of_rules(Number)’,
     ‘lines(Start,End)’, and ‘line_count(Start)’ with ‘Start’ being the
     begin line of the first multifile predicate clause)

‘provides(Predicate, Entity, Properties)’

     List of properties for other entity multifile predicate
     that are defined in the category (the properties include
     ‘number_of_clauses(Number)’, ‘number_of_rules(Number)’,
     ‘lines(Start,End)’, and''line_count(Start)'' with ‘Start’ being the
     begin line of the first multifile predicate clause)

‘alias(Entity, Properties)’

     List of properties for an entity alias.
     declared by the object (the properties include ‘object’ in case of
     an object alias, ‘module’ in case of a module alias,
     ‘for(Original)’, ‘lines(Start,End)’, and ‘line_count(Start)’ with
     ‘Start’ being the begin line of the ‘uses/1’ or ‘use_module/1’
     directive)

‘alias(Predicate, Properties)’

     List of properties for a predicate alias.
     declared by the category (the properties include ‘predicate’,
     ‘for(Original)’, ‘from(Entity)’, ‘non_terminal(NonTerminal)’,
     ‘lines(Start,End)’, and ‘line_count(Start)’ with ‘Start’ being the
     begin line of the alias directive)

‘calls(Call, Properties)’

     List of properties for predicate calls made by the
     category (‘Call’ is either a predicate indicator or a control
     construct such as ‘(::)/1-2’ or ‘(^^)/1’ with a predicate indicator
     as argument; note that ‘Call’ may not be ground in case of a call
     to a control construct where its argument is only know at runtime;
     the properties include ‘caller(Caller)’, ‘alias(Alias)’,
     ‘non_terminal(NonTerminal)’, ‘lines(Start,End)’, and
     ‘line_count(Start)’ with ‘Caller’, ‘Alias’, and ‘NonTerminal’ being
     predicate indicators and ‘Start’ being the begin line of the
     predicate clause or directive making the call)

‘updates(Predicate, Properties)’

     List of properties for dynamic predicate updates (and
     also access using the ‘clause/2’ predicate) made by the object
     (‘Predicate’ is either a predicate indicator or a control construct
     such as ‘(::)/1-2’ or ‘(:)/2’ with a predicate indicator as
     argument; note that ‘Predicate’ may not be ground in case of a
     control construct argument only know at runtime; the properties
     include ‘updater(Updater)’, ‘alias(Alias)’,
     ‘non_terminal(NonTerminal)’, ‘lines(Start,End)’, and
     ‘line_count(Start)’ with ‘Updater’ being a (possibly multifile)
     predicate indicator, ‘Alias’ and ‘NonTerminal’ being predicate
     indicators, and ‘Start’ being the begin line of the predicate
     clause or directive updating the predicate)

‘number_of_clauses(Number)’

     Total number of predicate clauses defined in the category (includes
     both user-defined clauses and auxiliary clauses generated by the
     compiler or by the expansion hooks but does not include
     clauses for multifile predicates defined for other entities or
     clauses for the category own multifile predicates contributed by
     other entities)

‘number_of_rules(Number)’

     Total number of predicate rules defined in the category (includes
     both user-defined rules and auxiliary rules generated by the
     compiler or by the expansion hooks but does not include
     rules for multifile predicates defined for other entities or rules
     for the category own multifile predicates contributed by other
     entities)

‘number_of_user_clauses(Number)’

     Total number of user-defined predicate clauses defined in the
     category (does not include clauses for multifile predicates defined
     for other entities or clauses for the category own multifile
     predicates contributed by other entities)

‘number_of_user_rules(Number)’

     Total number of user-defined predicate rules defined in the
     category (does not include rules for multifile predicates defined
     for other entities or rules for the category own multifile
     predicates contributed by other entities)

Some properties such as line numbers are only available when the
category is defined in a source file compiled with the
source_data flag turned on.  Moreover, line numbers are only
supported in backend Prolog compilers that provide access to
the start line of a read term.  When such support is not available, the
value ‘-1’ is returned for the start and end lines.

The properties that return the number of clauses (rules) report the
clauses (rules) 'textually defined in the object' for both multifile and
non-multifile predicates.  Thus, these numbers exclude clauses (rules)
for multifile predicates 'contributed' by other entities.

1.7.9 Importing categories
--------------------------

Any number of objects can import a category.  In addition, an object may
import any number of categories.  The syntax is very simple:

     :- object(Object,
         imports([Category1, Category2, ...])).
         ...
     :- end_object.

To make all public predicates imported via a category protected or to
make all public and protected predicates private we prefix the
category’s name with the corresponding keyword:

     :- object(Object,
         imports(private::Category)).
         ...
     :- end_object.

or:

     :- object(Object,
         imports(protected::Category)).
         ...
     :- end_object.

Omitting the scope keyword is equivalent to writing:

     :- object(Object,
         imports(public::Category)).
         ...
     :- end_object.

1.7.10 Calling category predicates
----------------------------------

Category predicates can be called from within an object by sending a
message to 'self' or using a 'super' call.  Consider the following
category:

     :- category(output).

         :- public(out/1).

         out(X) :-
             write(X), nl.

     :- end_category.

The predicate ‘out/1’ can be called from within an object importing the
category by simply sending a message to 'self'.  For example:

     :- object(worker,
         imports(output)).

         ...
         do(Task) :-
             execute(Task, Result),
             ::out(Result).
         ...

     :- end_object.

This is the recommended way of calling a category predicate that can be
specialized/overridden in a descendant object as the predicate
definition lookup will start from 'self'.

A direct call to a predicate definition found in an imported category
can be made using the (^^)/1 control construct.  For example:

     :- object(worker,
         imports(output)).

         ...
         do(Task) :-
             execute(Task, Result),
             ^^out(Result).
         ...

     :- end_object.

This alternative should only be used when the user knows a priori that
the category predicates will not be specialized or redefined by
descendant objects of the object importing the category.  Its advantage
is that, when the optimize flag is turned on, the Logtalk
compiler will try to optimize the calls by using static binding.
When dynamic binding is used due to e.g.  the lack of
sufficient information at compilation time, the performance is similar
to calling the category predicate using a message to self. (in
both cases a predicate lookup caching mechanism is used).

1.7.11 Parametric categories
----------------------------

Category predicates can be parameterized in the same way as object
predicates by using a compound term as the category identifier where all
the arguments of the compound term are variables.  These variables, the
'category parameters', can be accessed by calling the parameter/2
or this/1 built-in local methods in the category
predicate clauses or by using parameter variables.

Category parameter values can be defined by the importing objects.  For
example:

     :- object(speech(Season, Event),
         imports([dress(Season), speech(Event)])).
         ...
     :- end_object.

Note that access to category parameters is only possible from within the
category.  In particular, calls to the this/1 built-in local
method from category predicates always access the importing object
identifier (and thus object parameters, not category parameters).

1.7.12 Built-in categories
--------------------------

Logtalk defines a built-in category that is always available for any
application.

1.7.12.1 The built-in category ‘core_messages’
..............................................

The built-in core_messages(1) category provides default translations for
all compiler and runtime printed messages such as warnings and errors.
It does not define any public predicates.

   ---------- Footnotes ----------

   (1) ../../docs/core_messages_0.html#core-messages-0

1.8 Predicates
==============

Predicate directives and clauses can be encapsulated inside objects and
categories.  Protocols can only contain predicate directives.  From the
point-of-view of a traditional imperative object-oriented language,
predicates allows both object state and object behavior to be
represented.  Mutable object state can be represented using dynamic
object predicates but should only be used when strictly necessary as it
breaks declarative semantics.

1.8.1 Reserved predicate names
------------------------------

For practical and performance reasons, some predicate names have a fixed
interpretation.  These predicates are declared in the built-protocols.
They are: goal_expansion/2 and term_expansion/2,
declared in the expanding(1) protocol; see before/3 and
after/3, declared in the monitoring(2) protocol; and
forward/1, declared in the forwarding(3) protocol.  By default, the
compiler prints a warning when a definition for one of these predicates
is found but the reference to the corresponding built-in protocol is
missing.

   ---------- Footnotes ----------

   (1) ../../docs/expanding_0.html#expanding-0

   (2) ../../docs/monitoring_0.html#monitoring-0

   (3) ../../docs/forwarding_0.html#forwarding-0

1.8.2 Declaring predicates
--------------------------

Logtalk provides a clear distinction between 'declaring a predicate' and
'defining a predicate' and thus clear closed-world assumption.
semantics.  Messages or calls for declared but undefined predicates
fail.  Messages or calls for unknown (i.e.  non declared) predicates
throw an error.  Note that this is a fundamental requirement for
supporting protocols.: we must be able to declare a predicate
without necessarily defining it.

All object (or category) predicates that we want to access from other
objects (or categories) must be explicitly declared.  A predicate
declaration must contain, at least, a 'scope' directive.  Other
directives may be used to document the predicate or to ensure proper
compilation of the predicate clauses.

1.8.2.1 Scope directives
........................

A predicate scope directive specifies 'from where' the predicate can be
called, i.e.  its 'visibility'.  Predicates can be 'public',
'protected', 'private', or 'local'.  Public predicates can be called
from any object.  Protected predicates can only be called from the
container object or from a container descendant.  Private predicates can
only be called from the container object.  Predicates are local when
they are not declared in a scope directive.  Local predicates, like
private predicates, can only be called from the container object (or
category) but they are 'invisible' to the reflection built-in methods
(see current_predicate/1 and predicate_property/2)
and to the message error handling mechanisms (i.e.  sending a message
corresponding to a local predicate results in a ‘predicate_declaration’
existence error instead of a scope error).

The scope declarations are made using the directives public/1,
see protected/1, and private/1.  For example:

     :- public(init/1).

     :- protected(valid_init_option/1).

     :- private(process_init_options/1).

If a predicate does not have a (local or inherited) scope declaration,
it is assumed that the predicate is local.  Note that we do not need to
write scope declarations for all defined predicates.  One exception is
local dynamic predicates: declaring them as private predicates may allow
the Logtalk compiler to generate optimized code for asserting and
retracting clauses.

Note that a predicate scope directive doesn’t specify 'where' a
predicate is, or can be, defined.  For example, a private predicate can
only be called from an object holding its scope directive.  But it can
be defined in descendant objects.  A typical example is an object
playing the role of a class defining a private (possibly dynamic)
predicate for its descendant instances.  Only the class can call (and
possibly assert/retract clauses for) the predicate but its clauses can
be found/defined in the instances themselves.

Scope directives may also be used to declare grammar rule non-terminals
and operators.  For example:

     :- public(url//1).

     :- public(op(800, fx, tag)).

Note that, in the case of operators, the operator definitions don’t
become global when the entity containing the directives is compiled and
loaded.  This prevents an application breaking when e.g.  an updated
third-party library adds new operators.  It also allows loading entities
that provide conflicting operator definitions.  Here the usual
programming idiom is to copy the operator definitions to a ‘uses/2’
directive.  For example, the ‘lgtunit’ tool makes available a ‘(=~=)/2’
predicate (for approximate float equality) that is intended to be used
as an infix operator:

     :- uses(lgtunit, [
         op(700, xfx, =~=), (=~=)/2
     ]).

Thus, in practice, the solution to use library entity operators in
client entities is the same for using library entity predicates with
implicit message sending.

1.8.2.2 Mode directive
......................

Often predicates can only be called using specific argument patterns.
The valid arguments and instantiation modes of those arguments can be
documented by using the mode/2 directive.  For example:

     :- mode(member(?term, ?list), zero_or_more).
The first directive argument describes a valid calling mode.  The
minimum information will be the instantiation mode of each argument.
The first four possible values are described in [ISO95]).
The remaining two can also be found in use in some Prolog systems.

‘+’

     Argument must be instantiated (but not necessarily ground).

‘-’

     Argument should be a free (non-instantiated) variable (when bound,
     the call will unify the returned term with the given term).

‘?’

     Argument can either be instantiated or free.

‘@’

     Argument will not be further instantiated (modified).

‘++’

     Argument must be ground.

‘--’

     Argument must be unbound.  Used mainly when returning an opaque
     term.

These six mode atoms are also declared as prefix operators by the
Logtalk compiler.  This makes it possible to include type information
for each argument like in the example above.  Some possible type values
are: ‘event’, ‘object’, ‘category’, ‘protocol’, ‘callable’, ‘term’,
‘nonvar’, ‘var’, ‘atomic’, ‘atom’, ‘number’, ‘integer’, ‘float’,
‘compound’, and ‘list’.  The first four are Logtalk specific.  The
remaining are common Prolog types.  We can also use our own types that
can be either atoms or ground compound terms.  See the ‘types’ library
documentation for details.  The second directive argument documents the
number of proofs, but not necessarily distinct solutions, for the
specified mode.  As an example, the ‘member(X, [1,1,1,1])’ goal have
only one distinct solution but four proofs for that solution.  Note that
different modes for the same predicate often have different determinism.
The possible values are:

‘zero’

     Predicate always fails.

‘one’

     Predicate always succeeds once.

‘zero_or_one’

     Predicate either fails or succeeds.

‘zero_or_more’

     Predicate has zero or more proofs.

‘one_or_more’

     Predicate has one or more proofs.

‘zero_or_error’

     Predicate either fails or throws an error.

‘one_or_error’

     Predicate either succeeds once or throws an error.

‘zero_or_one_or_error’

     Predicate succeeds once or fails or throws an error.

‘zero_or_more_or_error’

     Predicate may fail or succeed multiple times or throw an error.

‘one_or_more_or_error’

     Predicate may succeed one or more times or throw an error.

‘error’

     Predicate will throw an error.

The last six values support documenting that some call modes may throw
an error or will throw an error despite the calls complying with the
argument types and instantiation modes.  As an example, consider the
‘open/3’ ISO Prolog built-in predicate.  We may write:

     :- mode(open(@source_sink, @io_mode, --stream), one_or_error).

In this case, the mode directive tells the user that a valid call can
still throw an error (there may be e.g.  a permission error opening the
specified source or sink).

Note that some predicates have more than one valid mode implying several
mode directives.  For example, to document the possible use modes of the
ISO Prolog ‘atom_concat/3’ built-in predicate we would write:

     :- mode(atom_concat(?atom, ?atom, +atom), one_or_more).
     :- mode(atom_concat(+atom, +atom, -atom), zero_or_one).

Some old Prolog compilers supported some sort of mode directives to
improve performance.  To the best of my knowledge, there is no modern
Prolog compiler supporting this kind of directive for that purpose.  The
current Logtalk version simply parses this directive for collecting its
information for use in the reflection API. (assuming the
source_data flag is turned on).  In any case, the use of mode
directives is a good starting point for documenting your predicates.

1.8.2.3 Meta-predicate directive
................................

Some predicates may have arguments that will be called as goals or
interpreted as closures that will be used for constructing
goals.  To ensure that these goals will be executed in the correct
context (i.e.  in the calling context, not in the
meta-predicate definition context) we need to use the
meta_predicate/1 directive (in the case of 'meta non-terminals',
there’s also a meta_non_terminal/1 directive).  For example:

     :- meta_predicate(findall(*, 0, *)).
     :- meta_predicate(map(2, *, *)).

The meta-predicate mode arguments in this directive have the following
meaning:

‘0’

     Meta-argument that will be called as a goal.

‘N’

     Meta-argument that will be a closure used to construct a call by
     extending it with ‘N’ arguments.  The value of ‘N’ must be a
     positive integer.

‘::’

     Argument that is context-aware but that will not be called as a
     goal or a closure.  It can contain, however, sub-terms that will be
     called as goals or closures.

‘^’

     Goal that may be existentially quantified (‘Vars^Goal’).

‘*’

     Normal argument.

The following meta-predicate mode arguments are for use only when
writing backend Prolog adapter files to deal with
proprietary built-in meta-predicates and meta-directives:

‘/’

     Predicate indicator (‘Name/Arity’), list of predicate indicators,
     or conjunction of predicate indicators.

‘//’

     Non-terminal indicator (‘Name//Arity’), list of predicate
     indicators, or conjunction of predicate indicators.

‘[0]’

     List of goals.

‘[N]’

     List of closures.

‘[/]’

     List of predicate indicators.

‘[//]’

     List of non-terminal indicators.

To the best of my knowledge, the use of non-negative integers to specify
closures has first introduced on Quintus Prolog for providing
information for predicate cross-reference tools.

     Warning: As each Logtalk entity is independently compiled, this
     directive must be included in every object or category that
     contains a definition for the described meta-predicate, even if the
     meta-predicate declaration is inherited from another entity, to
     ensure proper compilation of meta-arguments.

1.8.2.4 Discontiguous directive
...............................

The clause of an object (or category) predicate may not be contiguous.
In that case, we must declare the predicate discontiguous by using the
discontiguous/1 directive:

     :- discontiguous(foo/1).

This is a directive that we should avoid using: it makes your code
harder to read and it is not supported by some Prolog backends.

     Warning: As each Logtalk entity is compiled independently of other
     entities, this directive must be included in every object or
     category that contains a definition for the described predicate
     (even if the predicate declaration is inherited from other entity).

1.8.2.5 Dynamic directive
.........................

An object predicate can be static or dynamic.  By default, all
predicates (and non-terminals) of static objects defined in source files
are static.  To declare a dynamic predicate (or non-terminal) we use the
dynamic/1 directive.  For example:

     :- dynamic(foo/1).

Predicates of objects dynamically created at runtime (using the
create_object/4 built-in predicate) and predicates of dynamic
objects defined in source files (using the dynamic/0.
directive) are implicitly dynamic.

Dynamic predicates can be used to represent persistent mutable object
state.  Note that static objects may declare and define dynamic
predicates.  Categories can only declare dynamic predicates (with the
importing objects holding the predicate definitions).

     Warning: As each Logtalk entity is compiled independently from
     other entities, this directive must be included in every object
     that contains a definition for the described predicate (even if the
     predicate declaration is inherited from other object or imported
     from a category).  If we omit the dynamic declaration then the
     predicate definition will be compiled static.

1.8.2.6 Operator directive
..........................

An object (or category) predicate can be declared as an operator using
the familiar op/3 directive:

     :- op(Priority, Specifier, Operator).

Operators are local to the object (or category) where they are declared.
This means that, if you declare a public predicate as an operator, you
cannot use operator notation when sending to an object (where the
predicate is visible) the respective message (as this would imply
visibility of the operator declaration in the context of the 'sender' of
the message).  If you want to declare global operators and, at the same
time, use them inside an entity, just write the corresponding directives
at the top of your source file, before the entity opening directive.

Note that operators can also be declared using a scope directive.  Only
these operators are visible to the current_op/3 reflection
method.

When the same operators are used on several entities within the same
source file, the corresponding directives must either be repeated in
each entity or appear before any entity that uses them.  But in the
later case, this results in a global scope for the operators.  If you
prefer the operators to be local to the source file, just 'undefine'
them at the end of the file.  For example:

     % before any entity that uses the operator
     :- op(400, xfx, results).

     ...

     % after all entities that used the operator
     :- op(0, xfx, results).

Global operators can be declared in the application loader file.

1.8.2.7 Uses directive
......................

When a predicate makes heavy use of predicates defined on other objects,
its predicate clauses can be verbose due to all the necessary message
sending goals.  Consider the following example:

     foo :-
         ...,
         findall(X, list::member(X, L), A),
         list::append(A, B, C),
         list::select(Y, C, R),
         ...

Logtalk provides a directive, see uses/2, which allows us to
simplify the code above.  One of the usage templates for this directive
is:

     :- uses(Object, [
         Name1/Arity1, Name2/Arity2, ...
     ]).

Rewriting the code above using this directive results in a simplified
and more readable predicate definition:

     :- uses(list, [
         append/3, member/2, select/3
     ]).

     foo :-
         ...,
         findall(X, member(X, L), A),
         append(A, B, C),
         select(Y, C, R),
         ...

Logtalk also supports an extended version of this directive that allows
the declaration of predicate aliases using the notation
‘Predicate as Alias’ (or the alternative notation ‘Predicate::Alias’).
For example:

     :- uses(btrees, [new/1 as new_btree/1]).
     :- uses(queues, [new/1 as new_queue/1]).

You may use this extended version for solving conflicts between
predicates declared on several ‘uses/2’ directives or just for giving
new names to the predicates that will be more meaningful on their using
context.  It’s also possible to define predicate aliases that are also
predicate shorthands.  For example:

     :- uses(pretty_printer, [
         indent(4, Term) as indent(Term)
     ]).

See the directive documentation for details and other examples.

The ‘uses/2’ directive allows simpler predicate definitions as long as
there are no conflicts between the predicates declared in the directive
and the predicates defined in the object (or category) containing the
directive.  A predicate (or its alias if defined) cannot be listed in
more than one ‘uses/2’ directive.  In addition, a ‘uses/2’ directive
cannot list a predicate (or its alias if defined) which is defined in
the object (or category) containing the directive.  Any conflicts are
reported by Logtalk as compilation errors.

The object identifier argument can also be a parameter variable
when using the directive in a parametric object or a parametric
category.  In this case, see dynamic binding will necessarily be
used for all listed predicates (and non-terminals).  The parameter
variable must be instantiated at runtime when the messages are sent.
This feature simplifies experimenting with multiple implementations of
the same protocol (for example, to evaluate the performance of each
implementation for a particular case).  It also simplifies writing tests
that check multiple implementations of the same protocol.

An object (or category) can make a predicate listed in a ‘uses/2’ (or
‘use_module/2’) directive part of its protocol by simply adding a scope
directive for the predicate.  For example, in the ‘statistics’ library
we have:

     :- public(modes/2).
     :- uses(numberlist, [modes/2]).

Therefore, a goal such as ‘sample::modes(Sample, Modes)’ implicitly
calls ‘numberlist::modes(Sample, Modes)’ without requiring an explicit
local definition for the ‘modes/2’ predicate (which would trigger a
compilation error).

1.8.2.8 Alias directive
.......................

Logtalk allows the definition of an alternative name for an inherited or
imported predicate (or for an inherited or imported grammar rule
non-terminal) through the use of the alias/2 directive:

     :- alias(Entity, [
         Predicate1 as Alias1,
         Predicate2 as Alias2,
         ...
     ]).

This directive can be used in objects, protocols, or categories.  The
first argument, ‘Entity’, must be an entity referenced in the opening
directive of the entity containing the ‘alias/2’ directive.  It can be
an extended or implemented protocol, an imported category, an extended
prototype, an instantiated class, or a specialized class.  The second
argument is a list of pairs of predicate indicators (or grammar rule
non-terminal indicators) using the ‘as’ infix operator as connector.

A common use for the ‘alias/2’ directive is to give an alternative name
to an inherited predicate in order to improve readability.  For example:

     :- object(square,
         extends(rectangle)).

         :- alias(rectangle, [width/1 as side/1]).

         ...

     :- end_object.

The directive allows both ‘width/1’ and ‘side/1’ to be used as messages
to the object ‘square’.  Thus, using this directive, there is no need to
explicitly declare and define a “new” ‘side/1’ predicate.  Note that the
‘alias/2’ directive does not rename a predicate, only provides an
alternative, additional name; the original name continues to be
available (although it may be masked due to the default inheritance
conflict mechanism).

Another common use for this directive is to solve conflicts when two
inherited predicates have the same name and arity.  We may want to call
the predicate which is masked out by the Logtalk lookup algorithm (see
the Inheritance section) or we may need to call both
predicates.  This is simply accomplished by using the ‘alias/2’
directive to give alternative names to masked out or conflicting
predicates.  Consider the following example:

     :- object(my_data_structure,
         extends(list, set)).

         :- alias(list, [member/2 as list_member/2]).
         :- alias(set,  [member/2 as set_member/2]).

         ...

     :- end_object.

Assuming that both ‘list’ and ‘set’ objects define a ‘member/2’
predicate, without the ‘alias/2’ directives, only the definition of
‘member/2’ predicate in the object ‘list’ would be visible on the object
‘my_data_structure’, as a result of the application of the Logtalk
predicate lookup algorithm.  By using the ‘alias/2’ directives, all the
following messages would be valid (assuming a public scope for the
predicates):

     % uses list member/2
     | ?- my_data_structure::list_member(X, L).

      % uses set member/2
     | ?- my_data_structure::set_member(X, L).

     % uses list member/2
     | ?- my_data_structure::member(X, L).

When used this way, the ‘alias/2’ directive provides functionality
similar to programming constructs of other object-oriented languages
that support multi-inheritance (the most notable example probably being
the renaming of inherited features in Eiffel).

Note that the ‘alias/2’ directive never hides a predicate which is
visible on the entity containing the directive as a result of the
Logtalk lookup algorithm.  However, it may be used to make visible a
predicate which otherwise would be masked by another predicate, as
illustrated in the above example.

The ‘alias/2’ directive may also be used to give access to an inherited
predicate, which otherwise would be masked by another inherited
predicate, while keeping the original name as follows:

     :- object(my_data_structure,
         extends(list, set)).

         :- alias(list, [member/2 as list_member/2]).
         :- alias(set,  [member/2 as set_member/2]).

         member(X, L) :-
             ^^set_member(X, L).

         ...

     :- end_object.

Thus, when sending the message ‘member/2’ to ‘my_data_structure’, the
predicate definition in ‘set’ will be used instead of the one contained
in ‘list’.

1.8.2.9 Documenting directive
.............................

A predicate can be documented with arbitrary user-defined information by
using the info/2 directive:

     :- info(Name/Arity, List).

The second argument is a list of ‘Key is Value’ terms.  See the
Documenting section for details.

1.8.2.10 Multifile directive
............................

A predicate can be declared 'multifile' by using the multifile/1
directive:

     :- multifile(Name/Arity).

This allows clauses for a predicate to be defined in several objects
and/or categories.  This is a directive that should be used with care.
It’s commonly used in the definition of hook predicates.
Multifile predicates (and non-terminals) may also be declared dynamic
using the same predicate (or non-terminal) notation (multifile
predicates are static by default).

Logtalk precludes using a multifile predicate for breaking object
encapsulation by checking that the object (or category) declaring the
predicate (using a scope directive) defines it also as multifile.  This
entity is said to contain the 'primary declaration' for the multifile
predicate.  Entities containing primary multifile predicate declarations
must always be compiled before entities defining clauses for those
multifile predicates.  The Logtalk compiler will print a warning if the
scope directive is missing.  Note also that the ‘multifile/1’ directive
is mandatory when defining multifile predicates.

Consider the following simple example:

     :- object(main).

         :- public(a/1).
         :- multifile(a/1).
         a(1).

     :- end_object.

After compiling and loading the ‘main’ object, we can define other
objects (or categories) that contribute with clauses for the multifile
predicate.  For example:

     :- object(other).

         :- multifile(main::a/1).
         main::a(2).
         main::a(X) :-
             b(X).

         b(3).
         b(4).

     :- end_object.

After compiling and loading the above objects, you can use queries such
as:

     | ?- main::a(X).

     X = 1 ;
     X = 2 ;
     X = 3 ;
     X = 4
     yes

Note that the order of multifile predicate clauses depend on several
factors, including loading order and compiler implementation details.
Therefore, your code should never assume or rely on a specific order of
the multifile predicate clauses.

When a clause of a multifile predicate is a rule, its body is compiled
within the context of the object or category defining the clause.  This
allows clauses for multifile predicates to call local object or category
predicates.  But the values of the 'sender', 'this', and 'self' in the
implicit execution context are passed from the clause head to the clause
body.  This is necessary to ensure that these values are always valid
and to allow multifile predicate clauses to be defined in categories.  A
call to the ‘parameter/2’ execution context methods, however, retrieves
parameters of the entity defining the clause, not from the entity for
which the clause is defined.  The parameters of the entity for which the
clause is defined can be accessed by simple unification at the clause
head.

Multifile predicate rules should not contain cuts as these may prevent
other clauses for the predicate for being used by callers.  The compiler
prints by default a warning when a cut is found in a multifile predicate
definition.

Local calls to the database methods from multifile predicate clauses
defined in an object take place in the object own database instead of
the database of the entity holding the multifile predicate primary
declaration.  Similarly, local calls to the ‘expand_term/2’ and
‘expand_goal/2’ methods from a multifile predicate clause look for
clauses of the ‘term_expansion/2’ and ‘goal_expansion/2’ hook predicates
starting from the entity defining the clause instead of the entity
holding the multifile predicate primary declaration.  Local calls to the
‘current_predicate/1’, ‘predicate_property/2’, and ‘current_op/3’
methods from multifile predicate clauses defined in an object also
lookup predicates and their properties in the object own database
instead of the database of the entity holding the multifile predicate
primary declaration.

1.8.2.11 Coinductive directive
..............................

A predicate can be declared 'coinductive' by using the
coinductive/1 directive.  For example:

     :- coinductive(comember/2).

Logtalk support for coinductive predicates is experimental and requires
a backend Prolog compiler with minimal support for cyclic
terms.  The value of the read-only coinduction flag is set
to ‘supported’ for the backend Prolog compilers providing that support.

1.8.2.12 Synchronized directive
...............................

A predicate can be declared 'synchronized' by using the
synchronized/1 directive.  For example:

     :- synchronized(write_log_entry/2).
     :- synchronized([produce/1, consume/1]).

See the section on synchronized predicates for details.

1.8.3 Defining predicates
-------------------------

1.8.3.1 Object predicates
.........................

We define object predicates as we have always defined Prolog predicates,
the only difference be that we have four more control structures (the
three message sending operators plus the external call operator) to play
with.  For example, if we wish to define an object containing common
utility list predicates like ‘append/2’ or ‘member/2’ we could write
something like:

     :- object(list).

         :- public(append/3).
         :- public(member/2).

         append([], L, L).
         append([H| T], L, [H| T2]) :-
             append(T, L, T2).

         member(H, [H| _]).
         member(H, [_| T]) :-
             member(H, T).

     :- end_object.

Note that, abstracting from the opening and closing object directives
and the scope directives, what we have written is also valid Prolog
code.  Calls in a predicate definition body default to the local
predicates, unless we use the message sending operators or the external
call operator.  This enables easy conversion from Prolog code to Logtalk
objects: we just need to add the necessary encapsulation and scope
directives to the old code.

1.8.3.2 Category predicates
...........................

A category can only contain clauses for static predicates.  But there
are no restrictions in declaring and calling dynamic predicates from
inside a category.  Because a category can be imported by multiple
objects, dynamic predicates must be called either in the context of
self, using the message to self control structure,
see (;;)/1, or in the context of this. (i.e.  in the
context of the object importing the category).  For example, if we want
to define a category implementing attributes using the dynamic database
of 'self' we could write:

     :- category(attributes).

         :- public(get/2).
         :- public(set/2).

         :- private(attribute_/2).
         :- dynamic(attribute_/2).

         get(Var, Value) :-
             ::attribute_(Var, Value).

         set(Var, Value) :-
             ::retractall(attribute_(Var, _)),
             ::asserta(attribute_(Var, Value).

     :- end_category.

In this case, the ‘get/2’ and ‘set/2’ predicates will always
access/update the correct definition, contained in the object receiving
the messages.

In alternative, if we want a category implementing attributes using the
dynamic database of 'this', we would write instead:

     :- category(attributes).

         :- public(get/2).
         :- public(set/2).

         :- private(attribute_/2).
         :- dynamic(attribute_/2).

         get(Var, Value) :-
             attribute_(Var, Value).

         set(Var, Value) :-
             retractall(attribute_(Var, _)),
             asserta(attribute_(Var, Value).

     :- end_category.

In this case, each object importing the category will have its own
clauses for the ‘attribute_/2’ private dynamic predicate.

1.8.3.3 Meta-predicates
.......................

Meta-predicates may be defined inside objects and categories as any
other predicate.  A meta-predicate is declared using the
meta_predicate/1 directive as described earlier on this section.
When defining a meta-predicate, the arguments in the clause heads
corresponding to the meta-arguments must be variables.  All
meta-arguments are called in the context of the object or category
calling the meta-predicate.  In particular, when sending a message that
corresponds to a meta-predicate, the meta-arguments are called in the
context of the object or category sending the message.

The most simple example is a meta-predicate with a meta-argument that is
called as a goal.  E.g.  the ignore/1 built-in predicate
could be defined as:

     :- public(ignore/1).
     :- meta_predicate(ignore(0)).

     ignore(Goal) :-
        (Goal -> true; true).

The ‘0’ in the meta-predicate template tells us that the meta-argument
is a goal that will be called by the meta-predicate.

Some meta-predicates have meta-arguments which are not goals but
closures.  Logtalk supports the definition of meta-predicates that
are called with closures instead of goals as long as the definition uses
the call/1-N built-in predicate to call the closure with the
additional arguments.  A classical example is a list mapping predicate:

     :- public(map/2).
     :- meta_predicate(map(1, *)).

     map(_, []).
     map(Closure, [Arg| Args]) :-
         call(Closure, Arg),
         map(Closure, Args).

Note that in this case the meta-predicate directive specifies that the
closure will be extended with exactly one additional argument.  When
calling a meta-predicate, a closure can correspond to a user-defined
predicate, a built-in predicate, a lambda expression, or a
control construct.

In some cases, is not a meta-argument but one of its sub-terms that is
called as a goal or used as a closure.  For example:

     :- public(call_all/1).
     :- meta_predicate(call_all(::)).

     call_all([]).
     call_all([Goal| Goals]) :-
         call(Goal),
         call_all(Goals).

The ‘::’ mode indicator in the meta-predicate template allows the
corresponding argument in the meta-predicate definiton to be a
non-variable term and instructs the compiler to look into the argument
sub-terms for goal and closure meta-variables.

When a meta-predicate calls another meta-predicate, both predicates
require ‘meta_predicate/1’ directives.  For example, the ‘map/2’
meta-predicate defined above is usually implemented by exchanging the
argument order to take advantage of first-argument indexing:

     :- meta_predicate(map(1, *)).
     map(Closure, List) :-
         map_(List, Closure).

     :- meta_predicate(map_(*, 1)).
     map_([], _).
     map_([Head| Tail], Closure) :-
         call(Closure, Head),
         map_(Tail, Closure).

Note that Logtalk, unlike most Prolog module systems, is not based on a
predicate prefixing mechanism.  Thus, the meta-argument calling context
is not part of the meta-argument itself.

1.8.3.4 Lambda expressions
..........................

The use of lambda expressions(1) as meta-predicate goal and
closure arguments often saves writing auxiliary predicates for the
sole purpose of calling the meta-predicates.  A simple example of a
lambda expression is:

     | ?- meta::map([X,Y]>>(Y is 2*X), [1,2,3], Ys).
     Ys = [2,4,6]
     yes

In this example, a lambda expression, ‘[X,Y]>>(Y is 2*X)’, is used as an
argument to the ‘map/3’ list mapping predicate, defined in the library
object ‘meta’, in order to double the elements of a list of integers.
Using a lambda expression avoids writing an auxiliary predicate for the
sole purpose of doubling the list elements.  The lambda parameters are
represented by the list ‘[X,Y]’, which is connected to the lambda goal,
‘(Y is 2*X)’, by the ‘(>>)/2’ operator.

Currying is supported.  I.e.  it is possible to write a lambda
expression whose goal is another lambda expression.  The above example
can be rewritten as:

     | ?- meta::map([X]>>([Y]>>(Y is 2*X)), [1,2,3], Ys).
     Ys = [2,4,6]
     yes

Lambda expressions may also contain lambda free variables.  I.e.
variables that are global to the lambda expression.  For example, using
GNU Prolog as the backend compiler, we can write:

     | ?- meta::map({Z}/[X,Y]>>(Z#=X+Y), [1,2,3], Zs).
     Z = _#22(3..268435455)
     Zs = [_#3(2..268435454),_#66(1..268435453),_#110(0..268435452)]
     yes

The ISO Prolog construct ‘{}/1’ for representing the lambda free
variables as this representation is often associated with set
representation.  Note that the order of the free variables is of no
consequence (on the other hand, a list is used for the lambda parameters
as their order does matter).

Both lambda free variables and lambda parameters can be any Prolog term.
Consider the following example by Markus Triska:

     | ?- meta::map([A-B,B-A]>>true, [1-a,2-b,3-c], Zs).
     Zs = [a-1,b-2,c-3]
     yes

Lambda expressions can be used, as expected, in non-deterministic
queries as in the following example using SWI-Prolog as the backend
compiler and Markus Triska’s CLP(FD) library:

     | ?- meta::map({Z}/[X,Y]>>(clpfd:(Z#=X+Y)), Xs, Ys).
     Xs = [],
     Ys = [] ;
     Xs = [_G1369],
     Ys = [_G1378],
     _G1369+_G1378#=Z ;
     Xs = [_G1579, _G1582],
     Ys = [_G1591, _G1594],
     _G1582+_G1594#=Z,
     _G1579+_G1591#=Z ;
     Xs = [_G1789, _G1792, _G1795],
     Ys = [_G1804, _G1807, _G1810],
     _G1795+_G1810#=Z,
     _G1792+_G1807#=Z,
     _G1789+_G1804#=Z ;
     ...

As illustrated by the above examples, lambda expression syntax reuses
the ISO Prolog construct ‘{}/1’ and the standard operators ‘(/)/2’ and
‘(>>)/2’, thus avoiding defining new operators, which is always tricky
for a portable system such as Logtalk.  The operator ‘(>>)/2’ was chosen
as it suggests an arrow, similar to the syntax used in other languages
such as OCaml and Haskell to connect lambda parameters with lambda
functions.  This syntax was also chosen in order to simplify parsing,
error checking, and compilation of lambda expressions.  The full
specification of the lambda expression syntax can be found in the the
language grammar.

The compiler checks whenever possible that all variables in a lambda
expression are either classified as free variables or as lambda
parameters.  Non-classified variables in a lambda expression should be
regarded as a programming error.  The compiler also checks if a variable
is classified as both a free variable and a lambda parameter.  There are
a few cases where a variable playing a dual role is intended but, in
general, this also results from a programming error.  A third check
verifies that no lambda parameter variable is used elsewhere in a
clause.  Such cases are either programming errors, when the variable
appears before the lambda expression, or bad programming style, when the
variable is used after the lambda expression.  These linter warnings are
controlled by the lambda_variables flag.  Note, however,
that the dynamic features of the language and lack of sufficient
information at compile time may prevent the compiler of checking all
uses of lambda expressions.

     Warning: Variables listed in lambda parameters must not be shared
     with other goals in a clause.

An optimizing meta-predicate and lambda expression compiler, based on
the term-expansion mechanism, is provided as a standard
library for practical performance.

   ---------- Footnotes ----------

   (1) https://en.wikipedia.org/wiki/Lambda_calculus

1.8.3.5 Redefining built-in predicates
......................................

Logtalk built-in predicates and Prolog built-in predicates can be
redefined inside objects and categories.  Although the redefinition of
Logtalk built-in predicates should be avoided, the support for
redefining Prolog built-in predicates is a practical requirement given
the different sets of proprietary built-in predicates provided by
backend Prolog systems.

The compiler supports a redefined_built_ins flag, whose
default value is ‘silent’, that can be set to ‘warning’ to alert the
user of any redefined Logtalk or Prolog built-in predicate.

The redefinition of Prolog built-in predicates can be combined with the
conditional compilation directives when writing portable
applications where some of the supported backends don’t provide a
built-in predicate found in the other backends.  As an example, consider
the de facto standard ‘msort/2’ predicate (which sorts a list while
keeping duplicates).  This predicate is provided as a built-in predicate
in most but not all backends.  The ‘list’ library object includes the
code:

     :- if(predicate_property(msort(_, _), built_in)).

         msort(List, Sorted) :-
             {msort(List, Sorted)}.

     :- else.

         length(List, Length) :-
             ...

     :- endif.

I.e.  the object will use the built-in predicate when available.
Otherwise, it will use the predicate definition provided by the ‘list’
object.

The redefinition of built-in predicates can also be accomplished using
predicate shorthands.  This can be useful when porting code
while minimizing the changes.  For example, assume that existing code
uses the ‘format/2’ de facto standard predicate for writing messages.
To convert the code to use the message printing mechanism we
could write:

     :- uses(logtalk, [
         print_message(comment, core, Format+Arguments) as format(Format, Arguments)
     ]).

     process(Crate, Contents) :-
         format('Processing crate ~w...', [Crate]),
         ...,
         format('Filing with ~w...', [Contents]),
         ....

The predicate shorthand instructs the compiler to rewrite all ‘format/2’
goals as ‘logtalk::print_message/3’ goals, thus allowing us to reuse the
code without changes.

1.8.4 Definite clause grammar rules
-----------------------------------

Definite clause grammar rules (DCGs) provide a convenient notation to
represent the parsing and rewrite rules common of most grammars in
Prolog.  In Logtalk, definite clause grammar rules can be encapsulated
in objects and categories.  Currently, the ISO/IEC WG17 group is working
on a draft specification for a definite clause grammars Prolog standard.
Therefore, in the mean time, Logtalk follows the common practice of
Prolog compilers supporting definite clause grammars, extending it to
support calling grammar rules contained in categories and objects.  A
common example of a definite clause grammar is the definition of a set
of rules for parsing simple arithmetic expressions:

     :- object(calculator).

         :- public(parse/2).

         parse(Expression, Value) :-
             phrase(expr(Value), Expression).

         expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}.
         expr(Z) --> term(X), "-", expr(Y), {Z is X - Y}.
         expr(X) --> term(X).

         term(Z) --> number(X), "*", term(Y), {Z is X * Y}.
         term(Z) --> number(X), "/", term(Y), {Z is X / Y}.
         term(Z) --> number(Z).

         number(C) --> "+", number(C).
         number(C) --> "-", number(X), {C is -X}.
         number(X) --> [C], {0'0 =< C, C =< 0'9, X is C - 0'0}.

     :- end_object.

After compiling and loading this object, we can test the grammar rules
using the ‘parse/2’ message:

     | ?- calculator::parse("1+2-3*4", Result).

     Result = -9
     yes

The non-terminals can be called from predicates using the private
built-in methods phrase/2 and phrase/3 as shown
in the example above.  When we want to use the built-in methods
‘phrase/2’ and ‘phrase/3’, the non-terminal used as first argument must
be within the scope of the 'sender'.  For the above example, assuming
that we want the predicate corresponding to the ‘expr//1’ non-terminal
to be public, the corresponding scope directive would be:

     :- public(expr//1).

The ‘//’ infix operator used above tells the Logtalk compiler that the
scope directive refers to a grammar rule non-terminal, not to a
predicate.  The idea is that the predicate corresponding to the
translation of the ‘expr//1’ non-terminal will have a number of
arguments equal to one plus the number of additional arguments necessary
for processing the implicit difference list of tokens.

In the body of a grammar rule, we can call rules that are inherited from
ancestor objects, imported from categories, or contained in other
objects.  This is accomplished by using non-terminals as messages.
Using a non-terminal as a message to 'self' allows us to call grammar
rules in categories and ancestor objects.  To call grammar rules
encapsulated in other objects, we use a non-terminal as a message to
those objects.  Consider the following example, containing grammar rules
for parsing natural language sentences:

     :- object(sentence,
         imports(determiners, nouns, verbs)).

         :- public(parse/2).

         parse(List, true) :-
             phrase(sentence, List).
         parse(_, false).

         sentence --> noun_phrase, verb_phrase.

         noun_phrase --> ::determiner, ::noun.
         noun_phrase --> ::noun.

         verb_phrase --> ::verb.
         verb_phrase --> ::verb, noun_phrase.

     :- end_object.

The categories imported by the object would contain the necessary
grammar rules for parsing determiners, nouns, and verbs.  For example:

     :- category(determiners).

         :- private(determiner//0).

         determiner --> [the].
         determiner --> [a].

     :- end_category.

Along with the message sending operators (‘(::)/1’, ‘(::)/2’, and
‘(^^)/1’), we may also use other control constructs such as ‘(\+)/1’,
‘!/0’, ‘(;)/2’, ‘(->)/2’, and ‘{}/1’ in the body of a grammar.  When
using a backend Prolog compiler that supports modules, we may also use
the ‘(:)/2’ control construct.

     Warning: The semantics of ‘(\+)/1’ and ‘(->)/2’ control constructs
     in grammar rules with a terminal or a non-terminal in the 'first'
     argument are problematic due to unrestricted look ahead that may or
     may not be valid depending on the grammar rule implicit arguments.
     By default, the linter will print warnings for such calls
     (controlled by the grammar_rules flag).  Preferably
     restrit the use of the ‘(\+)/1’ control construct to ‘{}/1’
     arguments and the use of the ‘(->)/2’ control construct to ‘{}/1’
     test arguments.

In addition, grammar rules may contain meta-calls (a variable taking the
place of a non-terminal), which are translated to calls of the built-in
method ‘phrase/3’.  The meta_non_terminal/1 directive allows
the declaration of non-terminals that have arguments that are
meta-called from grammar rules.  For example:

     :- meta_non_terminal(zero_or_more(1, *)).

     zero_or_more(Closure, [Terminal| Terminals]) -->
         call(Closure, Terminal), !, zero_or_more(Closure, Terminals).
     zero_or_more(_, []) -->
         [].

You may have noticed that Logtalk defines {}/1 as a control
construct for bypassing the compiler when compiling a clause body goal.
As exemplified above, this is the same control construct that is used in
grammar rules for bypassing the expansion of rule body goals when a rule
is converted into a clause.  Both control constructs can be combined in
order to call a goal from a grammar rule body, while bypassing at the
same time the Logtalk compiler.  Consider the following example:

     bar :-
         write('bar predicate called'), nl.

     :- object(bypass).

         :- public(foo//0).

         foo --> {{bar}}.

     :- end_object.

After compiling and loading this code, we may try the following query:

     | ?- logtalk << phrase(bypass::foo, _, _).

     bar predicate called
     yes

This is the expected result as the expansion of the grammar rule into a
clause leaves the ‘{bar}’ goal untouched, which, in turn, is converted
into the goal ‘bar’ when the clause is compiled.  Note that we tested
the ‘bypass::foo//0’ non-terminal by calling the ‘phrase/3’ built-in
method in the context of the ‘logtalk’ built-in object.  This workaround
is necessary due to the Prolog backend implementation of the ‘phrase/3’
predicate no being aware of the Logtalk ‘::/2’ message-sending control
construct semantics.

A grammar rule non-terminal may be declared as dynamic or discontiguous,
as any object predicate, using the same ‘Name//Arity’ notation
illustrated above for the scope directives.  In addition, grammar rule
non-terminals can be documented using the info/2 directive,
as in the following example:

     :- public(sentence//0).

     :- info(sentence//0, [
         comment is 'Rewrites sentence into noun and verb phrases.'
     ]).

     Note: Future Logtalk versions may compile grammar rules differently
     from Prolog traditional compilation to prevent name clases between
     non-terminals and predicates.  Therefore, you should always call
     non-terminals from predicates using the ‘phrase/2-3’ built-in
     methods and always call predicates from grammar rules using the
     ‘call//1’ built-in method.  This recommended practice, besides
     making your code forward compatible with future Logtalk versions,
     also make the code more clear.  The linter prints warnings when
     these guidelines are not followed (notably, when a predicate is
     called as a non-terminal or a non-terminal is called as a
     predicate).

1.8.5 Built-in methods
----------------------

Built-in methods are built-in object and category predicates.  These
include methods to access message execution context, to find sets of
solutions, to inspect objects, for database handling, for term and goal
expansion, and for printing messages.  Some of them are counterparts to
standard Prolog built-in predicates that take into account Logtalk
semantics.  Similar to Prolog built-in predicates, built-in methods
cannot not be redefined.

1.8.5.1 Logic and control methods
.................................

The !/0, true/0, fail/0, false/0,
and repeat/0 standard control constructs and logic
predicates are interpreted as built-in public methods and thus can be
used as messages to any object.  In practice, they are only used as
messages when sending multiple messages to the same object (see the
section on message broadcasting).

1.8.5.2 Execution context methods
.................................

Logtalk defines five built-in private methods to access an object
execution context.  These methods are in the common usage scenarios
translated to a single unification performed at compile time with a
clause head context argument.  Therefore, they can be freely used
without worrying about performance penalties.  When called from inside a
category, these methods refer to the execution context of the object
importing the category.  These methods are private and cannot be used as
messages to objects.

To find the object that received the message under execution we may use
the self/1 method.  We may also retrieve the object that has
sent the message under execution using the sender/1 method.

The method this/1 enables us to retrieve the name of the
object for which the predicate clause whose body is being executed is
defined instead of using the name directly.  This helps to avoid
breaking the code if we decide to change the object name and forget to
change the name references.  This method may also be used from within a
category.  In this case, the method returns the object importing the
category on whose behalf the predicate clause is being executed.

Here is a short example including calls to these three object execution
context methods:

     :- object(test).

         :- public(test/0).

         test :-
             this(This),
             write('Calling predicate definition in '),
             writeq(This), nl,
             self(Self),
             write('to answer a message received by '),
             writeq(Self), nl,
             sender(Sender),
             write('that was sent by '),
             writeq(Sender), nl, nl.

     :- end_object.

     :- object(descendant,
         extends(test)).

     :- end_object.

After compiling and loading these two objects, we can try the following
goal:

     | ?- descendant::test.

     Calling predicate definition in test
     to answer a message received by descendant
     that was sent by user
     yes

Note that the goals ‘self(Self)’, ‘sender(Sender)’, and ‘this(This)’,
being translated to unifications with the clause head context arguments
at compile time, are effectively removed from the clause body.
Therefore, a clause such as:

     predicate(Arg) :-
         self(Self),
         atom(Arg),
         ... .

is compiled with the goal ‘atom(Arg)’ as the first condition on the
clause body.  As such, the use of these context execution methods do not
interfere with the optimizations that some Prolog compilers perform when
the first clause body condition is a call to a built-in type-test
predicate or a comparison operator.

For parametric objects and categories, the method parameter/2.
enables us to retrieve current parameter values (see the section on
parametric objects for a detailed description).  For example:

     :- object(block(_Color)).

         :- public(test/0).

         test :-
             parameter(1, Color),
             write('Color parameter value is '),
             writeq(Color), nl.

     :- end_object.

An alternative to the ‘parameter/2’ predicate is to use parameter variables.:

     :- object(block(_Color_)).

         :- public(test/0).

         test :-
             write('Color parameter value is '),
             writeq(_Color_), nl.

     :- end_object.

After compiling and loading either version of the object, we can try the
following goal:

     | ?- block(blue)::test.

     Color parameter value is blue
     yes

Calls to the ‘parameter/2’ method are translated to a compile time
unification when the second argument is a variable.  When the second
argument is bound, the calls are translated to a call to the built-in
predicate ‘arg/3’.

When type-checking predicate arguments, it is often useful to include
the predicate execution context when reporting an argument error.  The
context/1 method provides access to that context.  For
example, assume a predicate ‘foo/2’ that takes an atom and an integer as
arguments.  We could type-check the arguments by writing (using the
library ‘type’ object):

     foo(A, N) :-
         % type-check arguments
         context(Context),
         type::check(atom, A, Context),
         type::check(integer, N, Context),
         % arguments are fine; go ahead
         ... .

1.8.5.3 Error handling and throwing methods
...........................................

Besides the catch/3 and throw/1 methods
inherited from Prolog, Logtalk also provides a set of convenience
methods to throw standard ‘error/2’ exception terms:
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see permission_error/3, representation_error/1,
evaluation_error/1, resource_error/1,
syntax_error/1, and system_error/0.

1.8.5.4 Database methods
........................

Logtalk provides a set of built-in methods for object database
handling similar to the usual database Prolog predicates:
abolish/1, asserta/1, assertz/1,
clause/2, retract/1, and retractall/1.  These
methods always operate on the database of the object receiving the
corresponding message.  When called locally, these predicates take into
account any uses/2 or use_module/2 directives that
refer to the dynamic predicate being handled.  For example, in the
following object, the clauses for the ‘data/1’ predicate are retracted
and asserted in ‘user’ due to the ‘uses/2’ directive:

     :- object(an_object).

         :- uses(user, [data/1]).

         :- public(some_predicate/1).
         some_predicate(Arg) :-
             retractall(data(_)),
             assertz(data(Arg)).

     :- end_object.

When working with dynamic grammar rule non-terminals, you may use the
built-in method expand_term/2 convert a grammar rule into a
clause that can then be used with the database methods.

1.8.5.5 Meta-call methods
.........................

Logtalk supports the generalized call/1-N meta-predicate.
This built-in private meta-predicate must be used in the implementation
of meta-predicates which work with closures instead of
goals.  In addition, Logtalk supports the built-in private
meta-predicates ignore/1, once/1, and
(\+)/1.  These methods cannot be used as messages to objects.

1.8.5.6 All solutions methods
.............................

The usual all solutions meta-predicates are built-in private methods in
Logtalk: bagof/3, findall/3, findall/4,
and setof/3.  There is also a forall/2 method
that implements generate-and-test loops.  These methods cannot be used
as messages to objects.

1.8.5.7 Reflection methods
..........................

Logtalk provides a comprehensive set of built-in predicates and built-in
methods for querying about entities and predicates.  Some of the
information, however, requires that the source files are compiled with
the source_data flag turned on.

The reflection API supports two different views on entities
and their contents, which we may call the 'transparent box view' and the
'black box view'.  In the transparent box view, we look into an entity
disregarding how it will be used and returning all information available
on it, including predicate declarations and predicate definitions.  This
view is supported by the entity property built-in predicates.  In the
black box view, we look into an entity from a usage point-of-view using
built-in methods for inspecting object operators and predicates that are
within scope from where we are making the call: current_op/3,
which returns operator specifications, see predicate_property/2,
which returns predicate properties, and current_predicate/1,
which enables us to query about user-defined predicate definitions.  See
below for a more detailed description of these methods.

1.8.5.8 Definite clause grammar parsing methods and non-terminals
.................................................................

Logtalk supports two definite clause grammar parsing built-in private
methods, see phrase/2 and phrase/3, with definitions
similar to the predicates with the same name found on most Prolog
compilers that support definite clause grammars.  These methods cannot
be used as messages to objects.

Logtalk also supports phrase//1, call//1-N, and
eos//0 built-in non-terminals.  The ‘call//1-N’
non-terminals takes a closure. (which can be a lambda
expression) plus zero or more additional arguments and are processed by
appending the input list of tokens and the list of remaining tokens to
the arguments.

1.8.6 Predicate properties
--------------------------

We can find the properties of visible predicates by calling the
predicate_property/2 built-in method.  For example:

     | ?- bar::predicate_property(foo(_), Property).

Note that this method takes into account the predicate’s scope
declarations.  In the above example, the call will only return
properties for public predicates.

An object’s set of visible predicates is the union of all the predicates
declared for the object with all the built-in methods and all the
Logtalk and Prolog built-in predicates.

The following predicate properties are supported:

‘scope(Scope)’

     The predicate scope (useful for finding the predicate scope with a
     single call to ‘predicate_property/2’)

‘public’, ‘protected’, ‘private’

     The predicate scope (useful for testing if a predicate have a
     specific scope)

‘static’, ‘dynamic’

     All predicates are either static or dynamic (note, however, that a
     dynamic predicate can only be abolished if it was dynamically
     declared)

‘logtalk’, ‘prolog’, ‘foreign’

     A predicate can be defined in Logtalk source code, Prolog code, or
     in foreign code (e.g.  in C)

‘built_in’

     The predicate is a built-in predicate

‘multifile’

     The predicate is declared multifile (i.e.  it can have clauses
     defined in multiple files or entities)

‘meta_predicate(Template)’

     The predicate is declared as a meta-predicate with the specified
     template

‘coinductive(Template)’

     The predicate is declared as a coinductive predicate with the
     specified template

‘declared_in(Entity)’

     The predicate is declared (using a scope directive) in the
     specified entity

‘defined_in(Entity)’

     The predicate definition is looked up in the specified entity (note
     that this property does not necessarily imply that clauses for the
     predicate exist in ‘Entity’; the predicate can simply be false as
     per the closed-world assumption)

‘redefined_from(Entity)’

     The predicate is a redefinition of a predicate definition inherited
     from the specified entity

‘non_terminal(NonTerminal//Arity)’

     The predicate resulted from the compilation of the specified
     grammar rule non-terminal

‘alias_of(Predicate)’

     The predicate (name) is an alias for the specified predicate

‘alias_declared_in(Entity)’

     The predicate alias is declared in the specified entity

‘synchronized’

     The predicate is declared as synchronized (i.e.  it’s a
     deterministic predicate synchronized using a mutex when using a
     backend Prolog compiler supporting a compatible multi-threading
     implementation)

Some properties are only available when the entities are defined in
source files and when those source files are compiled with the
source_data flag turned on:

‘recursive’

     The predicate definition includes at least one recursive rule

‘inline’

     The predicate definition is inlined

‘auxiliary’

     The predicate is not user-defined but rather automatically
     generated by the compiler or the term-expansion mechanism.

‘mode(Mode, Solutions)’

     Instantiation, type, and determinism mode for the predicate (which
     can have multiple modes)

‘info(ListOfPairs)’

     Documentation key-value pairs as specified in the user-defined
     ‘info/2’ directive

‘number_of_clauses(N)’

     The number of clauses for the predicate existing at compilation
     time (note that this property is not updated at runtime when
     asserting and retracting clauses for dynamic predicates)

‘number_of_rules(N)’

     The number of rules for the predicate existing at compilation time
     (note that this property is not updated at runtime when asserting
     and retracting clauses for dynamic predicates)

‘declared_in(Entity, Line)’

     The predicate is declared (using a scope directive) in the
     specified entity in a source file at the specified line (if
     applicable)

‘defined_in(Entity, Line)’

     The predicate is defined in the specified entity in a source file
     at the specified line (if applicable)

‘redefined_from(Entity, Line)’

     The predicate is a redefinition of a predicate definition inherited
     from the specified entity, which is defined in a source file at the
     specified line (if applicable)

‘alias_declared_in(Entity, Line)’

     The predicate alias is declared in the specified entity
     in a source file at the specified line (if applicable)

The properties ‘declared_in/1-2’, ‘defined_in/1-2’, and
‘redefined_from/1-2’ do not apply to built-in methods and Logtalk or
Prolog built-in predicates.  Note that if a predicate is declared in a
category imported by the object, it will be the category name — not the
object name — that will be returned by the property ‘declared_in/1’.
The same is true for protocol declared predicates.

Some properties such as line numbers are only available when the entity
holding the predicates is defined in a source file compiled with the
source_data flag turned on.  Moreover, line numbers are only
supported in backend Prolog compilers that provide access to
the start line of a read term.  When such support is not available, the
value ‘-1’ is returned for the start and end lines.

1.8.7 Finding declared predicates
---------------------------------

We can find, by backtracking, all visible user predicates by calling the
current_predicate/1 built-in method.  This method takes into
account predicate scope declarations.  For example, the following call
will only return user predicates that are declared public:

     | ?- some_object::current_predicate(Name/Arity).

The predicate property ‘non_terminal/1’ may be used to retrieve all
grammar rule non-terminals declared for an object.  For example:

     current_non_terminal(Object, Name//Args) :-
         Object::current_predicate(Name/Arity),
         functor(Predicate, Functor, Arity),
         Object::predicate_property(Predicate, non_terminal(Name//Args)).

Usually, the non-terminal and the corresponding predicate share the same
functor but users should not rely on this always being true.

1.8.8 Calling Prolog predicates
-------------------------------

Logtalk is designed for both 'robustness' and 'portability'.  In the
context of calling Prolog predicates, robustness requires that the
compilation of Logtalk source code must not have 'accidental'
dependencies on Prolog code that happens to be loaded at the time of the
compilation.  One immediate consequence is that only Prolog 'built-in'
predicates are visible from within objects and categories.  But Prolog
systems provide a widely diverse set of built-in predicates, easily
rising portability issues.  Relying on non-standard predicates is often
unavoidable, however, due to the narrow scope of Prolog standards.
Logtalk applications may also require calling user-defined Prolog
predicates, either in ‘user’ or in Prolog modules.

1.8.8.1 Calling Prolog built-in predicates
..........................................

In predicate clauses and object ‘initialization/1’ directives, predicate
calls that are not prefixed with a message sending, super call, or
module qualification operator (‘::’, ‘^^’, or ‘:’), are compiled to
either calls to local predicates or as calls to Logtalk/Prolog built-in
predicates.  A predicate call is compiled as a call to a local predicate
if the object (or category) contains a scope directive, a multifile
directive, a dynamic directive, or a definition for the called
predicate.  When that is not the case, the compiler checks if the call
corresponds to a Logtalk or Prolog built-in predicate.  Consider the
following example:

     foo :-
         ...,
         write(bar),
         ...

The call to the ‘write/1’ predicate will be compiled as a call to the
corresponding Prolog standard built-in predicate unless the object (or
category) containing the above definition also contains a predicate
named ‘write/1’ or a directive for the predicate.

When calling non-standard Prolog built-in predicates or using
non-standard Prolog arithmetic functions, we may run into portability
problems while trying your applications with different backend Prolog
compilers.  We can use the compiler portability flag to
generate warnings for calls to non-standard predicates and arithmetic
functions.  We can also help document those calls using the
uses/2 directive.  For example, a few Prolog systems provide an
‘atom_string/2’ non-standard predicate.  We can write (in the object or
category calling the predicate):

     :- uses(user, [atom_string/2])

This directive is based on the fact that built-in predicates are visible
in plain Prolog (i.e.  in ‘user’).  Besides helping to document the
dependency on a non-standard built-in predicate, this directive will
also silence the compiler portability warning.

1.8.8.2 Calling Prolog non-standard built-in meta-predicates
............................................................

Prolog built-in meta-predicates may only be called locally within
objects or categories, i.e.  they cannot be used as messages.  Compiling
calls to non-standard, Prolog built-in meta-predicates can be tricky,
however, as there is no standard way of checking if a built-in predicate
is also a meta-predicate and finding out which are its meta-arguments.
But Logtalk supports overriding the original meta-predicate template
when not programmatically available or usable.  For example, assume a
‘det_call/1’ Prolog built-in meta-predicate that takes a goal as
argument.  We can add to the object (or category) calling it the
directive:

     :- meta_predicate(user::det_call(0)).

Another solution is to explicitly declare all non-standard built-in
Prolog meta-predicates in the corresponding adapter file using the
internal predicate ‘'$lgt_prolog_meta_predicate'/3’.  For example:

     '$lgt_prolog_meta_predicate'(det_call(_), det_call(0), predicate).

The third argument can be either the atom ‘predicate’ or the atom
‘control_construct’, a distinction that is useful when compiling in
debug mode.

1.8.8.3 Calling Prolog foreign predicates
.........................................

Prolog systems often support defining ‘foreign’ predicates, i.e.
predicates defined using languages other than Prolog using a ‘foreign
language interface’.  There isn’t, however, any standard for defining,
making available, and recognizing foreign predicates.  From a Logtalk
perspective, the two most common scenarios are calling a foreign
predicate (from within an object or a category) and making a set of
foreign predicates available as part of an object (or category)
protocol.  Assuming, as this is the most common case, that foreign
predicates are globally visible once made available (using a Prolog
system specific loading or linking procedure), we can simply call them
as user-defined plain predicates, as explained in the next section.
When defining an object (or category) that makes available foreign
predicates, the advisable solution is to name the predicates after the
object (or category) and then define object (or category) predicates
that call the foreign predicates.  Most backend adapter files include
support for recognizing foreign predicates that allows the Logtalk
compiler to inline calls to the predicates (thus avoiding call
indirection overheads).

1.8.8.4 Calling Prolog user-defined plain predicates
....................................................

User-defined Prolog plain predicates (i.e.  predicates that are not
defined in a Prolog module) can be called from within objects or
categories by sending the corresponding message to ‘user’.  For example:

     foo :-
         ...,
         user::bar,
         ...

In alternative, we can use the uses/2 directive and write:

     :- uses(user, [bar/0]).

     foo :-
         ...,
         bar,
         ...

Note that ‘user’ is a pseudo-object in Logtalk containing all predicate
definitions that are not encapsulated (either in a Logtalk entity or a
Prolog module).

When the Prolog predicate is not a meta-predicate, we can also use the
{}/1 compiler bypass control construct.  For example:

     foo :-
         ...,
         {bar},
         ...

But note that in this case the reflection API will not record
the dependency of the ‘foo/0’ predicate on the Prolog ‘bar/0’ predicate
as we are effectively bypassing the compiler.

1.8.8.5 Calling Prolog module predicates
........................................

Prolog module predicates can be called from within objects or categories
by using explicit qualification.  For example:

     foo :-
         ...,
         module:bar,
         ...

You can also use in alternative the use_module/2 directive to
call the module predicates using implicit qualification:

     :- use_module(module, [bar/0]).

     foo :-
         ...,
         bar,
         ...

Note that the first argument of the ‘use_module/2’ directive, when used
within an object or a category, is a 'module name', not a 'file
specification' (also be aware that Prolog modules are sometimes defined
in files with names that differ from the module names).

As loading a Prolog module varies between Prolog systems, the actual
loading directive or goal is preferably done from the application
loader file.  An advantage of this approach is that it contributes
to a clean separation between 'loading' and 'using' a resource with the
loader file being the central point that loads all application resources
(complex applications often use a 'hierarchy' of loader files but the
main idea remains the same).

As an example, assume that we need to call predicates defined in a
CLP(FD) Prolog library, which can be loaded using ‘library(clpfd)’ as
the file specification.  In the loader file, we would add:

     :- use_module(library(clpfd), []).

Specifying an empty import list is often used to avoid adding the module
exported predicates to plain Prolog.  In the objects and categories we
can then call the library predicates, using implicit or explicit
qualification, as explained.  For example:

     :- object(puzzle).

         :- public(puzzle/1).

         :- use_module(clpfd, [
             all_different/1, ins/2, label/1,
             (#=)/2, (#\=)/2,
             op(700, xfx, #=), op(700, xfx, #\=)
         ]).

         puzzle([S,E,N,D] + [M,O,R,E] = [M,O,N,E,Y]) :-
             Vars = [S,E,N,D,M,O,R,Y],
             Vars ins 0..9,
             all_different(Vars),
                       S*1000 + E*100 + N*10 + D +
                       M*1000 + O*100 + R*10 + E #=
             M*10000 + O*1000 + N*100 + E*10 + Y,
             M #\= 0, S #\= 0,
             label([M,O,N,E,Y]).

     :- end_object.

     Warning: The actual module code 'must' be loaded prior to
     compilation of Logtalk source code that uses it.  In particular,
     programmers should not expect that the module be auto-loaded
     (including when using a backend Prolog compiler that supports an
     auto-loading mechanism).

The module identifier argument can also be a parameter variable
when using the directive in a parametric object or a parametric
category.  In this case, dynamic binding will necessarily be used for
all listed predicates (and non-terminals).  The parameter variable must
be instantiated at runtime when the calls are made.

1.8.8.6 Calling Prolog module meta-predicates
.............................................

The Logtalk library provides implementations of common meta-predicates,
which can be used in place of module meta-predicates (e.g.  list mapping
meta-predicates).  If that is not the case, the Logtalk compiler may
need help to understand the module meta-predicate templates.  Despite
some recent progress in standardization of the syntax of
‘meta_predicate/1’ directives and of the ‘meta_predicate/1’ property
returned by the ‘predicate_property/2’ reflection predicate, portability
is still a major problem.  Thus, Logtalk allows the original
‘meta_predicate/1’ directive to be 'overridden' with a local directive
that Logtalk can make sense of.  Note that Logtalk is not based on a
predicate prefixing mechanism as found in module systems.  This
fundamental difference precludes an automated solution at the Logtalk
compiler level.

As an example, assume that you want to call from an object (or a
category) a module meta-predicate with the following meta-predicate
directive:

     :- module(foo, [bar/2]).

     :- meta_predicate(bar(*, :)).

The ‘:’ meta-argument specifier is ambiguous.  It tell us that the
second argument of the meta-predicate is module sensitive but it does
not tell us 'how'.  Some legacy module libraries and some Prolog systems
use ‘:’ to mean ‘0’ (i.e.  a meta-argument that will be meta-called).
Some others use ‘:’ for meta-arguments that are not meta-called but that
still need to be augmented with module information.  Whichever the case,
the Logtalk compiler doesn’t have enough information to unambiguously
parse the directive and correctly compile the meta-arguments in the
meta-predicate call.  Therefore, the Logtalk compiler will generate an
error stating that ‘:’ is not a valid meta-argument specifier when
trying to compile a ‘foo:bar/2’ goal.  There are two alternative
solutions for this problem.  The advised solution is to override the
meta-predicate directive by writing, inside the object (or category)
where the meta-predicate is called:

     :- meta_predicate(foo:bar(*, *)).

or:

     :- meta_predicate(foo:bar(*, 0)).

depending on the true meaning of the second meta-argument.  The second
alternative, only usable when the meta-argument can be handled as a
normal argument, is to simply use the {}/1 compiler bypass
control construct to call the meta-predicate as-is:

     ... :- {foo:bar(..., ...)}, ...

The downside of this alternative is that it hides the dependency on the
module library from the reflection API and thus from the developer
tools.

1.8.9 Defining Prolog multifile predicates
------------------------------------------

Some Prolog module libraries, e.g.  constraint packages, expect clauses
for some library predicates to be defined in other modules.  This is
accomplished by declaring the library predicate 'multifile' and by
explicitly prefixing predicate clause heads with the library module
identifier.  For example:

     :- multifile(clpfd:run_propagator/2).
     clpfd:run_propagator(..., ...) :-
         ...

Logtalk supports the definition of Prolog module multifile predicates in
objects and categories.  While the clause head is compiled as-is, the
clause body is compiled in the same way as a regular object or category
predicate, thus allowing calls to local object or category predicates.
For example:

     :- object(...).

         :- multifile(clpfd:run_propagator/2).
         clpfd:run_propagator(..., ...) :-
             % calls to local object predicates
             ...

     :- end_object.

The Logtalk compiler will print a warning if the ‘multifile/1’ directive
is missing.  These multifile predicates may also be declared dynamic
using the same ‘Module:Name/Arity’ notation.

1.8.10 Asserting and retracting Prolog predicates
-------------------------------------------------

To assert and retract clauses for Prolog dynamic predicates, we can use
an explicitly qualified module argument.  For example:

     :- object(...).

         :- dynamic(m:bar/1).

         foo(X) :-
             retractall(m:bar(_)),
             assertz(m:bar(X)),
             ...

     :- end_object.

In alternative, we can use use_module/2 directives to declare
the module predicates.  For example:

     :- object(...).

         :- use_module(m, [bar/1]).
         :- dynamic(m:bar/1).

         foo(X) :-
             % retract and assert bar/1 clauses in module m
             retractall(bar(_)),
             assertz(bar(X)),
             ...

     :- end_object.

When the Prolog dynamic predicates are defined in ‘user’, the
recommended and most portable practice (as not all backends support a
module system) is to use a uses/2 directive:

     :- object(...).

         :- uses(user, [bar/1]).
         :- dynamic(user::bar/1).

         foo(X) :-
             % retract and assert bar/1 clauses in user
             retractall(bar(_)),
             assertz(bar(X)),
             ...

     :- end_object.

Note that in the alternatives using ‘uses/2’ or ‘use_module/2’
directives, the argument of the database handling predicates must be
know at compile time.  If that is not the case, you must use instead
either an explicitly-qualified argument or the {}/1 control
construct.  For example:

     :- object(...).

         add(X) :-
             % assert clause X in module m
             assertz(m:X),
             ...

         remove(Y) :-
             % retract all clauses in user whose head unifies with Y
             {retractall(Y)},
             ...

     :- end_object.

1.9 Inheritance
===============

The inheritance mechanisms found on object-oriented programming
languages allow the specialization of previously defined objects,
avoiding the unnecessary repetition of code and allowing the definition
of common functionality for sets of objects.  In the context of logic
programming, we can interpret inheritance as a form of 'theory
extension': an object will virtually contain, besides its own
predicates, all the predicates inherited from other objects that are not
redefined locally.  Inheritance is not, however, the only mechanism for
theory extension.  Logtalk also supports 'composition' using
categories.

Logtalk uses a depth-first lookup procedure for finding predicate
declarations and predicate definitions, as explained below, when a
message is sent to an object.  The lookup procedures locate the
entity holding the predicate declaration and the entity
holding the predicate definition using the predicate name and
arity.  The alias/2 predicate directive may be used for
defining alternative names for inherited predicates, for solving
inheritance conflicts, and for giving access to all inherited
definitions (thus overriding the default lookup procedure).

The lookup procedures are used when sending a message (using the
(;;)/2, (;;)/1, and []/1 control constructs)
and when making 'super' calls (using the (^^)/1 control
construct).  The exact details of the lookup procedures depend on the
role played by the object receiving the message or making the
'super' call, as explained next.  The lookup procedures are also used by
the current_predicate/1 and predicate_property/2.
reflection predicates.

1.9.1 Protocol inheritance
--------------------------

Protocol inheritance refers to the inheritance of predicate declarations
(see scope directives).  These can be contained in objects,
protocols, or categories.  Logtalk supports single and multi-inheritance
of protocols: an object or a category may implement several protocols
and a protocol may extend several protocols.

1.9.1.1 Lookup order for prototype hierarchies
..............................................

The lookup order for predicate declarations is first the object, second
the implemented protocols (and the protocols that these may extend),
third the imported categories (and the protocols that they may
implement), and finally the objects that the object extends (following
their declaration order).  This lookup is performed in depth-first
order.  When an object inherits two different declarations for the same
predicate, by default, only the first one will be considered.

1.9.1.2 Lookup order for class hierarchies
..........................................

The lookup order for predicate declarations is first the object classes
(following their declaration order), second the classes implemented
protocols (and the protocols that these may extend), third the classes
imported categories (and the protocols that they may implement), and
finally the superclasses of the object classes.  This lookup is
performed in depth-first order.  If the object inherits two different
declarations for the same predicate, by default, only the first one will
be considered.

1.9.2 Implementation inheritance
--------------------------------

Implementation inheritance refers to the inheritance of predicate
definitions.  These can be contained in objects or in categories.
Logtalk supports multi-inheritance of implementation: an object may
import several categories or extend, specialize, or instantiate several
objects.

1.9.2.1 Lookup order for prototype hierarchies
..............................................

The lookup order for predicate definitions is similar to the lookup for
predicate declarations except that implemented protocols are ignored (as
they can only contain predicate directives).

1.9.2.2 Lookup order for class hierarchies
..........................................

The lookup order for predicate definitions is similar to the lookup for
predicate declarations except that implemented protocols are ignored (as
they can only contain predicate directives) and that the lookup starts
at the instance itself (that received the message) before proceeding, if
no predicate definition is found there, to the instance classes imported
categories and then to the class superclasses.

1.9.2.3 Redefining inherited predicate definitions
..................................................

When we define a predicate that is already inherited from an ancestor
object or an imported category, the inherited definition is hidden by
the new definition.  This is called inheritance overriding: a local
definition overrides any inherited definitions.  For example, assume
that we have the following two objects:

     :- object(root).

         :- public(bar/1).
         bar(root).

         :- public(foo/1).
         foo(root).

     :- end_object.

     :- object(descendant,
         extends(root)).

         foo(descendant).

     :- end_object.

After compiling and loading these objects, we can check the overriding
behavior by trying the following queries:

     | ?- root::(bar(Bar), foo(Foo)).

     Bar = root
     Foo = root
     yes

     | ?- descendant::(bar(Bar), foo(Foo)).

     Bar = root
     Foo = descendant
     yes

However, we can explicitly code other behaviors.  Some examples follow.

1.9.2.4 Specializing inherited predicate definitions
....................................................

Specialization of inherited definitions: the new definition calls the
inherited definition and makes additional calls.  This is accomplished
by calling the (^^)/1. 'super call' operator in the new
definition.  For example, assume a ‘init/0’ predicate that must account
for object specific initializations along the inheritance chain:

     :- object(root).

         :- public(init/0).

         init :-
             write('root init'), nl.

     :- end_object.

     :- object(descendant,
         extends(root)).

         init :-
             write('descendant init'), nl,
             ^^init.

     :- end_object.

     | ?- descendant::init.

     descendant init
     root init
     yes

1.9.2.5 Union of inherited and local predicate definitions
..........................................................

Union of the new with the inherited definitions: all the definitions are
taken into account, the calling order being defined by the inheritance
mechanisms.  This can be accomplished by writing a clause that just
calls, using the (^^)/1. 'super call' operator, the inherited
definitions.  The relative position of this clause among the other
definition clauses sets the calling order for the local and inherited
definitions.  For example:

     :- object(root).

         :- public(foo/1).

         foo(1).
         foo(2).

     :- end_object.

     :- object(descendant,
         extends(root)).

         foo(3).
         foo(Foo) :-
             ^^foo(Foo).

     :- end_object.

     | ?- descendant::foo(Foo).

     Foo = 3 ;
     Foo = 1 ;
     Foo = 2 ;
     no

1.9.2.6 Selective inheritance of predicate definitions
......................................................

The selective inheritance of predicate definitions (also known as
differential inheritance) is normally used in the representation of
exceptions to inherited default definitions.  We can use the
(^^)/1. 'super call' operator to test and possibly reject some of
the inherited definitions.  A common example is representing flightless
birds:

     :- object(bird).

         :- public(mode/1).

         mode(walks).
         mode(flies).

     :- end_object.

     :- object(penguin,
         extends(bird)).

         mode(swims).
         mode(Mode) :-
             ^^mode(Mode),
             Mode \== flies.

     :- end_object.

     | ?- penguin::mode(Mode).

     Mode = swims ;
     Mode = walks ;
     no

1.9.3 Public, protected, and private inheritance
------------------------------------------------

To make all public predicates declared via implemented
protocols, imported categories, or ancestor objects protected predicates
or to make all public and protected predicates
private predicates we prefix the entity’s name with the
corresponding keyword.  For example:

     :- object(Object,
         implements(private::Protocol)).

         % all the Protocol public and protected
         % predicates become private predicates
         % for the Object clients

         ...

     :- end_object.

or:

     :- object(Class,
         specializes(protected::Superclass)).

         % all the Superclass public predicates become
         % protected predicates for the Class clients

         ...

     :- end_object.

Omitting the scope keyword is equivalent to using the public scope
keyword.  For example:

     :- object(Object,
         imports(public::Category)).

         ...

     :- end_object.

This is the same as:

     :- object(Object,
         imports(Category)).

         ...

     :- end_object.

This way we ensure backward compatibility with older Logtalk versions
and a simplified syntax when protected or private inheritance are not
used.

1.9.4 Multiple inheritance
--------------------------

Logtalk supports multi-inheritance by enabling an object to extend,
instantiate, or specialize more than one object.  Likewise, a protocol
may extends multiple protocols and a category may extend multiple
categories.  In this case, the depth-first lookup algorithms described
above traverse the list of entities per relation from left to right.
Consider as an example the following object opening directive:

     :- object(foo,
         extends((bar, baz))).

The lookup procedure will look first into the parent object ‘bar’ and
its related entities before looking into the parent object ‘baz’.  The
alias/2 predicate directive can always be used to solve
multi-inheritance conflicts.  It should also be noted that the
multi-inheritance support does not affect performance when we use
single-inheritance.

1.9.5 Composition versus multiple inheritance
---------------------------------------------

It is not possible to discuss inheritance mechanisms without referring
to the long and probably endless debate on single versus multiple
inheritance.  The single inheritance mechanism can be implemented
efficiently but it imposes several limitations on reusing, even if the
multiple characteristics we intend to inherit are orthogonal.  On the
other hand, the multiple inheritance mechanisms are attractive in their
apparent capability of modeling complex situations.  However, they
include a potential for conflict between inherited definitions whose
variety does not allow a single and satisfactory solution for all the
cases.

No solution that we might consider satisfactory for all the problems
presented by the multiple inheritance mechanisms has been found.  From
the simplicity of some extensions that use the Prolog search strategy
like [McCabe92] or [Moss94] and to the
sophisticated algorithms of CLOS [Bobrow_et_al_88], there is
no adequate solution for all the situations.  Besides, the use of
multiple inheritance carries some complex problems in the domain of
software engineering, particularly in the reuse and maintenance of the
applications.  All these problems are substantially reduced if we
preferably use in our software development composition mechanisms
instead of specialization mechanisms [Taenzer89].  Multiple
inheritance is best used as an analysis and project abstraction, rather
than as an implementation technique [Shan_et_al_93].  Note
that Logtalk provides first-class support for composition using
categories.

1.10 Event-driven programming
=============================

The addition of event-driven programming capacities to the Logtalk
language [Moura94] is based on a simple but powerful idea:

     The computations must result, not only from message sending, but
     also from the 'observation' of message sending.

The need to associate computations to the occurrence of events was very
early recognized in knowledge representation languages, programming
languages [Stefik_et_al_86], [Moon86], operative
systems [Tanenbaum87], and graphical user interfaces.

With the integration between object-oriented and event-driven
programming, we intend to achieve the following goals:

   * Minimize the 'coupling' between objects.  An object should only
     contain what is intrinsic to it.  If an object observes another
     object, that means that it should depend only on the public
     protocol of the object observed and not on the implementation of
     that protocol.

   * Provide a mechanism for building 'reflexive systems' in Logtalk
     based on the dynamic behavior of objects in complement to the
     reflective information on object predicates and relations.

   * Provide a mechanism for easily defining method 'pre- and
     post-conditions' that can be toggled using the events.
     compiler flag.  The pre- and post-conditions may be defined in the
     same object containing the methods or distributed between several
     objects acting as method monitors.

   * Provide a 'publish-subscribe' mechanism where public messages play
     the role of events.

1.10.1 Definitions
------------------

The words 'event' and 'monitor' have multiple meanings in computer
science.  To avoid misunderstandings, we start by defining them in the
Logtalk context.

1.10.1.1 Event
..............

In an object-oriented system, all computations start through message
sending.  It thus becomes quite natural to declare that the only event
that can occur in this kind of system is precisely the sending of a
message.  An event can thus be represented by the ordered tuple
‘(Object, Message, Sender)’.

If we consider message processing an indivisible activity, we can
interpret the sending of a message and the return of the control to the
object that has sent the message as two distinct events.  This
distinction allows us to have a more precise control over a system
dynamic behavior.  In Logtalk, these two types of events have been named
‘before’ and ‘after’, respectively for sending a message and for
returning of control to the sender.  Therefore, we refine our event
representation using the ordered tuple ‘(Event, Object, Message,
Sender)’.

The implementation of events in Logtalk enjoys the following properties:

Independence between the two types of events

     We can choose to watch only one event type or to process each one
     of the events associated to a message sending in an independent
     way.

All events are automatically generated by the message sending mechanism

     The task of generating events is transparently accomplished by the
     message sending mechanism.  The user only needs to define the
     events that will be monitored.

The events watched at any moment can be dynamically changed during program execution

     The notion of event allows the user not only to have the
     possibility of observing, but also of controlling and modifying an
     application behavior, namely by dynamically changing the observed
     events during program execution.  It is our goal to provide the
     user with the possibility of modeling the largest number of
     situations.

1.10.1.2 Monitor
................

Complementary to the notion of event is the notion of monitor.  A
monitor is an object that is automatically notified by the message
sending mechanism whenever a registered event occurs.  Any object that
defines the event-handling predicates can play the role of a monitor.

The implementation of monitors in Logtalk enjoys the following
properties:

Any object can act as a monitor

     The monitor status is a role that any object can perform during its
     existence.  The minimum protocol necessary is declared in the
     built-in monitoring(1) protocol.  Strictly speaking, the reference
     to this protocol is only needed when specializing event handlers.
     Nevertheless, it is considered good programming practice to always
     refer the protocol when defining event handlers.

Unlimited number of monitors for each event

     Several monitors can observe the same event because of distinct
     reasons.  Therefore, the number of monitors per event is bounded
     only by the available computing resources.

The monitor status of an object can be dynamically changed in runtime

     This property does not imply that an object must be dynamic to act
     as a monitor (the monitor status of an object is not stored in the
     object).

Event handlers cannot modify the event arguments

     Notably, if the message contains unbound variables, these cannot be
     bound by the calls to the monitor event handlers.

   ---------- Footnotes ----------

   (1) ../../docs/monitoring_0.html#monitoring-0

1.10.2 Event generation
-----------------------

Assuming that the events flag is set to ‘allow’ for the
object (or category) sending the messages we want to observe, for each
message that is sent using the (;;)/2 control construct, the
runtime system automatically generates two events.  The first — 'before
event' — is generated when the message is sent.  The second — 'after
event' — is generated after the message has successfully been executed.

Note that 'self' messages (using the (;;)/1 control
construct) or 'super' calls (using the (^^)/1 control
construct) don’t generate events.

1.10.3 Communicating events to monitors
---------------------------------------

Whenever a spied event occurs, the message sending mechanism calls the
corresponding event handlers directly for all registered monitors.
These calls are internally made bypassing the message sending primitives
in order to avoid potential endless loops.  The event handlers consist
in user definitions for the public predicates declared in the built-in
monitoring(1) protocol (see below for more details).

   ---------- Footnotes ----------

   (1) ../../docs/monitoring_0.html#monitoring-0

1.10.4 Performance concerns
---------------------------

Ideally, the existence of monitored messages should not affect the
processing of the remaining messages.  On the other hand, for each
message that has been sent, the system must verify if its respective
event is monitored.  Whenever possible, this verification should be
performed in constant time and independently of the number of monitored
events.  The events representation takes advantage of the first argument
indexing performed by most Prolog compilers, which ensure — in the
general case — access in constant time.

Event-support can be turned off on a per-object (or per-category) basis
using the events compiler flag.  With event-support turned
off, Logtalk uses optimized code for processing message sending calls
that skips the checking of monitored events, resulting in a small but
measurable performance improvement.

1.10.5 Monitor semantics
------------------------

The established semantics for monitors actions consists on considering
its success as a necessary condition so that a message can succeed:

   - All actions associated to events of type ‘before’ must succeed, so
     that the message processing can start.

   - All actions associated to events of type ‘after’ also have to
     succeed so that the message itself succeeds.  The failure of any
     action associated to an event of type ‘after’ forces backtracking
     over the message execution (the failure of a monitor never causes
     backtracking over the preceding monitor actions).

Note that this is the most general choice.  If we wish a transparent
presence of monitors in a message processing, we just have to define the
monitor actions in such a way that they never fail (which is very simple
to accomplish).

1.10.6 Activation order of monitors
-----------------------------------

Ideally, whenever there are several monitors defined for the same event,
the calling order should not interfere with the result.  However, this
is not always possible.  In the case of an event of type ‘before’, the
failure of a monitor prevents a message from being sent and prevents the
execution of the remaining monitors.  In case of an event of type
‘after’, a monitor failure will force backtracking over message
execution.  Different orders of monitor activation can therefore lead to
different results if the monitor actions imply object modifications
unrecoverable in case of backtracking.  Therefore, the order for monitor
activation should be assumed as arbitrary.  In effect, to assume or to
try to impose a specific sequence requires a global knowledge of an
application dynamics, which is not always possible.  Furthermore, that
knowledge can reveal itself as incorrect if there is any changing in the
execution conditions.  Note that, given the independence between
monitors, it does not make sense that a failure forces backtracking over
the actions previously executed.

1.10.7 Event handling
---------------------

Logtalk provides three built-in predicates for event handling.  These
predicates support defining, enumerating, and abolishing events.
Applications that use events extensively usually define a set of objects
that use these built-in predicates to implement more sophisticated and
higher-level behavior.

1.10.7.1 Defining new events
............................

New events can be defined using the define_events/5 built-in
predicate:

     | ?- define_events(Event, Object, Message, Sender, Monitor).

Note that if any of the ‘Event’, ‘Object’, ‘Message’, and ‘Sender’
arguments is a free variable or contains free variables, this call will
define a 'set' of matching events.

1.10.7.2 Abolishing defined events
..................................

Events that are no longer needed may be abolished using the
abolish_events/5 built-in predicate:

     | ?- abolish_events(Event, Object, Message, Sender, Monitor).

If called with free variables, this goal will remove all matching
events.

1.10.7.3 Finding defined events
...............................

The events that are currently defined can be retrieved using the
current_event/5 built-in predicate:

     | ?- current_event(Event, Object, Message, Sender, Monitor).

Note that this predicate will return 'sets' of matching events if some
of the returned arguments are free variables or contain free variables.

1.10.7.4 Defining event handlers
................................

The monitoring(1) built-in protocol declares two public predicates,
see before/3 and after/3, that are automatically
called to handle ‘before’ and ‘after’ events.  Any object that plays the
role of monitor must define one or both of these event handler methods:

     before(Object, Message, Sender) :-
         ... .

     after(Object, Message, Sender) :-
         ... .

The arguments in both methods are instantiated by the message sending
mechanism when a monitored event occurs.  For example, assume that we
want to define a monitor called ‘tracer’ that will track any message
sent to an object by printing a describing text to the standard output.
Its definition could be something like:

     :- object(tracer,
         % built-in protocol for event handler methods
         implements(monitoring)).

         before(Object, Message, Sender) :-
             write('call: '), writeq(Object),
             write(' <-- '), writeq(Message),
             write(' from '), writeq(Sender), nl.

         after(Object, Message, Sender) :-
             write('exit: '), writeq(Object),
             write(' <-- '), writeq(Message),
             write(' from '), writeq(Sender), nl.

     :- end_object.

Assume that we also have the following object:

     :- object(any).

         :- public(bar/1).
         bar(bar).

         :- public(foo/1).
         foo(foo).

     :- end_object.

After compiling and loading both objects and setting the events
flag to ‘allow’, we can start tracing every message sent to any
object by calling the define_events/5 built-in predicate:

     | ?- set_logtalk_flag(events, allow).

     yes

     | ?- define_events(_, _, _, _, tracer).

     yes

From now on, every message sent from ‘user’ to any object will be traced
to the standard output stream:

     | ?- any::bar(X).

     call: any <-- bar(X) from user
     exit: any <-- bar(bar) from user
     X = bar

     yes

To stop tracing, we can use the abolish_events/5 built-in
predicate:

     | ?- abolish_events(_, _, _, _, tracer).

     yes

The monitoring(2) protocol declares the event handlers as public
predicates.  If necessary, see protected or private implementation of the protocol
may be used in order to change the scope of the event
handler predicates.  Note that the message sending processing mechanism
is able to call the event handlers irrespective of their scope.
Nevertheless, the scope of the event handlers may be restricted in order
to prevent other objects from calling them.

The pseudo-object user can also act as a monitor.  This
object expects the ‘before/3’ and ‘after/3’ predicates to be defined in
the plain Prolog database.  To avoid predicate existence errors when
setting ‘user’ as a monitor, this object declares the predicates
multifile.  Thus, any plain Prolog code defining the predicates should
include the directives:

     :- multifile(before/3).
     :- multifile(after/3).

   ---------- Footnotes ----------

   (1) ../../docs/monitoring_0.html#monitoring-0

   (2) ../../docs/monitoring_0.html#monitoring-0

1.11 Multi-threading programming
================================

Logtalk provides 'experimental' support for multi-threading programming
on selected Prolog compilers.  Logtalk makes use of the low-level Prolog
built-in predicates that implement message queues and interface with
POSIX threads and mutexes (or a suitable emulation), providing a small
set of high-level predicates and directives that allows programmers to
easily take advantage of modern multi-processor and multi-core computers
without worrying about the tricky details of creating, synchronizing, or
communicating with threads, mutexes, and message queues.  Logtalk
multi-threading programming integrates with object-oriented programming
providing a 'threaded engines' API, enabling objects and categories to
prove goals concurrently, and supporting synchronous and asynchronous
messages.

1.11.1 Enabling multi-threading support
---------------------------------------

Multi-threading support may be disabled by default.  It can be enabled
on the Prolog adapter files of supported compilers by setting the
read-only threads compiler flag to ‘supported’.

1.11.2 Enabling objects to make multi-threading calls
-----------------------------------------------------

The threaded/0 object directive is used to enable an object
to make multi-threading calls:

     :- threaded.

1.11.3 Multi-threading built-in predicates
------------------------------------------

Logtalk provides a small set of built-in predicates for multi-threading
programming.  For simple tasks where you simply want to prove a set of
goals, each one in its own thread, Logtalk provides a threaded/1
built-in predicate.  The remaining predicates allow for
fine-grained control, including postponing retrieving of thread goal
results at a later time, supporting non-deterministic thread goals, and
making 'one-way' asynchronous calls.  Together, these predicates provide
high-level support for multi-threading programming, covering most common
use cases.

1.11.3.1 Proving goals concurrently using threads
.................................................

A set of goals may be proved concurrently by calling the Logtalk
built-in predicate threaded/1.  Each goal in the set runs in
its own thread.

When the ‘threaded/1’ predicate argument is a 'conjunction' of goals,
the predicate call is akin to 'and-parallelism'.  For example, assume
that we want to find all the prime numbers in a given interval, ‘[N,
M]’.  We can split the interval in two parts and then span two threads
to compute the prime numbers in each sub-interval:

     prime_numbers(N, M, Primes) :-
         M > N,
         N1 is N + (M - N) // 2,
         N2 is N1 + 1,
         threaded((
             prime_numbers(N2, M, [], Acc),
             prime_numbers(N, N1, Acc, Primes)
         )).

     prime_numbers(N, M, Acc, Primes) :-
         ...

The ‘threaded/1’ call terminates when the two implicit threads
terminate.  In a computer with two or more processors (or with a
processor with two or more cores) the code above can be expected to
provide better computation times when compared with single-threaded code
for sufficiently large intervals.

When the ‘threaded/1’ predicate argument is a 'disjunction' of goals,
the predicate call is akin to 'or-parallelism', here reinterpreted as a
set of goals 'competing' to find a solution.  For example, consider the
different methods that we can use to find the roots of real functions.
Depending on the function, some methods will faster than others.  Some
methods will converge into the solution while others may diverge and
never find it.  We can try all the methods simultaneously by writing:

     find_root(Function, A, B, Error, Zero) :-
         threaded((
             bisection::find_root(Function, A, B, Error, Zero)
         ;   newton::find_root(Function, A, B, Error, Zero)
         ;   muller::find_root(Function, A, B, Error, Zero)
         )).

The above ‘threaded/1’ goal succeeds when one of the implicit threads
succeeds in finding the function root, leading to the termination of all
the remaining competing threads.

The ‘threaded/1’ built-in predicate is most useful for lengthy,
independent deterministic computations where the computational costs of
each goal outweigh the overhead of the implicit thread creation and
management.

1.11.3.2 Proving goals asynchronously using threads
...................................................

A goal may be proved asynchronously using a new thread by calling the
threaded_call/1-2 built-in predicate .  Calls to this
predicate are always true and return immediately (assuming a callable
argument).  The term representing the goal is copied, not shared with
the thread.  The thread computes the first solution to the goal, posts
it to the implicit message queue of the object from where the
‘threaded_call/1’ predicate was called, and suspends waiting for either
a request for an alternative solution or for the program to commit to
the current solution.

The results of proving a goal asynchronously in a new thread may be
later retrieved by calling the threaded_exit/1-2 built-in
predicate within the same object where the call to the ‘threaded_call/1’
predicate was made.  The ‘threaded_exit/1’ calls suspend execution until
the results of the ‘threaded_call/1’ calls are sent back to the object
message queue.

The ‘threaded_exit/1’ predicate allow us to retrieve alternative
solutions through backtracking (if you want to commit to the first
solution, you may use the threaded_once/1-2 predicate
instead of the ‘threaded_call/1’ predicate).  For example, assuming a
‘lists’ object implementing the usual ‘member/2’ predicate, we could
write:

     | ?- threaded_call(lists::member(X, [1,2,3])).

     X = _G189
     yes

     | ?- threaded_exit(lists::member(X, [1,2,3])).

     X = 1 ;
     X = 2 ;
     X = 3 ;
     no

In this case, the ‘threaded_call/1’ and the ‘threaded_exit/1’ calls are
made within the pseudo-object ‘user’.  The implicit thread running the
‘lists::member/2’ goal suspends itself after providing a solution,
waiting for a request to an alternative solution; the thread is
automatically terminated when the runtime engine detects that
backtracking to the ‘threaded_exit/1’ call is no longer possible.

Calls to the ‘threaded_exit/1’ predicate block the caller until the
object message queue receives the reply to the asynchronous call.  The
predicate threaded_peek/1-2 may be used to check if a reply
is already available without removing it from the thread queue.  The
‘threaded_peek/1’ predicate call succeeds or fails immediately without
blocking the caller.  However, keep in mind that repeated use of this
predicate is equivalent to polling a message queue, which may hurt
performance.

Be careful when using the ‘threaded_exit/1’ predicate inside
failure-driven loops.  When all the solutions have been found (and the
thread generating them is therefore terminated), re-calling the
predicate will generate an exception.  Note that failing instead of
throwing an exception is not an acceptable solution as it could be
misinterpreted as a failure of the ‘threaded_call/1’ argument.

The example on the previous section with prime numbers could be
rewritten using the ‘threaded_call/1’ and ‘threaded_exit/1’ predicates:

     prime_numbers(N, M, Primes) :-
         M > N,
         N1 is N + (M - N) // 2,
         N2 is N1 + 1,
         threaded_call(prime_numbers(N2, M, [], Acc)),
         threaded_call(prime_numbers(N, N1, Acc, Primes)),
         threaded_exit(prime_numbers(N2, M, [], Acc)),
         threaded_exit(prime_numbers(N, N1, Acc, Primes)).

     prime_numbers(N, M, Acc, Primes) :-
         ...

When using asynchronous calls, the link between a ‘threaded_exit/1’ call
and the corresponding ‘threaded_call/1’ call is established using
unification.  If there are multiple ‘threaded_call/1’ calls for a
matching ‘threaded_exit/1’ call, the connection can potentially be
established with any of them (this is akin to what happens with
tabling).  Nevertheless, you can easily use a call 'tag' by using in
alternative threaded_call/2, threaded_once/2, and
threaded_exit/2 built-in predicates.  For example:

     ?- threaded_call(member(X, [1,2,3]), Tag).

     Tag = 1
     yes

     ?- threaded_call(member(X, [1,2,3]), Tag).

     Tag = 2
     yes

     ?- threaded_exit(member(X, [1,2,3]), 2).

     X = 1 ;
     X = 2 ;
     X = 3
     yes

When using these predicates, the tags shall be considered as an opaque
term; users shall not rely on its type.  Tagged asynchronous calls can
be canceled by using the threaded_cancel/1 predicate.

1.11.4 One-way asynchronous calls
---------------------------------

Sometimes we want to prove a goal in a new thread without caring about
the results.  This may be accomplished by using the built-in predicate
threaded_ignore/1.  For example, assume that we are
developing a multi-agent application where an agent may send an “happy
birthday” message to another agent.  We could write:

     ..., threaded_ignore(agent::happy_birthday), ...

The call succeeds with no reply of the goal success, failure, or even
exception ever being sent back to the object making the call.  Note that
this predicate implicitly performs a deterministic call of its argument.

1.11.5 Asynchronous calls and synchronized predicates
-----------------------------------------------------

Proving a goal asynchronously using a new thread may lead to problems
when the goal results in side effects such as input/output operations or
modifications to an object database.  For example, if a new
thread is started with the same goal before the first one finished its
job, we may end up with mixed output, a corrupted database, or
unexpected goal failures.  In order to solve this problem, predicates
(and grammar rule non-terminals) with side effects can be declared as
'synchronized' by using the synchronized/1 predicate
directive.  Proving a query to a synchronized predicate (or synchronized
non-terminal) is internally protected by a mutex, thus allowing for easy
thread synchronization.  For example:

     % ensure thread synchronization
     :- synchronized(db_update/1).

     db_update(Update) :-
         % predicate with side-effects
         ...

A second example: assume an object defining two predicates for writing,
respectively, even and odd numbers in a given interval to the standard
output.  Given a large interval, a goal such as:

     | ?- threaded_call(obj::odd_numbers(1,100)),
          threaded_call(obj::even_numbers(1,100)).

     1 3 2 4 6 8 5 7 10 ...
     ...

will most likely result in a mixed up output.  By declaring the
‘odd_numbers/2’ and ‘even_numbers/2’ predicates synchronized:

     :- synchronized([
         odd_numbers/2,
         even_numbers/2]).

one goal will only start after the other one finished:

     | ?- threaded_ignore(obj::odd_numbers(1,99)),
          threaded_ignore(obj::even_numbers(1,99)).

     1 3 5 7 9 11 ...
     ...
     2 4 6 8 10 12 ...
     ...

Note that, in a more realistic scenario, the two ‘threaded_ignore/1’
calls would be made concurrently from different objects.  Using the same
synchronized directive for a set of predicates imply that they all use
the same mutex, as required for this example.

As each Logtalk entity is independently compiled, this directive must be
included in every object or category that contains a definition for the
described predicate, even if the predicate declaration is inherited from
another entity, in order to ensure proper compilation.  Note that a
synchronized predicate cannot be declared dynamic.  To ensure atomic
updates of a dynamic predicate, declare as synchronized the predicate
performing the update.

Synchronized predicates may be used as wrappers to messages sent to
objects that are not multi-threading aware.  For example, assume a ‘log’
object defining a ‘write_log_entry/2’ predicate that writes log entries
to a file, thus using side effects on its implementation.  We can
specify and define e.g.  a ‘sync_write_log_entry/2’ predicate as
follows:

     :- synchronized(sync_write_log_entry/2).

     sync_write_log_entry(File, Entry) :-
         log::write_log_entry(File, Entry).

and then call the ‘sync_write_log_entry/2’ predicate instead of the
‘write_log_entry/2’ predicate from multi-threaded code.

The synchronization directive may be used when defining objects that may
be reused in both single-threaded and multi-threaded Logtalk
applications.  The directive simply make calls to the synchronized
predicates deterministic when the objects are used in a single-threaded
application.

1.11.6 Synchronizing threads through notifications
--------------------------------------------------

Declaring a set of predicates as synchronized can only ensure that they
are not executed at the same time by different threads.  Sometimes we
need to suspend a thread not on a synchronization lock but on some
condition that must hold true for a thread goal to proceed.  I.e.  we
want a thread goal to be suspended until a condition becomes true
instead of simply failing.  The built-in predicate
threaded_wait/1 allows us to suspend a predicate execution
(running in its own thread) until a notification is received.
Notifications are posted using the built-in predicate
threaded_notify/1.  A notification is a Prolog term that a
programmer chooses to represent some condition becoming true.  Any
Prolog term can be used as a notification argument for these predicates.
Related calls to the ‘threaded_wait/1’ and ‘threaded_notify/1’ must be
made within the same object, 'this', as the object message queue is used
internally for posting and retrieving notifications.

Each notification posted by a call to the ‘threaded_notify/1’ predicate
is consumed by a single ‘threaded_wait/1’ predicate call (i.e.  these
predicates implement a peer-to-peer mechanism).  Care should be taken to
avoid deadlocks when two (or more) threads both wait and post
notifications to each other.

1.11.7 Threaded engines
-----------------------

Threaded engines provide an alternative to the multi-threading
predicates described in the previous sections.  An 'engine' is a
computing thread whose solutions can be lazily computed and retrieved.
In addition, an engine also supports a term queue that allows passing
arbitrary terms to the engine.

An engine is created by calling the threaded_engine_create/3.
built-in predicate.  For example:

     | ?- threaded_engine_create(X, member(X, [1,2,3]), worker).
     yes

The first argument is an 'answer template' to be used for retrieving
solution bindings.  The user can name the engine, as in this example
where the atom ‘worker’ is used, or have the runtime generate a name,
which should be treated as an opaque term.

Engines are scoped by the object within which the
‘threaded_engine_create/3’ call takes place.  Thus, different objects
can create engines with the same names with no conflicts.  Moreover,
engines share the visible predicates of the object creating them.

The engine computes the first solution of its goal argument and suspends
waiting for it to be retrieved.  Solutions can be retrieved one at a
time using the threaded_engine_next/2 built-in predicate:

     | ?- threaded_engine_next(worker, X).
     X = 1
     yes

The call blocks until a solution is available and fails if there are no
solutions left.  After returning a solution, this predicate signals the
engine to start computing the next one.  Note that this predicate is
deterministic.  In contrast with the ‘threaded_exit/1-2’ built-in
predicates, retrieving the next solution requires calling the predicate
again instead of by backtracking into its call.  For example:

     collect_all(Engine, [Answer| Answers]) :-
         threaded_engine_next(Engine, Answer),
         !,
         collect_all(Engine, Answers).
     collect_all(_, []).

There is also a reified alternative version of the predicate, see
threaded_engine_next_reified/2, which returns ‘the(Answer)’, ‘no’,
and ‘exception(Error)’ terms as answers.  Using this predicate,
collecting all solutions to an engine uses a different programming
pattern:

     ... :-
         ...,
         threaded_engine_next_reified(Engine, Reified),
         collect_all_reified(Reified, Engine, Answers),
         ...

     collect_all_reified(no, _, []).
     collect_all_reified(the(Answer), Engine, [Answer| Answers]) :-
         threaded_engine_next_reified(Engine, Reified),
         collect_all_reified(Reified, Engine, Answers).

Engines must be explicitly terminated using the
threaded_engine_destroy/1 built-in predicate:

     | ?- threaded_engine_destroy(worker).
     yes

A common usage pattern for engines is to define a recursive predicate
that uses the engine term queue to retrieve a task to be performed.  For
example, assume we define the following predicate:

     loop :-
         threaded_engine_fetch(Task),
         handle(Task),
         loop.

The threaded_engine_fetch/1 built-in predicate fetches a
task for the engine term queue.  The engine clients would use the
threaded_engine_post/2 built-in predicate to post tasks into the
engine term queue.  The engine would be created using the call:

     | ?- threaded_engine_create(none, loop, worker).

     yes

The ‘handle/1’ predicate, after performing a task, can use the
threaded_engine_yield/1 built-in predicate to make the task
results available for consumption using the ‘threaded_engine_next/2’ and
‘threaded_engine_next_reified/2’ built-in predicates.  Blocking
semantics are used by these two predicates: the
‘threaded_engine_yield/1’ predicate blocks until the returned solution
is consumed while the ‘threaded_engine_next/2’ predicate blocks until a
solution becomes available.

1.11.8 Multi-threading performance
----------------------------------

The performance of multi-threading applications is highly dependent on
the backend Prolog compiler, on the operating-system, and on
the use of dynamic binding and dynamic predicates.  All
compatible backend Prolog compilers that support multi-threading
features make use of POSIX threads or 'pthreads'.  The performance of
the underlying pthreads implementation can exhibit significant
differences between operating systems.  An important point is
synchronized access to dynamic predicates.  As different threads may try
to simultaneously access and update dynamic predicates, these operations
may used a lock-free algorithm or be protected by a lock, usually
implemented using a mutex.  In the latter case, poor mutex lock
operating-system performance, combined with a large number of collisions
by several threads trying to acquire the same lock, can result in severe
performance penalties.  Thus, whenever possible, avoid using dynamic
predicates and dynamic binding.

1.12 Error handling
===================

Error handling is accomplished in Logtalk by using the standard
‘catch/3’ and ‘throw/1’ predicates [ISO95] together with a
set of built-in methods that simplify generating errors decorated with
expected context.

Errors thrown by Logtalk have, whenever possible, the following format:

     error(Error, logtalk(Goal, ExecutionContext))

In this exception term, ‘Goal’ is the goal that triggered the error
‘Error’ and ‘ExecutionContext’ is the context in which ‘Goal’ is called.
For example:

     error(
         permission_error(modify,private_predicate,p),
         logtalk(foo::abolish(p/0), _)
     )

Note, however, that ‘Goal’ and ‘ExecutionContext’ can be unbound or only
partially instantiated when the corresponding information is not
available (e.g.  due to compiler optimizations that throw away the
necessary error context information).  The ‘ExecutionContext’ argument
is an opaque term that can be decoded using the
logtalk::execution_context/7(1) predicate.

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0-execution-context-7

1.12.1 Raising Exceptions
-------------------------

The error handling section in the reference manual lists a
set of convenient built-in methods that generate ‘error/2’ exception
terms with the expected context argument.  For example, instead of
manually constructing a type error as in:

     ...,
     context(Context),
     throw(error(type_error(atom, 42), Context)).

we can simply write:

     ...,
     type_error(atom, 42).

The provided error built-in methods cover all standard error types found
in the ISO Prolog Core standard.

1.12.2 Type-checking
--------------------

One of the most common case where errors may be generated is when
type-checking predicate arguments and input data before processing it.
The standard library includes a type(1) object that defines an extensive
set of types, together with predicates for validating and checking
terms.  The set of types is user extensible and new types can be defined
by adding clauses for the ‘type/1’ and ‘check/2’ multifile predicates.
For example, assume that we want to be able to check 'temperatures'
expressed in Celsius, Fahrenheit, or Kelvin scales.  We start by
declaring (in an object or category) the new type:

     :- multifile(type::type/1).
     type::type(temperature(_Unit)).

Next, we need to define the actual code that would verify that a
temperature is valid.  As the different scales use a different value for
absolute zero, we can write:

     :- multifile(type::check/2).
     type::check(temperature(Unit), Term) :-
         check_temperature(Unit, Term).

     % given that temperature has only a lower bound, we make use of the library
     % property/2 type to define the necessary test expression for each unit
     check_temperature(celsius, Term) :-
         type::check(property(float, [Temperature]>>(Temperature >= -273.15)), Term).
     check_temperature(fahrenheit, Term) :-
         type::check(property(float, [Temperature]>>(Temperature >= -459.67)), Term).
     check_temperature(kelvin, Term) :-
         type::check(property(float, [Temperature]>>(Temperature >= 0.0)), Term).

With this definition, a term is first checked that it is a float value
before checking that it is in the expected open interval.  But how do we
use this new type?  If we want just to test if a temperature is valid,
we can write:

     ..., type::valid(temperature(celsius), 42.0), ...

The type::valid/2(2) predicate succeeds or fails depending on the second
argument being of the type specified in the first argument.  If instead
of success or failure we want to generate an error for invalid values,
we can use the type::check/2(3) predicate instead:

     ..., type::check(temperature(celsius), 42.0), ...

If we require an ‘error/2’ exception term with the error context, we can
use instead the type::check/3(4) predicate:

     ...,
     context(Context),
     type::check(temperature(celsius), 42.0, Context),
     ...

Note that ‘context/1’ calls are inlined and messages to the library
‘type’ object use static binding when compiling with the
optimize flag turned on, thus enabling efficient
type-checking.

   ---------- Footnotes ----------

   (1) ../../docs/type_0.html#type-0

   (2) ../../docs/type_0.html#type-0-valid-2

   (3) ../../docs/type_0.html#type-0-check-2

   (4) ../../docs/type_0.html#type-0-check-3

1.12.3 Expected terms
---------------------

Support for representing and handling 'expected terms' is provided by
the expecteds library.  Expected terms allows defering
errors to later stages of an application in alternative to raising an
exception as soon as an error is detected.

1.12.4 Compiler warnings and errors
-----------------------------------

The current Logtalk compiler uses the standard ‘read_term/3’ built-in
predicate to read and compile a Logtalk source file.  This improves the
compatibility with backend Prolog compilers and their
proprietary syntax extensions and standard compliance quirks.  But one
consequence of this design choice is that invalid Prolog terms or syntax
errors may abort the compilation process with limited information given
to the user (due to the inherent limitations of the ‘read_term/3’
predicate).

Assuming that all the terms in a source file are valid, there is a set
of errors and potential errors, described below, that the compiler will
try to detect and report, depending on the used compiler flags (see the
Compiler flags section of this manual on lint flags for
details).

1.12.4.1 Unknown entities
.........................

The Logtalk compiler warns about any referenced entity that is not
currently loaded.  The warning may reveal a misspell entity name or just
an entity that it will be loaded later.  Out-of-oder loading should be
avoided when possible as it prevents some code optimizations such as
static binding of messages to methods.

1.12.4.2 Singleton variables
............................

Singleton variables in a clause are often misspell variables and, as
such, one of the most common errors when programming in Prolog.
Assuming that the backend Prolog compiler implementation of
the ‘read_term/3’ predicate supports the standard ‘singletons/1’ option,
the compiler warns about any singleton variable found while compiling a
source file.

1.12.4.3 Redefinition of Prolog built-in predicates
...................................................

The Logtalk compiler will warn us of any redefinition of a Prolog
built-in predicate inside an object or category.  Sometimes the
redefinition is intended.  In other cases, the user may not be aware
that a particular backend Prolog compiler may already provide
the predicate as a built-in predicate or may want to ensure code
portability among several Prolog compilers with different sets of
built-in predicates.

1.12.4.4 Redefinition of Logtalk built-in predicates
....................................................

Similar to the redefinition of Prolog built-in predicates, the Logtalk
compiler will warn us if we try to redefine a Logtalk built-in.  But the
redefinition will probably be an error in most (if not all) cases.

1.12.4.5 Redefinition of Logtalk built-in methods
.................................................

An error will be thrown if we attempt to redefine a Logtalk built-in
method inside an entity.  The default behavior is to report the error
and abort the compilation of the offending entity.

1.12.4.6 Misspell calls of local predicates
...........................................

A warning will be reported if Logtalk finds (in the body of a predicate
definition) a call to a local predicate that is not defined, built-in
(either in Prolog or in Logtalk) or declared dynamic.  In most cases
these calls are simple misspell errors.

1.12.4.7 Portability warnings
.............................

A warning will be reported if a predicate clause contains a call to a
non-standard built-in predicate or arithmetic function, Portability
warnings are also reported for non-standard flags or flag values.  These
warnings often cannot be avoided due to the limited scope of the ISO
Prolog standard.

1.12.4.8 Deprecated elements
............................

A warning will be reported if a deprecated directive, control construct,
or predicate is used.  These warnings should be fixed as soon as
possible as support for any deprecated features will likely be
discontinued in future versions.

1.12.4.9 Missing directives
...........................

A warning will be reported for any missing dynamic, discontiguous,
meta-predicate, and public predicate directive.

1.12.4.10 Duplicated directives
...............................

A warning will be reported for any duplicated scope, multifile, dynamic,
discontiguous, meta-predicate, and meta-non-terminal directives.  Note
that conflicting directives for the same predicate are handled as
errors, not as duplicated directive warnings.

1.12.4.11 Duplicated clauses
............................

A warning will be reported for any duplicated entity clauses.  This
check is computationally heavy, however, and usually turned off by
default.

1.12.4.12 Goals that are always true or false
.............................................

A warning will be reported for any goal that is always true or false.
This is usually caused by typos in the code.  For example, writing ‘X ==
y’ instead of ‘X == Y’.

1.12.4.13 Trivial fails
.......................

A warning will be reported for any call to a local static predicate with
no matching clause.

1.12.4.14 Suspicious calls
..........................

A warning will be reported for calls that are syntactically correct but
most likely a semantic error.  An example is (;;)/1 calls in
clauses that apparently are meant to implement recursive predicate
definitions where the user intention is to call the local predicate
definition.

1.12.4.15 Lambda variables
..........................

A warning will be reported for lambda expressions with
unclassified variables (not listed as either lambda free or
lambda parameter variables), for variables playing a dual
role (as both lambda free and lambda parameter variables), and for
lambda parameters used elsewhere in a clause.

1.12.4.16 Redefinition of predicates declared in ‘uses/2’ or ‘use_module/2’ directives
......................................................................................

A error will be reported for any attempt to define locally a predicate
that is already declared in an uses/2 or use_module/2
directive.

1.12.4.17 Other warnings and errors
...................................

The Logtalk compiler will throw an error if it finds a predicate clause
or a directive that cannot be parsed.  The default behavior is to report
the error and abort the compilation.

1.12.5 Runtime errors
---------------------

This section briefly describes runtime errors that result from misuse of
Logtalk built-in predicates, built-in methods or from message sending.
For a complete and detailed description of runtime errors please consult
the Reference Manual.

1.12.5.1 Logtalk built-in predicates
....................................

Most Logtalk built-in predicates checks the type and mode of the calling
arguments, throwing an exception in case of misuse.

1.12.5.2 Logtalk built-in methods
.................................

Most Logtalk built-in method checks the type and mode of the calling
arguments, throwing an exception in case of misuse.

1.12.5.3 Message sending
........................

The message sending mechanisms always check if the receiver of a message
is a defined object and if the message corresponds to a declared
predicate within the scope of the sender.  The built-in protocol
forwarding(1) declares a predicate, see forward/1, which is
automatically called (if defined) by the runtime for any message that
the receiving object does not understand.  The usual definition for this
error handler is to delegate or forward the message to another object
that might be able to answer it:

     forward(Message) :-
         % forward the message while preserving the sender
         [Object::Message].

If preserving the original sender is not required, this definition can
be simplified to:

     forward(Message) :-
         Object::Message.

More sophisticated definitions are, of course, possible.

   ---------- Footnotes ----------

   (1) ../../docs/forwarding_0.html#forwarding-0

1.13 Reflection
===============

Logtalk provides support for both 'structural' and 'behavioral'
reflection.  Structural reflection supports computations over an
application structure while behavioral reflection computations over what
an application does while running.  The structural and behavioral
reflection APIs are used by all the developer tools, which
are regular applications.

1.13.1 Structural reflection
----------------------------

Structural reflection allows querying the properties of objects,
categories, protocols, and predicates.  This API provides two views on
the structure of an application: a 'transparent-box view' and a
'black-box view', described next.

1.13.1.1 Transparent-box view
.............................

The transparent-box view provides a structural view of the contents and
properties of entities, predicates, and source files akin to accessing
the corresponding source code.  I.e.  this is the view we use when
asking questions such as: 'What predicates are declared in this
protocol?'  'Which predicates are called by this predicate?'  'Where are
clauses for this multifile predicate defined?'

For entities, built-in predicates are provided for enumerating entities,
see enumerating entity properties. (including
entity declared, defined, called, and updated predicates; i.e.  full
predicate cross-referencing data), and enumerating entity relations.
(for full entity cross-referencing data).  For a
detailed description of the supported entity properties, see the
sections on object properties, protocol properties,
and category properties.  For examples of querying entity
relations, see the sections on object relations,
protocol relations, and category relations.

     Note: Some entity and predicate properties are only available when
     the source files are compiled with the source_data flag
     turned on.

The logtalk(1) built-in object provides predicates for querying loaded
source files and their properties.

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0

1.13.1.2 Black-box view
.......................

The black-box view provides a view that takes into account entity
encapsulation and thus only allow querying about predicates and
operators that are within scope of the entity calling the reflection
methods.  This is the view we use and asking questions such as: 'What
messages can be sent to this object?'

Built-in methods are provided for querying the predicates that are declared and can be called or used as messages
and for querying
the predicate properties.  It is also possible to enumerate
entity operators.  See the sections on finding declared predicates
and on predicate properties for more
details.

1.13.2 Behavioral reflection
----------------------------

Behavioral reflection provides insight on what an application does when
running.  Specifically, by observing and acting on the messages being
exchanged between objects.  See the section on event-driven programming
for details.  There is also a dependents.
library that provides an implementation of Smalltalk dependents
mechanism.

For use in debugging tools, there is also a small reflection API
providing trace and debug event predicates provided by the
logtalk(1) built-in object.

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0

1.14 Writing and running applications
=====================================

For a successful programming in Logtalk, you need a good working
knowledge of Prolog and an understanding of the principles of
object-oriented programming.  Most guidelines for writing good Prolog
code apply as well to Logtalk programming.  To those guidelines, you
should add the basics of good object-oriented design.

One of the advantages of a system like Logtalk is that it enable us to
use the currently available object-oriented methodologies, tools, and
metrics [Champaux92] in logic programming.  That said,
writing applications in Logtalk is similar to writing applications in
Prolog: we define new predicates describing what is true about our
domain objects, about our problem solution.  We encapsulate our
predicate directives and definitions inside new objects, categories, and
protocols that we create by hand with a text editor or by using the
Logtalk built-in predicates.  Some of the information collected during
the analysis and design phases can be integrated in the objects,
categories and protocols that we define by using the available entity
and predicate documenting directives.

1.14.1 Starting Logtalk
-----------------------

We run Logtalk inside a normal Prolog session, after loading the
necessary files.  Logtalk extends but does not modify your Prolog
compiler.  We can freely mix Prolog queries with the sending of messages
and our applications can be made of both normal Prolog clauses and
object definitions.

Depending on your Logtalk installation, you may use a script or a
shortcut to start Logtalk with your chosen Prolog compiler.  On POSIX
operating-systems, Bash shell integration scripts should be available
from the command-line.  On Windows, PowerShell integration scripts
should be available from the command-line and integration shortcuts
should be available from the Start Menu.  Scripts are named upon the
used backend Prolog compilers.

For example, assuming a POSIX operating-system and GNU Prolog as the
backend:

     $ gplgt
     ...

Depending on your Logtalk installation, you may need to type instead
‘gplgt.sh’.  On Windows, using PowerShell 7.2 or later version and
ECLiPSe as the backend:

     PS> eclipselgt.ps1
     ...

1.14.2 Running parallel Logtalk processes
-----------------------------------------

Running parallel Logtalk processes is enabled by setting the
clean flag to ‘on’.  This is the default flag value in the backend
adapter files.  With this setting, the intermediate Prolog files
generated by the Logtalk compiler include the processes identifier in
the names, thus preventing file names clashes when running parallel
processes.  When the flag is turned off, the generated intermediate
Prolog file names don’t include the process identifier and are kept
between runs.  This is usually done to avoid repeated recompilation of
stable code when developing large applications or when running multiple
test sets for performance (by avoiding repeated recompilation of the
‘lgtunit’ tool).

To run parallel Logtalk processes with the ‘clean’ flag turned off, each
process must use its own scratch directory.  This is
accomplished by defining the ‘scratch_directory’ library alias to a per
process location.  For example, assuming we’re using GNU Prolog as the
backend, a possible definition could be:

     :- multifile(logtalk_library_path/2).
     :- dynamic(logtalk_library_path/2).

     logtalk_library_path(scratch_directory, Directory) :-
         temporary_name(lgtXXXXXX, Name),
         decompose_file_name(Name, _, Prefix, _),
         atom_concat('/tmp/', Prefix, Directory),
         (   file_exists(Directory) ->
             true
         ;   make_directory(Directory)
         ).

Assuming the code above is saved in a
‘parallel_logtalk_processes_setup.pl’ file, we would then start Logtalk
using:

     $ gplgt --init-goal "consult('parallel_logtalk_processes_setup.pl')"

The details on how to define and load the definition of the
‘scratch_directory’ library alias are, however, backend specific (due to
the lack of Prolog standardization) and possibly also operating-system
specific (different locations for the temporary directory).  The Logtalk
library contains support for selected backends.

1.14.3 Source files
-------------------

Logtalk source files may define any number of entities (objects,
categories, or protocols).  Source files may also contain Prolog code
interleaved with Logtalk entity definitions.  Plain Prolog code is
usually copied as-is to the corresponding Prolog output file (except, of
course, if subject to the term-expansion mechanism).  Prolog
modules are compiled as objects.  The following Prolog directives are
processed when read (thus affecting the compilation of the source code
that follows): ‘ensure_loaded/1’, ‘use_module/1-2’, ‘op/3’, and
‘set_prolog_flag/2’.  The initialization/1 directive may be
used for defining an initialization goal to be executed when loading a
source file.

Logtalk source files can include the text of other files by using the
include/1 directive.  Although there is also a standard
Prolog ‘include/1’ directive, any occurrences of this directive in a
Logtalk source file is handled by the Logtalk compiler, not by the
backend Prolog compiler, to improve portability.

When writing a Logtalk source file the following advice applies:

   - When practical and when performance is critical, define each entity
     on its own source file.

   - Source file loading order can impact performance (e.g.  if an
     object imports a category defined in a source file loaded after the
     object source file, no static binding optimizations will be
     possible).

   - Initialization directives that result in the compilation and
     loading of other source files (e.g.  libraries) should preferably
     be written in the application loader file to ensure the
     availability of the entities they define when compiling the
     application source files (thus enabling static binding
     optimizations).

1.14.3.1 Naming conventions
...........................

When defining each entity in its own source file, it is recommended that
the source file be named after the entity identifier.  For parametric
objects, the identifier arity can be appended to the identifier functor.
By default, all Logtalk source files use the extension ‘.lgt’ but this
is optional and can be set in the adapter files.  For example, we may
define an object named ‘vehicle’ and save it in a ‘vehicle.lgt’ source
file.  A ‘sort(_)’ parametric object would be saved it on a ‘sort_1.lgt’
source file.

1.14.3.2 Source file text encoding
..................................

The text encoding used in a source file may be declared using the
encoding/1 directive when running Logtalk with backend Prolog
compilers that support multiple encodings (check the
encoding_directive flag in the adapter file of your Prolog
compiler).

1.14.4 Multi-pass compiler
--------------------------

Logtalk is implemented using a 'multi-pass' compiler.  In comparison,
some Prolog systems use a multi-pass compiler while others use a
single-pass compiler.  While there are pros and cons with each solution,
the most relevant consequence in this context is for the content of
source files.  In Logtalk, entities and predicates only become available
(for the runtime system) after the source file is successfully compiled
and loaded.  This may prevent some compiler optimizations, notably
static binding, if some of the referred entities are defined in the
same source file.  On the other hand, the order of predicate directives
and predicate definitions is irrelevant.  In contrast, in a system
implemented using a single-pass compiler, the order of the source file
terms can and often is significant for proper and successful
compilation.  In these systems, predicates may become available for
calling as soon as they are compiled even if the remaining of the source
file is yet to be compiled.

The Logtalk compiler reads source files using the Prolog standard
‘read_term/3’ predicate.  This ensures compatibility with any syntax
extensions that the used backend may implement.  In the first compiler
stage, all source file terms are read and data about all defined
entities, directives, predicates, and grammar rules is collected.  Any
defined term-expansion rules are applied to the read terms.
Grammar rules are expanded into predicate clauses unless expanded by
user-defined term-expansion rules.  The second stage compiles all
initialization goals and clause bodies, taking advantage of the data
collected in the first stage, and applying any defined goal-expansion
rules.  Depending on the compilation mode, the generated code can be
instrumented for debugging tools or optimized for performance.  Linter
checks are performed during these two first stages.  The final step in
the second stage is to write the generated intermediate Prolog code into
a temporary file.  In the third and final stage, this intermediate
Prolog file is compiled and loaded by the used backend.  These
intermediate files are deleted by default after loading (see the
clean flag description for details).

1.14.5 Compiling and loading your applications
----------------------------------------------

Your applications will be made of source files containing your objects,
protocols, and categories.  The source files can be compiled to disk by
calling the logtalk_compile/1 built-in predicate:

     | ?- logtalk_compile([source_file1, source_file2, ...]).

This predicate runs the compiler on each file and, if no fatal errors
are found, outputs Prolog source files that can then be consulted or
compiled in the usual way by your Prolog compiler.

To compile to disk and also load into memory the source files we can use
the logtalk_load/1 built-in predicate:

     | ?- logtalk_load([source_file1, source_file2, ...]).

This predicate works in the same way of the predicate
‘logtalk_compile/1’ but also loads the compiled files into memory.

Both predicates expect a source file name or a list of source file names
as an argument.  The Logtalk source file name extension, as defined in
the adapter file (by default, ‘.lgt’), can be omitted.

If you have more than a few source files then you may want to use a
loader file helper file containing the calls to the
‘logtalk_load/1-2’ predicates.  Consulting or compiling the loader file
will then compile and load all your Logtalk entities into memory (see
below for details).

With most backend Prolog compilers, you can use the shorthands
‘{File}’ for ‘logtalk_load(File)’ and ‘{File1, File2, ...}’ for
‘logtalk_load([File1, File2, ...])’.  The use these shorthands should be
restricted to the Logtalk/Prolog top-level interpreter as they are not
part of the language specification and may be commented out in case of
conflicts with backend Prolog compiler features.

The built-in predicate logtalk_make/0 can be used to reload
all modified source files.  With most backend Prolog compilers, you can
also use the ‘{*}’ top-level shortcut.  Files are also reloaded when the
compilation mode changes.  An extended version of this predicate, see
logtalk_make/1, accepts multiple targets including ‘all’, ‘clean’,
‘check’, ‘circular’, ‘documentation’, ‘caches’, ‘debug’, ‘normal’, and
‘optimal’.  For example, assume that you have loaded your application
files and found a bug.  You can easily recompile the files in debug mode
by using the ‘logtalk_make(debug)’ goal.  After debugging and fixing the
bug, you can reload the files in normal mode using the
‘logtalk_make(normal)’ or in optimized mode using the
‘logtalk_make(optimal)’ goal.  See the predicates documentation for a
complete list of targets and top-level shortcuts.  In particular, the
‘logtalk_make(clean)’ goal can be specially useful before switching
backend Prolog compilers as the generated intermediate files may not be
compatible.  The ‘logtalk_make(caches)’ goal is usually used when
benchmarking compiler performance improvements.

1.14.6 Compiler errors, warnings, and comments
----------------------------------------------

Following a Prolog tradition inherited from Quintus Prolog, the compiler
prefixes (by default) errors with a ‘!’ and warnings with a ‘*’.  For
example:

     !     Existence error: directive object/1 does not exist
     !       in directive end_object/0
     !       in file /home/jdoe/logtalk/examples/errors/unmatched_directive.lgt at or above line 27

     *     No matching clause for goal: baz(a)
     *       while compiling object main_include_compiler_warning
     *       in file /home/jdoe/logtalk/examples/errors/include_compiler_warning.lgt between lines 38-39

Compiler comments are prefixed by ‘%’.  For example:

     ?- {ack(loader)}.
     % [ /home/jdoe/logtalk/examples/ack/ack.lgt loaded ]
     % [ /home/jdoe/logtalk/examples/ack/loader.lgt loaded ]
     % (0 warnings)
     true.

1.14.7 Loader files
-------------------

If you look into the Logtalk distribution, you will notice that most
source code directories (e.g.  of tools, libraries, and examples)
contain a 'driver file' that can be used to load all included source
files and any required libraries.  These loader files are usually named
‘loader.lgt’ or contain the word 'loader' in their name.  Loader files
are ordinary source files and thus compiled and loaded like any source
file.  By also defining a loader file for your project, you can then
load it by simply typing:

     | ?- {loader}.

Another driver file, usually named ‘tester.lgt’ (or containing the word
'tester' in its name) is commonly used to load and run tests.  By also
defining a tester file for your project, you can then run its tests by
simply typing:

     | ?- {tester}.

Usually these driver files contain calls to the built-in predicates
set_logtalk_flag/2. (e.g.  for setting global,
'project-specific', flag values) and logtalk_load/1 or
logtalk_load/2. (for loading project files), wrapped inside a Prolog
‘initialization/1’ directive for portability.  For instance, if your
code is split in three source files named ‘source1.lgt’, ‘source2.lgt’,
and ‘source3.lgt’, then the contents of your loader file could be:

     :- initialization((
         % set project-specific global flags
         set_logtalk_flag(events, allow),
         % load the project source files
         logtalk_load([source1, source2, source3])
     )).

Another example of directives that are often used in a loader file would
be ‘op/3’ directives declaring global operators needed by your project.
Loader files are also often used for setting source file-specific
compiler flags (this is useful even when you only have a single source
file if you always load it with using the same set of compiler flags).
For example:

     :- initialization((
         % set project-specific global flags
         set_logtalk_flag(underscore_variables, dont_care),
         set_logtalk_flag(source_data, off),
         % load the project source files
         logtalk_load(
             [source1, source2, source3],
             % source file-specific flags
             [portability(warning)]),
         logtalk_load(
             [source4, source5],
             % source file-specific flags
             [portability(silent)])
     )).

To take the best advantage of loader and tester files, define a clause
for the multifile and dynamic ‘logtalk_library_path/2’ predicate for the
directory containing your source files as explained in the next section.

When your project also uses Prolog module resources, the loader file is
also the advised place to load them, preferably without any exports.
For example:

     :- use_module(library(clpfd), []).
     ...

     :- initialization((
         ...
     )).

Complex projects often use a main loader file that loads the loader
files of each of the project components.  Thus, loader files provide a
central point to understand a project organization and dependencies.

Worth mentioning here a common mistake when first starting working with
loader files.  New users sometimes try to set compiler flags using
‘logtalk_load/2’ when loading a loader file.  For example, by writing:

     | ?- logtalk_load(loader, [optimize(on)]).

This will not work as you might expect as the compiler flags will only
be used in the compilation of the ‘loader.lgt’ file itself and will not
affect the compilation of files loaded through the ‘initialization/1’
directive contained on the loader file.

1.14.8 Libraries of source files
--------------------------------

Logtalk defines a 'library' simply as a directory containing source
files.  Library locations can be specified by defining or asserting
clauses for the dynamic and multifile predicate
logtalk_library_path/2.  For example:

     :- multifile(logtalk_library_path/2).
     :- dynamic(logtalk_library_path/2).

     logtalk_library_path(shapes, '$LOGTALKUSER/examples/shapes/').

The first argument of the predicate is used as an alias for the path on
the second argument.  Library aliases may also be used on the second
argument.  For example:

     :- multifile(logtalk_library_path/2).
     :- dynamic(logtalk_library_path/2).

     logtalk_library_path(lgtuser, '$LOGTALKUSER/').
     logtalk_library_path(examples, lgtuser('examples/')).
     logtalk_library_path(viewpoints, examples('viewpoints/')).

This allows us to load a library source file without the need to first
change the current working directory to the library directory and then
back to the original directory.  For example, in order to load a
‘loader.lgt’ file, contained in a library named ‘viewpoints’, we just
need to type:

     | ?- logtalk_load(viewpoints(loader)).

The best way to take advantage of this feature is to load at startup a
source file containing clauses for the ‘logtalk_library_path/2’
predicate needed for all available libraries (typically, using a
settings file, as discussed below).  This allows us to load library
source files or entire libraries without worrying about libraries paths,
improving code portability.  The directory paths on the second argument
should always end with the path directory separator character.  Most
backend Prolog compilers allows the use of environment variables in the
second argument of the ‘logtalk_library_path/2’ predicate.  Use of POSIX
relative paths (e.g.  ‘'../'’ or ‘'./'’) for top-level library
directories (e.g.  ‘lgtuser’ in the example above) is not advised as
different backend Prolog compilers may start with different initial
working directories, which may result in portability problems of your
loader files.

This library notation provides functionality inspired by the
‘file_search_path/2’ mechanism introduced by Quintus Prolog and later
adopted by some other Prolog compilers but with a key difference: there
is no fragile search mechanism and the Logtalk ‘make’ can be used to
check for duplicated library aliases.  Multiple definitions for the same
alias are problematic when using external dependencies as any
third-party update to those dependencies can introduce file name
clashes.  Note that the potential for these clashes cannot be reliably
minimized by a careful ordering of the ‘logtalk_library_path/2’
predicate clauses due to this predicate being multifile and dynamic.

1.14.9 Settings files
---------------------

Although is always possible to edit the backend Prolog compiler
adapter files, the recommended solution to customize compiler flags
is to create a ‘settings.lgt’ file in the Logtalk user folder or in the
user home folder.  Depending on the backend Prolog compiler and on the
operating-system, is also possible to define per-project settings files
by creating a ‘settings.lgt’ file in the project directory and by
starting Logtalk from this directory.  At startup, Logtalk tries to load
a ‘settings.lgt’ file from the following directories, searched in
sequence:

   - Startup directory (‘$LOGTALK_STARTUP_DIRECTORY’)

   - Logtalk user directory (‘$LOGTALKUSER’)

   - User home directory (‘$HOME’; ‘%USERPROFILE%’ on Windows if
     ‘%HOME%’ is not defined)

   - Application data directory (‘%APPDATA%\Logtalk’; only on Windows)

   - Config directory (‘$XDG_CONFIG_HOME/logtalk’)

   - Default config directory (‘$HOME/.config/logtalk/’)

The startup directory is only searched when the read-only
settings_file flag is set to ‘allow’.  When no settings files are
found, Logtalk will use the default compiler flag values set on the
backend Prolog compiler adapter files.  When limitations of the backend
Prolog compiler or on the operating-system prevent Logtalk from finding
the settings files, these can always be loaded manually after Logtalk
startup.

Settings files are normal Logtalk source files (although when
automatically loaded by Logtalk they are compiled and loaded silently
with any errors being reported but otherwise ignored).  The usual
contents is an ‘initialization/1’ Prolog directive containing calls to
the set_logtalk_flag/2. Logtalk built-in predicate and
asserting clauses for the logtalk_library_path/2 multifile
dynamic predicate.  Note that the set_logtalk_flag/2.
directive cannot be used as its scope is local to the source file being
compiled.

One of the troubles of writing portable applications is the different
feature sets of Prolog compilers.  Using the Logtalk support for
conditional compilation and the prolog_dialect flag we can
write a single settings file that can be used with several backend Prolog compilers.:

     :- if(current_logtalk_flag(prolog_dialect, yap)).

         % YAP specific settings
         ...

     :- elif(current_logtalk_flag(prolog_dialect, gnu)).

         % GNU Prolog specific settings
         ...

     :- else.

         % generic Prolog settings

     :- endif.

The Logtalk distribution includes a ‘settings-sample.lgt’ sample file
with commented out code snippets for common settings.

1.14.10 Compiler linter
-----------------------

The compiler includes a linter that checks for a wide range
of possible problems in source files.  Notably, the compiler checks for
unknown entities, unknown predicates, undefined predicates (i.e.
predicates that are declared but not defined), missing directives
(including missing ‘dynamic/1’ and ‘meta_predicate/1’ directives),
redefined built-in predicates, calls to non-portable predicates,
singleton variables, goals that are always true or always false (i.e.
goals that are can be replaced by ‘true’ or ‘fail’), and trivial fails
(i.e.  calls to predicates with no match clauses).  Most of the linter
warnings are controlled by compiler flags.  See the next
section for details.

1.14.11 Compiler flags
----------------------

The logtalk_load/1 and logtalk_compile/1 always
use the current set of default compiler flags as specified in your
settings file and the Logtalk adapter files or changed for the current
session using the built-in predicate set_logtalk_flag/2.
Although the default flag values cover the usual cases, you may want to
use a different set of flag values while compiling or loading some of
your Logtalk source files.  This can be accomplished by using the
logtalk_load/2 or the logtalk_compile/2 built-in
predicates.  These two predicates accept a list of options affecting how
a Logtalk source file is compiled and loaded:

     | ?- logtalk_compile(Files, Options).

or:

     | ?- logtalk_load(Files, Options).

In fact, the ‘logtalk_load/1’ and ‘logtalk_compile/1’ predicates are
just shortcuts to the extended versions called with the default compiler
flag values.  The options are represented by a compound term where the
functor is the flag name and the sole argument is the flag value.

We may also change the default flag values from the ones loaded from the
adapter file by using the set_logtalk_flag/2 built-in
predicate.  For example:

     | ?- set_logtalk_flag(unknown_entities, silent).

The current default flags values can be enumerated using the
current_logtalk_flag/2 built-in predicate:

     | ?- current_logtalk_flag(unknown_entities, Value).

     Value = silent
     yes

Logtalk also implements a set_logtalk_flag/2 directive, which
can be used to set flags within a source file or within an entity.  For
example:

     % compile objects in this source file with event support
     :- set_logtalk_flag(events, allow).

     :- object(foo).

         % compile this object with support
         % for dynamic predicate declarations
         :- set_logtalk_flag(dynamic_declarations, allow).
         ...

     :- end_object.

     ...

Note that the scope of the ‘set_logtalk_flag/2’ directive is local to
the entity or to the source file containing it.

     Note: Applications should never rely on default flag values for
     working properly.  Whenever the compilation of a source file or an
     entity requires a specific flag value, the flag should be set
     explicitly in the entity, in the source file, or in the loader
     file.

1.14.11.1 Read-only flags
.........................

Some flags have read-only values and thus cannot be changed at runtime.
Their values are defined in the Prolog backend adapter files.
These are:

‘settings_file’

     Allows or disables loading of a settings file at
     startup.  Possible values are ‘allow’, ‘restrict’, and ‘deny’.  The
     usual default value is ‘allow’ but it can be changed by editing the
     adapter file when e.g.  embedding Logtalk in a compiled
     application.  With a value of ‘allow’, settings files are searched
     in the startup directory, in the Logtalk user directory, in the
     user home directory, in the ‘APPDATA’ if running on Windows, and in
     the XDG configuration directory.  With a value of ‘restrict’, the
     search for the settings files skips the startup directory.

‘prolog_dialect’

     Identifier of the backend Prolog compiler. (an atom).
     This flag can be used for conditional compilation of
     Prolog compiler specific code.

‘prolog_version’

     Version of the backend Prolog compiler. (a compound term,
     ‘v(Major, Minor, Patch)’, whose arguments are integers).  This flag
     availability depends on the Prolog compiler.  Checking the value of
     this flag fails for any Prolog compiler that does not provide
     access to version data.

‘prolog_compatible_version’

     Compatible version of the backend Prolog compiler. (a
     compound term, usually with the format ‘@>=(v(Major, Minor,
     Patch))’, whose arguments are integers).  This flag availability
     depends on the Prolog compiler.  Checking the value of this flag
     fails for any Prolog compiler that does not provide access to
     version data.

‘unicode’

     Informs Logtalk if the backend Prolog compiler supports
     the Unicode standard.  Possible flag values are ‘unsupported’,
     ‘full’ (all Unicode planes supported), and ‘bmp’ (supports only the
     Basic Multilingual Plane).

‘encoding_directive’

     Informs Logtalk if the backend Prolog compiler supports
     the encoding/1 directive.  This directive is used for
     declaring the text encoding of source files.  Possible flag values
     are ‘unsupported’, ‘full’ (can be used in both Logtalk source files
     and compiler generated Prolog files), and ‘source’ (can be used
     only in Logtalk source files).

‘tabling’

     Informs Logtalk if the backend Prolog compiler provides
     tabling programming support.  Possible flag values are
     ‘unsupported’ and ‘supported’.

‘engines’

     Informs if the backend Prolog compiler provides the
     required low level multi-threading programming support for Logtalk
     threaded engines.  Possible flag values are
     ‘unsupported’ and ‘supported’.

‘threads’

     Informs if the backend Prolog compiler provides the
     required low level multi-threading programming support for all
     high-level Logtalk multi-threading features.  Possible
     flag values are ‘unsupported’ and ‘supported’.

‘modules’

     Informs Logtalk if the backend Prolog compiler provides
     suitable module support.  Possible flag values are ‘unsupported’
     and ‘supported’ (independently of this flag, Logtalk provides
     limited support for compiling Prolog modules as objects).

‘coinduction’

     Informs Logtalk if the backend Prolog compiler provides
     the required minimal support for cyclic terms necessary for working
     with coinductive predicates.  Possible flag values are
     ‘unsupported’ and ‘supported’.

1.14.11.2 Version flags
.......................

‘version_data(Value)’

     Read-only flag whose value is the compound term
     ‘logtalk(Major,Minor,Patch,Status)’.  The first three arguments are
     integers and the last argument is an atom, possibly empty,
     representing version status: ‘aN’ for alpha versions, ‘bN’ for beta
     versions, ‘rcN’ for release candidates (with ‘N’ being a natural
     number), and ‘stable’ for stable versions.  The ‘version_data’ flag
     is also a de facto standard for Prolog compilers.

1.14.11.3 Lint flags
....................

‘unknown_entities(Option)’

     Controls the unknown entity warnings, resulting from loading an
     entity that references some other entity that is not currently
     loaded.  Possible option values are ‘warning’ (the usual default)
     and ‘silent’.  Note that these warnings are not always avoidable,
     specially when using reflective designs of class-based hierarchies.

‘unknown_predicates(Option)’

     Defines the compiler behavior when unknown messages or calls to
     unknown predicates (or non-terminals) are found.  An unknown
     message is a message sent to an object that is not part of the
     object protocol.  An unknown predicate is a called predicate that
     is neither locally declared or defined.  Possible option values are
     ‘error’, ‘warning’ (the usual default), and ‘silent’ (not
     recommended).

‘undefined_predicates(Option)’

     Defines the compiler behavior when calls to declared but undefined
     predicates (or non-terminals) are found.  Note that these calls
     will fail at runtime as per closed-world assumption.  Possible
     option values are ‘error’, ‘warning’ (the usual default), and
     ‘silent’ (not recommended).

‘steadfastness(Option)’

     Controls warnings about 'possible' non steadfast.
     predicate definitions due to variable aliasing at a clause head and
     a cut in the clause body.  Possible option values are ‘warning’ and
     ‘silent’ (the usual default due to the possibility of false
     positives).

‘portability(Option)’

     Controls the non-ISO specified Prolog built-in predicate and
     non-ISO specified Prolog built-in arithmetic function calls
     warnings plus use of non-standard Prolog flags and/or flag values.
     Possible option values are ‘warning’ and ‘silent’ (the usual
     default).

‘deprecated(Option)’

     Controls the deprecated predicate warnings.  Possible option values
     are ‘warning’ (the usual default) and ‘silent’.

‘missing_directives(Option)’

     Controls the missing predicate directive warnings.  Possible option
     values are ‘warning’ (the usual default) and ‘silent’ (not
     recommended).

‘duplicated_directives(Option)’

     Controls the duplicated predicate directive warnings.  Possible
     option values are ‘warning’ (the usual default) and ‘silent’ (not
     recommended).  Note that conflicting directives for the same
     predicate are handled as errors, not as duplicated directive
     warnings.

‘trivial_goal_fails(Option)’

     Controls the printing of warnings for calls to local static
     predicates with no matching clauses.  Possible option values are
     ‘warning’ (the usual default) and ‘silent’ (not recommended).

‘always_true_or_false_goals(Option)’

     Controls the printing of warnings for goals that are always true or
     false.  Possible option values are ‘warning’ (the usual default)
     and ‘silent’ (not recommended).

‘grammar_rules(Option)’

     Controls the printing of grammar rules related warnings.  Possible
     option values are ‘warning’ (the usual default) and ‘silent’ (not
     recommended).

‘arithmetic_expressions(Option)’

     Controls the printing of arithmetic expressions related warnings.
     Possible option values are ‘warning’ (the usual default) and
     ‘silent’ (not recommended).

‘lambda_variables(Option)’

     Controls the printing of lambda variable related warnings.
     Possible option values are ‘warning’ (the usual default) and
     ‘silent’ (not recommended).

‘suspicious_calls(Option)’

     Controls the printing of suspicious call warnings.  Possible option
     values are ‘warning’ (the usual default) and ‘silent’ (not
     recommended).

‘redefined_built_ins(Option)’

     Controls the Logtalk and Prolog built-in predicate redefinition
     warnings.  Possible option values are ‘warning’ and ‘silent’ (the
     usual default).  Warnings about redefined Prolog built-in
     predicates are often the result of running a Logtalk application on
     several Prolog compilers as each Prolog compiler defines its set of
     built-in predicates.

‘redefined_operators(Option)’

     Controls the Logtalk and Prolog built-in operator redefinition
     warnings.  Possible option values are ‘warning’ (the usual default)
     and ‘silent’.  Redefining Logtalk operators or standard Prolog
     operators can break term parsing causing syntax errors or change
     how terms are parsed introducing bugs.

‘singleton_variables(Option)’

     Controls the singleton variable warnings.  Possible option values
     are ‘warning’ (the usual default) and ‘silent’ (not recommended).

‘underscore_variables(Option)’

     Controls the interpretation of variables that start with an
     underscore (excluding the anonymous variable) that occur once in a
     term as either don’t care variables or singleton variables.
     Possible option values are ‘dont_care’ and ‘singletons’ (the usual
     default).  Note that, depending on your Prolog compiler, the
     ‘read_term/3’ built-in predicate may report variables that start
     with an underscore as singleton variables.  There is no standard
     behavior, hence this option.

‘naming(Option)’

     Controls warnings about entity, predicate, and variable names per
     official coding guidelines (which advise using underscores for
     entity and predicate names and camel case for variable names).
     Additionally, variable names should not differ only on case.
     Possible option values are ‘warning’ and ‘silent’ (the usual
     default due to the current limitation to ASCII names and the
     computational cost of the checks).

‘duplicated_clauses(Option)’

     Controls warnings of duplicated entity clauses (and duplicated
     entity grammar rules).  Possible option values are ‘warning’ and
     ‘silent’ (the usual default due to the required heavy
     computations).  When the term-expansion mechanism is used and
     results in duplicated clauses, the reported line numbers are for
     lines of the original clauses that were expanded.

‘disjunctions(Option)’

     Controls warnings on clauses where the body is a disjunction.
     Possible option values are ‘warning’ (the usual default) and
     ‘silent’.  As per coding guidelines, in most cases, these clauses
     can be rewritten using a clause per disjunction branch for improved
     code readability.

‘conditionals(Option)’

     Controls warnings on if-then-else and soft-cut control constructs.
     Possible option values are ‘warning’ (the usual default) and
     ‘silent’.  Warnings include misuse of cuts, potential bugs in the
     test part, and missing else part (lack of compliance with coding
     guidelines).

‘catchall_catch(Option)’

     Controls warnings on ‘catch/3’ goals that catch all exceptions.
     Possible option values are ‘warning’ and ‘silent’ (the usual
     default).  Lack of standardization often makes it tricky or
     cumbersome to avoid too generic ‘catch/3’ goals when writing
     portable code.

‘left_recursion(Option)’

     Controls warnings of left-recursion on clauses and grammar rules.
     Specifically, when the clause or grammar rule head and the left
     most goal in the body are variants.  Possible option values are
     ‘warning’ (the usual default) and ‘silent’.

‘tail_recursive(Option)’

     Controls warnings of non-tail recursive predicate (and
     non-terminal) definitions.  The lint check does not detect all
     cases of non-tail recursive predicate definitions, however.  Also,
     definitions that make two or more recursive calls are not reported
     as usually they cannot be changed to be tail recursive.  Possible
     option values are ‘warning’ and ‘silent’ (the usual default).

1.14.11.4 Optional features compilation flags
.............................................

‘complements(Option)’

     Allows objects to be compiled with support for complementing
     categories turned off in order to improve performance and security.
     Possible option values are ‘allow’ (allow complementing categories
     to override local object predicate declarations and definitions),
     ‘restrict’ (allow complementing categories to add predicate
     declarations and definitions to an object but not to override
     them), and ‘deny’ (ignore complementing categories; the usual
     default).  This option can be used on a per-object basis.  Note
     that changing this option is of no consequence for objects already
     compiled and loaded.

‘dynamic_declarations(Option)’

     Allows objects to be compiled with support for dynamic declaration
     of new predicates turned off in order to improve performance and
     security.  Possible option values are ‘allow’ and ‘deny’ (the usual
     default).  This option can be used on a per-object basis.  Note
     that changing this option is of no consequence for objects already
     compiled and loaded.  This option is only checked when sending an
     asserta/1 or assertz/1 message to an
     object.  Local asserting of new predicates is always allowed.

‘events(Option)’

     Allows message sending calls to be compiled with or without
     event-driven programming support.  Possible option values are
     ‘allow’ and ‘deny’ (the usual default).  Objects (and categories)
     compiled with this option set to ‘deny’ use optimized code for
     message-sending calls that does not trigger events.  As such, this
     option can be used on a per-object (or per-category) basis.  Note
     that changing this option is of no consequence for objects already
     compiled and loaded.

‘context_switching_calls(Option)’

     Allows context switching calls (‘(<<)/2’) to be either allowed or
     denied.  Possible option values are ‘allow’ and ‘deny’.  The
     default flag vale is ‘allow’.  Note that changing this option is of
     no consequence for objects already compiled and loaded.

1.14.11.5 Backend Prolog compiler and loader flags
..................................................

‘prolog_compiler(Flags)’

     List of compiler flags for the generated Prolog files.  The valid
     flags are specific to the used Prolog backend compiler.  The usual
     default is the empty list.  These flags are passed to the backend
     Prolog compiler built-in predicate that is responsible for
     compiling to disk a Prolog file.  For Prolog compilers that don’t
     provide separate predicates for compiling and loading a file, use
     instead the prolog_loader flag.

‘prolog_loader(Flags)’

     List of loader flags for the generated Prolog files.  The valid
     flags are specific to the used Prolog backend compiler.  The usual
     default is the empty list.  These flags are passed to the backend
     Prolog compiler built-in predicate that is responsible for loading
     a (compiled) Prolog file.

1.14.11.6 Other flags
.....................

‘scratch_directory(Directory)’

     Sets the directory to be used to store the temporary files
     generated when compiling Logtalk source files.  This directory can
     be specified using an atom or using library notation.
     The directory must always end with a slash.  The default value is a
     sub-directory of the source files directory, either ‘'./lgt_tmp/'’
     or ‘'./.lgt_tmp/'’ (depending on the backend Prolog compiler and
     operating-system).  Relative directories must always start with
     ‘'./'’ due to the lack of a portable solution to check if a path is
     relative or absolute.  The default value set on the backend Prolog compiler
     adapter file can be overriden by defining the
     ‘scratch_directory’ library alias (see the
     logtalk_library_path/2 predicate documentation for details).

‘report(Option)’

     Controls the default printing of messages.  Possible option values
     are ‘on’ (by usual default, print all messages that are not
     intercepted by the user), ‘warnings’ (only print warning and error
     messages that are not intercepted by the user), and ‘off’ (do not
     print any messages that are not intercepted by the user).

‘code_prefix(Character)’

     Enables the definition of prefix for all functors of Prolog code
     generated by the Logtalk compiler.  The option value must be a
     single character atom.  Its default value is ‘'$'’.  Specifying a
     code prefix provides a way to solve possible conflicts between
     Logtalk compiled code and other Prolog code.  In addition, some
     Prolog compilers automatically hide predicates whose functor start
     with a specific prefix such as the character ‘$’.  Although this is
     not a read-only flag, it should only be changed at startup time and
     'before' loading any source files.  When changing this flag (e.g.
     from a settings file), restart with the clean
     flag turned on to ensure that any compiled files using the old
     ‘code_prefix’ value will be recompiled.

‘optimize(Option)’

     Controls the compiler optimizations.  Possible option values are
     ‘on’ (used by default for deployment) and ‘off’ (used by default
     for development).  Compiler optimizations include the use of static
     binding whenever possible, the removal of redundant calls to
     ‘true/0’ from predicate clauses, the removal of redundant
     unifications when compiling grammar rules, and inlining of
     predicate definitions with a single clause that links to a local
     predicate, to a plain Prolog built-in (or foreign) predicate, or to
     a Prolog module predicate with the same arguments.  Care should be
     taken when developing applications with this flag turned on as
     changing and reloading a file may render static binding.
     optimizations invalid for code defining in other loaded files.
     Turning on this flag automatically turns off the debug.
     flag.

‘source_data(Option)’

     Defines how much information is retained when compiling a source
     file.  Possible option values are ‘on’ (the usual default for
     development) and ‘off’.  With this flag set to ‘on’, Logtalk will
     keep the information represented using documenting directives plus
     source location data (including source file names and line
     numbers).  This information can be retrieved using the
     reflection API and is useful for documenting, debugging, and
     integration with third-party development tools.  This flag can be
     turned off in order to generate more compact code.

‘debug(Option)’

     Controls the compilation of source files in debug mode (the Logtalk
     default debugger can only be used with files compiled in this
     mode).  Also controls, by default, printing of ‘debug>’ and
     ‘debug(Topic)’ messages.  Possible option values are ‘on’ and ‘off’
     (the usual default).  Turning on this flag automatically turns off
     the optimize flag.

‘reload(Option)’

     Defines the reloading behavior for source files.  Possible option
     values are ‘skip’ (skip reloading of already loaded files; this
     value can be used to get similar functionality to the Prolog
     directive ‘ensure_loaded/1’ but should be used only with fully
     debugged code), ‘changed’ (the usual default; reload files only
     when they are changed since last loaded provided that any explicit
     flags and the compilation mode are the same as before), and
     ‘always’ (always reload files).

‘relative_to(Directory)’

     Defines a base directory for resolving relative source file paths.
     The default value is the directory of the source file being
     compiled.

‘hook(Object)’

     Allows the definition of an object (which can be the pseudo-object
     user(1)) implementing the expanding(2) built-in protocol.  The hook
     object must be compiled and loaded when this option is used.  It’s
     also possible to specify a Prolog module instead of a Logtalk
     object but the module must be pre-loaded and its identifier must be
     different from any object identifier.

‘clean(Option)’

     Controls cleaning of the intermediate Prolog files generated when
     compiling Logtalk source files.  Possible option values are ‘off’
     and ‘on’ (the usual default).  When turned on, intermediate files
     are deleted after loading and all source files are recompiled
     disregarding any existing intermediate files.  When turned off, the
     intermediate files are kept.  This is useful when embedding
     applications, which requires collecting the intermediate code, and
     when working on large applications to avoid repeated recompilation
     of stable code.  The flag must be turned on when changing
     compilation modes, changing flags such as code_prefix,
     or when turning on linter flags that are off by default without at
     the same time making changes to the application source files
     themselves as any existing intermediate files would not be
     recompiled as necessary due to file timestamps not changing.

   ---------- Footnotes ----------

   (1) ../../docs/user_0.html#user-0

   (2) ../../docs/expanding_0.html#expanding-0

1.14.11.7 User-defined flags
............................

Logtalk provides a create_logtalk_flag/3 predicate that can
be used for defining new flags.

1.14.12 Reloading source files
------------------------------

As a general rule, reloading source files should never occur in
production code and should be handled with care in development code.
Reloading a Logtalk source file usually requires reloading the
intermediate Prolog file that is generated by the Logtalk compiler.  The
problem is that there is no standard behavior for reloading Prolog
files.  For static predicates, almost all Prolog compilers replace the
old definitions with the new ones.  However, for dynamic predicates, the
behavior depends on the Prolog compiler.  Most compilers replace the old
definitions but some of them simply append the new ones, which usually
leads to trouble.  See the compatibility notes for the backend Prolog
compiler you intend to use for more information.  There is an additional
potential problem when using multi-threading programming.  Reloading a
threaded object does not recreate from scratch its old message queue,
which may still be in use (e.g.  threads may be waiting on it).

When using library entities and stable code, you can avoid reloading the
corresponding source files (and, therefore, recompiling them) by setting
the reload compiler flag to ‘skip’.  For code under
development, you can turn off the clean flag to avoid
recompiling files that have not been modified since last compilation
(assuming that backend Prolog compiler that you are using supports
retrieving of file modification dates).  You can disable deleting the
intermediate files generated when compiling source files by changing the
default flag value in your settings file, by using the corresponding
compiler flag with the compiling and loading built-in predicates, or,
for the remaining of a working session, by using the call:

     | ?- set_logtalk_flag(clean, off).

Some caveats that you should be aware.  First, some warnings that might
be produced when compiling a source file will not show up if the
corresponding object file is up-to-date because the source file is not
being (re)compiled.  Second, if you are using several Prolog compilers
with Logtalk, be sure to perform the first compilation of your source
files with the ‘clean’ flag turned off: the intermediate Prolog files
generated by the Logtalk compiler may be not compatible across Prolog
compilers or even for the same Prolog compiler across operating systems
(e.g.  due to the use of different character encodings or end-of-line
characters).

1.14.13 Batch processing
------------------------

When doing batch processing, you probably want to turn off the
report flag to suppress all messages of type ‘banner’, ‘comment’,
‘comment(_)’, ‘warning’, and ‘warning(_)’ that are normally printed.
Note that error messages and messages providing information requested by
the user will still be printed.

1.14.14 Optimizing performance
------------------------------

The default compiler flag settings are appropriated for the
'development' but not necessarily for the 'deployment' of applications.
To minimize the generated code size, turn the source_data.
flag off.  To optimize runtime performance, turn on the optimize
flag.  Your chosen backend Prolog compiler may also provide
performance related flags; check its documentation.

Pay special attention to file compilation/loading order.  Whenever
possible, compile and load your files taking into account file
dependencies.  By default, the compiler will print a warning whenever a
file references an entity that is not yet loaded.  Solving these
warnings is key for optimal performance by enabling static binding
optimizations.  For a clear picture of file dependencies,
use the diagrams tool to generate a file dependency diagram
for your application.

Minimize the use of dynamic predicates.  Parametric objects can often be
used in alternative.  When dynamic predicates cannot be avoided, try to
make them private.  Declaring a dynamic predicate also as a private
predicate allows the compiler to optimize local calls to the database
methods (e.g.  see assertz/1 and retract/1) that
modify the predicate.

Sending a message to self implies dynamic binding.
but there are often cases where (;;)/1 is misused to call an
imported or inherited predicate that is never going to be redefined in a
descendant.  In these cases, a super call, (^^)/1,
can be used instead with the benefit of often enabling static binding.
Most of the guidelines for writing efficient Prolog code also apply to
Logtalk code.  In particular, define your predicates to take advantage
of first-argument indexing.  In the case of recursive predicates, define
them as tail-recursive predicates whenever possible.

See the section on performance for a detailed discussion on
Logtalk performance.

1.14.15 Portable applications
-----------------------------

Logtalk is compatible with most modern standards compliant Prolog
compilers.  However, this does not necessarily imply that your Logtalk
applications will have the same level of portability.  If possible, you
should only use in your applications Logtalk built-in predicates and ISO
Prolog specified built-in predicates and arithmetic functions.  If you
need to use built-in predicates (or built-in arithmetic functions) that
may not be available in other Prolog compilers, you should try to
encapsulate the non-portable code in a small number of objects and
provide a portable 'interface' for that code through the use of Logtalk
protocols.  An example will be code that access operating-system
specific features.  The Logtalk compiler can warn you of the use of
non-ISO specified built-in predicates and arithmetic functions by using
the portability compiler flag.

1.14.16 Conditional compilation
-------------------------------

Logtalk supports conditional compilation within source files using the
if/1, elif/1, else/0, and
endif/0 directives.  This support is similar to the support found
in several Prolog systems such as ECLiPSe, GNU Prolog, SICStus Prolog,
SWI-Prolog, XSB, and YAP.

1.14.17 Avoiding common errors
------------------------------

Try to write objects and protocol documentation 'before' writing any
other code; if you are having trouble documenting a predicate perhaps we
need to go back to the design stage.

Try to avoid lengthy hierarchies.  Composition is often a better choice
over inheritance for defining new objects (Logtalk supports
component-based programming through the use of categories).
In addition, prototype-based hierarchies are semantically simpler than
class-based hierarchies.

Dynamic predicates or dynamic entities are sometimes needed, but we
should always try to minimize the use of non-logical features such as
asserts and retracts.

Since each Logtalk entity is independently compiled, if an object
inherits a dynamic or a meta-predicate predicate, then the respective
directives must be repeated to ensure a correct compilation.

In general, Logtalk does not verify if a user predicate call/return
arguments comply with the declared modes.  On the other hand, Logtalk
built-in predicates, built-in methods, and message sending control
structures are fully checked for calling mode errors.

Logtalk error handling strongly depends on the ISO compliance of the
chosen Prolog compiler.  For instance, the error terms that are
generated by some Logtalk built-in predicates assume that the Prolog
built-in predicates behave as defined in the ISO standard regarding
error conditions.  In particular, if your Prolog compiler does not
support a ‘read_term/3’ built-in predicate compliant with the ISO Prolog
Standard definition, then the current version of the Logtalk compiler
may not be able to detect misspell variables in your source code.

1.14.18 Coding style guidelines
-------------------------------

It is suggested that all code between an entity opening and closing
directives be indented by one tab stop.  When defining entity code, both
directives and predicates, Prolog coding style guidelines may be
applied.  All Logtalk source files, examples, and standard library
entities use tabs (the recommended setting is a tab width equivalent to
4 spaces) for laying out code.  Closed related entities can be defined
in the same source file.  However, for best performance, is often
necessary to have an entity per source file.  Entities that might be
useful in different contexts (such as library entities) are best defined
in their own source files.

A detailed coding style guide is available at the Logtalk official
website.

1.15 Printing messages and asking questions
===========================================

Applications, components, and libraries often print all sorts of
messages.  These include banners, logging, debugging, and computation
results messages but also, in some cases, user interaction messages.
However, the authors of applications, components, and libraries often
cannot anticipate the context where their software will be used and thus
decide which and when messages should be displayed, suppressed, or
diverted.  Consider the different components in a Logtalk application
development and deployment.  At the base level, you have the Logtalk
compiler and runtime.  The compiler writes messages related to e.g.
compiling and loading files, compiling entities, compilation warnings
and errors.  The runtime may write banner messages or throw execution
errors that may result in printing human-level messages.  The
development environment can be console-based or you may be using a GUI
tool such as PDT. In the latter case, PDT needs to intercept the Logtalk
compiler and runtime messages to present the relevant information using
its GUI. Then you have all the other components in a typical
application.  For example, your own libraries and third-party libraries.
The libraries may want to print messages on its own, e.g.  banners,
debugging information, or logging information.  As you assemble all your
application components, you want to have the final word on which
messages are printed, where, and when.  Uncontrolled message printing by
libraries could potentially disturb application flow, expose
implementation details, spam the user with irrelevant details, or break
user interfaces.

The solution is to decouple the calls to print a message from the actual
printing of the output text.  The same is true for calls to read user
input.  By decoupling the call to input some data from the actual read
of the data, we can easily switched e.g.  from a command-line interface
to a GUI input dialog or even automate providing the data (e.g.  when
automating testing of user interaction).

Logtalk provides a solution based on the 'structured message printing
mechanism' that was introduced by Quintus Prolog, where it was
apparently implemented by Dave Bowen (thanks to Richard O’Keefe for the
historical bits).  This mechanism gives the programmer full control of
message printing, allowing it to filter, rewrite, or redirect any
message.  Variations of this mechanism can also be found in some Prolog
systems including SICStus Prolog, SWI-Prolog, and YAP. Based on this
mechanism, Logtalk introduces an extension that also allows abstracting
asking a user for input.  Both mechanisms are implemented by the
logtalk(1) built-in object and described in this section.  The message
printing mechanism is extensively used by the Logtalk compiler itself
and by the developer tools.  The question asking mechanism is used e.g.
in the debugger tool.

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0

1.15.1 Printing messages
------------------------

The main predicate for printing a message is
logtalk;;print_message/3.  A simple example, using the Logtalk
runtime is:

     | ?- logtalk::print_message(banner, core, banner).

     Logtalk 3.23.0
     Copyright (c) 1998-2018 Paulo Moura
     yes

The first argument of the predicate is the kind of message that we want
to print.  In this case, we use ‘banner’ to indicate that we are
printing a product name and copyright banner.  An extensive list of
message kinds is supported by default:

‘banner’

     banner messages (used e.g.  when loading tools or main application
     components; can be suppressed by setting the report.
     flag to ‘warnings’ or ‘off’)

‘help’

     messages printed in reply for the user asking for help (mostly for
     helping port existing Prolog code)

‘information’ and ‘information(Group)’

     messages usually printed in reply to a user request for information

‘silent’ and ‘silent(Group)’

     not printed by default (but can be intercepted using the
     ‘message_hook/4’ predicate)

‘comment’ and ‘comment(Group)’

     useful but usually not essential messages (can be suppressed by
     setting the report flag to ‘warnings’ or ‘off’)

‘warning’ and ‘warning(Group)’

     warning messages (generated e.g.  by the compiler; can be
     suppressed by turning off the report flag)

‘error’ and ‘error(Group)’

     error messages (generated e.g.  by the compiler)

‘debug, debug(Group)’

     debugging messages (by default, only printed when the debug
     flag is turned on; the ‘print_message/3’ goals for these
     messages are suppressed by the compiler when the optimize
     flag is turned on)

‘question, question(Group)’

     questions to a user

Using a compound term allows easy partitioning of messages of the same
kind in different groups.  Note that you can define your own alternative
message kind identifiers, for your own components, together with
suitable definitions for their associated prefixes and output streams.

The second argument of ‘print_message/3’ represents the 'component'
defining the message being printed.  Here 'component' is a generic term
that can designate e.g a tool, a library, or some sub-system in a large
application.  In our example, the component name is ‘core’, identifying
the Logtalk compiler/runtime.  This argument was introduced to provide
multiple namespaces for message terms and thus simplify
programming-in-the-large by allowing easy filtering of all messages from
a specific component and also avoiding conflicts when two components
happen to define the same message term (e.g.  ‘banner’).  Users should
choose and use a unique name for a component, which usually is the name
of the component itself.  For example, all messages from the
lgtunit tool use ‘lgtunit’ for the component argument.  The
compiler and runtime are interpreted as a single component designated as
‘core’.

The third argument of ‘print_message/3’ is the message itself,
represented by a term.  In the above example, the message term is
‘banner’.  Using a term to represent a message instead of a string with
the message text itself have significant advantages.  Notably, it allows
using a compound term for easy parameterization of the message text and
simplifies machine-processing, localization of applications, and message
interception.  For example:

     | ?- logtalk::print_message(comment, core, redefining_entity(object, foo)).

     % Redefining object foo
     yes

1.15.2 Message tokenization
---------------------------

The advantages of using message terms require a solution for generating
the actual messages text.  This is supported by defining grammar rules
for the logtalk;;message_tokens//2 multifile non-terminal,
which translates a message term, for a given component, to a list of
tokens.  For example:

     :- multifile(logtalk::message_tokens//2).
     :- dynamic(logtalk::message_tokens//2).

     logtalk::message_tokens(redefining_entity(Type, Entity), core) -->
         ['Redefining ~w ~q'-[Type, Entity], nl].

The following tokens can be used when translating a message:

‘at_same_line’

     Signals a following part to a multi-part message with no line break
     in between; this token is ignored when it’s not the first in the
     list of tokens

‘tab(Expression)’

     Evaluate the argument as an arithmetic expression and write the
     resulting number of spaces; this token is ignored when the number
     of spaces is not positive

‘nl’

     Change line in the output stream

‘flush’

     Flush the output stream (by calling the ‘flush_output/1’ standard
     predicate)

‘Format-Arguments’

     ‘Format’ must be an atom and ‘Arguments’ must be a list of format
     arguments (the token arguments are passed to a call to the
     ‘format/3’ de facto standard predicate)

‘term(Term, Options)’

     ‘Term’ can be any term and ‘Options’ must be a list of valid
     ‘write_term/3’ output options (the token arguments are passed to a
     call to the ‘write_term/3’ standard predicate)

‘ansi(Attributes, Format, Arguments)’

     Taken from SWI-Prolog; by default, do nothing; can be used for
     styled output

‘begin(Kind, Var)’

     Taken from SWI-Prolog; by default, do nothing; can be used together
     with ‘end(Var)’ to wrap a sequence of message tokens

‘end(Var)’

     Taken from SWI-Prolog; by default, do nothing

The ‘logtalk’ object also defines public predicates for printing a list
of tokens, for hooking into printing an individual token, and for
setting default output stream and message prefixes.  For example, the
SWI-Prolog adapter file uses the print message token hook predicate to
enable coloring of messages printed on a console.

1.15.3 Meta-messages
--------------------

Defining tokenization rules for every message is not always necessary,
however.  Logtalk defines several 'meta-messages' that are handy for
simple cases and temporary messages only used to help developing,
notably debugging messages.  See the Debugging messages.
section and the logtalk built-in object(1) remarks section for details.

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0

1.15.4 Intercepting messages
----------------------------

Calls to the logtalk;;print_message/3 predicate can be
intercepted by defining clauses for the logtalk;;message_hook/4
multifile hook predicate.  This predicate can suppress, rewrite,
and divert messages.

As a first example, assume that you want to make Logtalk startup less
verbose by suppressing printing of the default compiler flag values.
This can be easily accomplished by defining the following category in a
settings file:

     :- category(my_terse_logtalk_startup_settings).

         :- multifile(logtalk::message_hook/4).
         :- dynamic(logtalk::message_hook/4).

         logtalk::message_hook(default_flags, comment(settings), core, _).

     :- end_category.

The printing message mechanism automatically calls the ‘message_hook/4’
hook predicate.  When this call succeeds, the mechanism assumes that the
message have been successfully handled.

As another example, assume that you want to print all otherwise silent
compiler messages:

     :- category(my_verbose_logtalk_message_settings).

         :- multifile(logtalk::message_hook/4).
         :- dynamic(logtalk::message_hook/4).

         logtalk::message_hook(_Message, silent, core, Tokens) :-
             logtalk::message_prefix_stream(comment, core, Prefix, Stream),
             logtalk::print_message_tokens(Stream, Prefix, Tokens).

         logtalk::message_hook(_Message, silent(Key), core, Tokens) :-
             logtalk::message_prefix_stream(comment(Key), core, Prefix, Stream),
             logtalk::print_message_tokens(Stream, Prefix, Tokens).

     :- end_category.

This example calls the logtalk;;message_prefix_stream/4 hook
predicate, which can be used to define a message line prefix and an
output stream for printing messages for a given component.

1.15.5 Asking questions
-----------------------

Logtalk 'structured question asking' mechanism complements the message
printing mechanism.  It provides an abstraction for the common task of
asking a user a question and reading back its reply.  By default, this
mechanism writes the question, writes a prompt, and reads the answer
using the current user input and output streams but allows all steps to
be intercepted, filtered, rewritten, and redirected.  Two typical
examples are using a GUI dialog for asking questions and automatically
providing answers to specific questions.

The question asking mechanism works in tandem with the message printing
mechanism, using it to print the question text and a prompt.  It
provides an asking predicate and a hook predicate, both declared and
defined in the ‘logtalk’ built-in object.  The asking predicate, see
logtalk;;ask_question/5, is used for ask a question and read the
answer.  Assume that we defined the following message tokenization and
question prompt and stream:

     :- category(hitchhikers_guide_to_the_galaxy).

         :- multifile(logtalk::message_tokens//2).
         :- dynamic(logtalk::message_tokens//2).

         % abstract the question text using the atom ultimate_question;
         % the second argument, hitchhikers, is the application component
         logtalk::message_tokens(ultimate_question, hitchhikers) -->
             ['The answer to the ultimate question of life, the universe and everything is?'-[], nl].

        :- multifile(logtalk::question_prompt_stream/4).
        :- dynamic(logtalk::question_prompt_stream/4).

        % the prompt is specified here instead of being part of the question text
        % as it will be repeated if the answer doesn't satisfy the question closure
        logtalk::question_prompt_stream(question, hitchhikers, '> ', user_input).

     :- end_category.

After compiling and loading this category, we can now ask the ultimate
question:

     | ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N).

     The answer to the ultimate question of life, the universe and everything is?
     > 42.

     N = 42
     yes

Note that the fourth argument, ‘'=='(42)’ in our example, is a
closure that is used to check the answers provided by the user.
The question is repeated until the goal constructed by extending the
closure with the user answer succeeds.  For example:

     | ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N).
     The answer to the ultimate question of life, the universe and everything is?
     > icecream.
     > tea.
     > 42.

     N = 42
     yes

Practical usage examples of this mechanism can be found e.g.  in the
‘debugger’ tool where it’s used to abstract the user interaction when
tracing a goal execution in debug mode.

1.15.6 Intercepting questions
-----------------------------

Calls to the logtalk;;ask_question/5 predicate can be
intercepted by defining clauses for the logtalk;;question_hook/6
multifile hook predicate.  This predicate can suppress, rewrite,
and divert questions.  For example, assume that we want to automate
testing and thus cannot rely on someone manually providing answers:

     :- category(hitchhikers_fixed_answers).

         :- multifile(logtalk::question_hook/6).
         :- dynamic(logtalk::question_hook/6).

         logtalk::question_hook(ultimate_question, question, hitchhikers, _, _, 42).

     :- end_category.

After compiling and loading this category, trying the question again
will now skip asking the user:

     | ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N).

     N = 42
     yes

In a practical case, the fixed answer would be used for followup goals
being tested.  The question answer read loop (which calls the question
check closure) is not used when a fixed answer is provided using the
‘logtalk::question_hook/6’ predicate thus preventing the creation of
endless loops.  For example, the following query succeeds:

     | ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(41), N).

     N = 42
     yes

Note that the ‘logtalk::question_hook/6’ predicate takes as argument the
closure specified in the ‘logtalk::ask_question/5’ call, allowing a
fixed answer to be checked before being returned.

1.16 Term and goal expansion
============================

Logtalk supports a 'term and goal expansion mechanism' that can be used
to define source-to-source transformations.  Two common uses are the
definition of language extensions and domain-specific languages.

Logtalk improves upon the term-expansion mechanism found on some Prolog
systems by providing the user with fine-grained control on 'if', 'when',
and 'how' expansions are applied.  It allows declaring in a source file
itself which expansions, if any, will be used when compiling it.  It
allows declaring which expansions will be used when compiling a file
using compile and loading predicate options.  It also allows defining a
default expansion for all source files.  It defines a concept of 'hook
objects' that can be used as building blocks to create custom and
reusable 'expansion workflows' with explicit and well defined semantics.
It prevents the simply act of loading expansion rules affecting
subsequent compilation of files.  It prevents conflicts between groups
of expansion rules of different origins.  It avoids a set of buggy
expansion rules from breaking other sets of expansions rules.

1.16.1 Defining expansions
--------------------------

Term and goal expansions are defined using, respectively, the predicates
term_expansion/2 and goal_expansion/2, which are
declared in the expanding(1) built-in protocol.  Note that, unlike
Prolog systems also providing these two predicates, they are 'not'
declared as multifile predicates in the protocol.  This
design decision is key to give the programmer full control of the
expansion process and prevent the problems that inflict most Prolog
system providing a term-expansion mechanism.

An example of an object defining expansion rules:

     :- object(an_object,
         implements(expanding)).

         term_expansion(ping, pong).
         term_expansion(
             colors,
             [white, yellow, blue, green, read, black]
         ).

         goal_expansion(a, b).
         goal_expansion(b, c).
         goal_expansion(X is Expression, true) :-
             catch(X is Expression, _, fail).

     :- end_object.

These predicates can be explicitly called using the expand_term/2
and expand_goal/2 built-in methods or called
automatically by the compiler when compiling a source file (see the
section below on 'hook objects').

In the case of source files referenced in include/1.
directives, expansions are only applied automatically when the
directives are found in source files, not when used as arguments in the
create_object/4, create_protocol/3, and
create_category/4, predicates.  This restriction prevents
inconsistent results when, for example, an expansion is defined for a
predicate with clauses found in both an included file and as argument in
a call to the ‘create_object/4’ predicate.

Clauses for the ‘term_expansion/2’ predicate are called until of them
succeeds.  The returned expansion can be a single term or a list of
terms (including the empty list).  For example:

     | ?- an_object::expand_term(ping, Term).

     Term = pong
     yes

     | ?- an_object::expand_term(colors, Colors).

     Colors = [white, yellow, blue, green, read, black]
     yes

When no ‘term_expansion/2’ clause applies, the same term that we are
trying to expand is returned:

     | ?- an_object::expand_term(sounds, Sounds).

     Sounds = sounds
     yes

Clauses for the ‘goal_expansion/2’ predicate are recursively called on
the expanded goal until a fixed point is reached.  For example:

     | ?- an_object::expand_goal(a, Goal).

     Goal = c
     yes

     | ?- an_object::expand_goal(X is 3+2*5, Goal).

     X = 13,
     Goal = true
     yes

When no ‘goal_expansion/2’ clause applies, the same goal that we are
trying to expand is returned:

     | ?- an_object::expand_goal(3 =:= 5, Goal).

     Goal = (3=:=5)
     yes

The goal-expansion mechanism prevents an infinite loop when expanding a
goal by checking that a goal to be expanded does not resulted from a
previous expansion of the same goal.  For example, consider the
following object:

     :- object(fixed_point,
         implements(expanding)).

         goal_expansion(a, b).
         goal_expansion(b, c).
         goal_expansion(c, (a -> b; c)).

     :- end_object.

The expansion of the goal ‘a’ results in the goal ‘(a -> b; c)’ with no
attempt to further expand the ‘a’, ‘b’, and ‘c’ goals as they have
already been expanded.

Goal-expansion applies to goal arguments of control constructs,
meta-arguments in built-in or ‘user’ defined meta-predicates,
meta-arguments in local user-defined meta-predicates, meta-arguments in
meta-predicate messages when static binding is possible, and
‘initialization/1’, ‘if/1’, and ‘elif/1’ directives.

   ---------- Footnotes ----------

   (1) ../../docs/expanding_0.html#expanding-0

1.16.2 Expanding grammar rules
------------------------------

A common term expansion is the translation of grammar rules into
predicate clauses.  This transformation is performed automatically by
the compiler when a source file entity defines grammar rules.  It can
also be done explicitly by calling the ‘expand_term/2’ built-in method.
For example:

     | ?- logtalk::expand_term((a --> b, c), Clause).

     Clause = (a(A,B) :- b(A,C), c(C,B))
     yes

Note that the default translation of grammar rules can be overridden by
defining clauses for the term_expansion/2 predicate.

1.16.3 Bypassing expansions
---------------------------

Terms and goals wrapped by the {}/1 control construct are
not expanded.  For example:

     | ?- an_object::expand_term({ping}, Term).

     Term = {ping}
     yes

     | ?- an_object::expand_goal({a}, Goal).

     Goal = {a}
     yes

This also applies to source file terms and source file goals when using
hook objects (discussed next).

1.16.4 Hook objects
-------------------

Term and goal expansion of a source file during its compilation is
performed by using 'hook objects'.  A hook object is simply an object
implementing the expanding(1) built-in protocol and defining clauses for
the term and goal expansion hook predicates.  Hook objects must be
compiled and loaded prior to be used to expand a source file.

To compile a source file using a hook object, we can use the hook
compiler flag in the second argument of the logtalk_compile/2
and logtalk_load/2 built-in predicates.  For example:

     | ?- logtalk_load(source_file, [hook(hook_object)]).
     ...

In alternative, we can use a set_logtalk_flag/2 directive in
the source file itself.  For example:

     :- set_logtalk_flag(hook, hook_object).

To use multiple hook objects in the same source file, simple write each
directive before the block of code that it should handle.  For example:

     :- object(h1,
         implements(expanding)).

         term_expansion((:- public(a/0)), (:- public(b/0))).
         term_expansion(a, b).

     :- end_object.

     :- object(h2,
         implements(expanding)).

         term_expansion((:- public(a/0)), (:- public(c/0))).
         term_expansion(a, c).

     :- end_object.

     :- set_logtalk_flag(hook, h1).

     :- object(s1).

         :- public(a/0).
         a.

     :- end_object.

     :- set_logtalk_flag(hook, h2).

     :- object(s2).

         :- public(a/0).
         a.

     :- end_object.

     | ?- {h1, h2, s}.
     ...

     | ?- s1::b.
     yes

     | ?- s2::c.
     yes

It is also possible to define a default hook object by defining a global
value for the ‘hook’ flag by calling the set_logtalk_flag/2.
predicate.  For example:

     | ?- set_logtalk_flag(hook, hook_object).

     yes

Note that, due to the ‘set_logtalk_flag/2’ directive being local to a
source, file, using it to specify a hook object will override any
defined default hook object or any hook object specified as a
‘logtalk_compile/2’ or ‘logtalk_load/2’ predicate compiler option for
compiling or loading the source file.

     Note: Clauses for the ‘term_expansion/2’ and ‘goal_expansion/2’
     predicates defined within an object or a category are never used in
     the compilation of the object or the category itself.

   ---------- Footnotes ----------

   (1) ../../docs/expanding_0.html#expanding-0

1.16.5 Virtual source file terms and loading context
----------------------------------------------------

When using a hook object to expand the terms of a source file, two
virtual file terms are generated: ‘begin_of_file’ and ‘end_of_file’.
These terms allow the user to define term-expansions before and after
the actual source file terms.

Logtalk also provides a logtalk_load_context/2 built-in
predicate that can be used to access the compilation/loading context
when performing expansions.  The logtalk built-in object also
provides a set of predicates that can be useful, notably when adding
Logtalk support for languages extensions originally developed for
Prolog.

As an example of using the virtual terms and the
‘logtalk_load_context/2’ predicate, assume that you want to convert
plain Prolog files to Logtalk by wrapping the Prolog code in each file
using an object (named after the file) that implements a given protocol.
This could be accomplished by defining the following hook object:

     :- object(wrapper(_Protocol_),
         implements(expanding)).

         term_expansion(begin_of_file, (:- object(Name,implements(_Protocol_)))) :-
             logtalk_load_context(file, File),
             os::decompose_file_name(File,_ , Name, _).

         term_expansion(end_of_file, (:- end_object)).

     :- end_object.

Assuming e.g.  ‘my_car.pl’ and ‘lease_car.pl’ files to be wrapped and a
‘car_protocol’ protocol, we could then load them using:

     | ?- logtalk_load(
              ['my_car.pl', 'lease_car.pl'],
              [hook(wrapper(car_protocol))]
          ).

     yes

     Note: When a source file also contains plain Prolog directives and
     predicates, these are term-expanded but not goal-expanded (with the
     exception of the ‘initialization/1’, ‘if/1’, and ‘elif/1’
     directives, where the goal argument is expanded to improve code
     portability across backends).

1.16.6 Default compiler expansion workflow
------------------------------------------

When compiling a source file, the compiler will first try, by
default, the source file specific hook object specified using a local
‘set_logtalk_flag/2’ directive, if defined.  If that expansion fails, it
tries the hook object specified using the ‘hook/1’ compiler option in
the ‘logtalk_compile/2’ or ‘logtalk_load/2’ goal that compiles or loads
the file, if defined.  If that expansion fails, it tries the default
hook object, if defined.  If that expansion also fails, the compiler
tries the Prolog dialect specific expansion rules found in the
adapter file. (which are used to support non-standard Prolog
features).

1.16.7 User defined expansion workflows
---------------------------------------

Sometimes we have multiple hook objects that we need to combine and use
in the compilation of a source file.  Logtalk includes a
hook_flows library that supports two basic expansion workflows: a
pipeline(1) of hook objects, where the expansion results from a hook
object are feed to the next hook object in the pipeline, and a set(2) of
hook objects, where expansions are tried until one of them succeeds.
These workflows are implemented as parametric objects allowing combining
them to implement more sophisticated expansion workflows.  There is also
a hook_objects library that provides convenient hook objects
for defining custom expansion workflows.  This library includes an hook
object that can be used to restore the default expansion workflow used
by the compiler.

For example, assuming that you want to apply the Prolog backend specific
expansion rules defined in its adapter file, using the
backend_adapter_hook(3) library object, passing the resulting terms to
your own expansion when compiling a source file, we could use the goal:

     | ?- logtalk_load(
              source,
              [hook(hook_pipeline([backend_adapter_hook, my_expansion]))]
          ).

As a second example, we can prevent expansion of a source file using the
library object identity_hook(4) by adding as the first term in a source
file the directive:

     :- set_logtalk_flag(hook, identity_hook).

The file will be compiled as-is as any hook object (specified as a
compiler option or as a default hook object) and any backend adapter
expansion rules are overriden by the directive.

   ---------- Footnotes ----------

   (1) ../../docs/hook_pipeline_1.html#hook-pipeline-1

   (2) ../../docs/hook_set_1.html#hook-set-1

   (3) ../../docs/backend_adapter_hook_0.html#backend-adapter-hook-0

   (4) ../../docs/identity_hook_0.html#identity-hook-0

1.16.8 Using Prolog defined expansions
--------------------------------------

In order to use clauses for the ‘term_expansion/2’ and
‘goal_expansion/2’ predicates defined in plain Prolog, simply specify
the pseudo-object ‘user’ as the hook object when compiling source files.
When using backend Prolog compilers that support a module
system, it can also be specified a module containing clauses for the
expanding predicates as long as the module name doesn’t coincide with an
object name.  When defining a custom workflow, the library object
prolog_module_hook/1(1) can be used as a workflow step.  For example,
assuming a module ‘functions’ defining expansion rules that we want to
use:

     | ?- logtalk_load(
              source,
              [hook(hook_set([prolog_module_hook(functions), my_expansion]))]
          ).

But note that Prolog module libraries may provide definitions of the
expansion predicates that are not compatible with the Logtalk compiler.
Specially when setting the hook object to ‘user’, be aware of any Prolog
library that is loaded, possibly by default or implicitly by the Prolog
system, that may be contributing definitions of the expansion
predicates.  It is usually safer to define a specific hook object for
combining multiple expansions in a fully controlled way.

     Note: The ‘user’ object declares ‘term_expansion/2’ and
     ‘goal_expansion/2’ as multifile and dynamic predicates.  This helps
     in avoiding predicate existence errors when compiling source files
     with the ‘hook’ flag set to ‘user’ as these predicates are only
     natively declared in some of the supported backend Prolog
     compilers.

   ---------- Footnotes ----------

   (1) ../../docs/prolog_module_hook_1.html#prolog-module-hook-1

1.16.9 Debugging expansions
---------------------------

The ‘term_expansion/2’ and ‘goal_expansion/2’ predicates can be
debugged as any other object predicates.  Note that expansions can
often be manually tested by sending expand_term/2 and
expand_goal/2 messages to a hook object with the term or goal
whose expansion you want to check as argument.  An alternative to the
debugging tools is to use a monitor for the runtime messages
that call the predicates.  For example, assume a ‘expansions_debug.lgt’
file with the contents:

     :- initialization(
         define_events(after, edcg, _, _, expansions_debug)
     ).

     :- object(expansions_debug,
         implements(monitoring)).

         after(edcg, term_expansion(T,E), _) :-
             writeq(term_expansion(T,E)), nl.

     :- end_object.

We can use this monitor to help debug the expansion rules of the
edcg library when applied to the ‘edcgs’ example using the
queries:

     | ?- {expansions_debug}.
     ...

     | ?- set_logtalk_flag(events, allow).
     yes

     | ?- {edcgs(loader)}.
     ...
     term_expansion(begin_of_file,begin_of_file)
     term_expansion((:-object(gemini)),[(:-object(gemini)),(:-op(1200,xfx,-->>))])
     term_expansion(acc_info(castor,A,B,C,true),[])
     term_expansion(pass_info(pollux),[])
     term_expansion(pred_info(p,1,[castor,pollux]),[])
     term_expansion(pred_info(q,1,[castor,pollux]),[])
     term_expansion(pred_info(r,1,[castor,pollux]),[])
     term_expansion((p(A)-->>B is A+1,q(B),r(B)),(p(A,C,D,E):-B is A+1,q(B,C,F,E),r(B,F,D,E)))
     term_expansion((q(A)-->>[]),(q(A,B,B,C):-true))
     term_expansion((r(A)-->>[]),(r(A,B,B,C):-true))
     term_expansion(end_of_file,end_of_file)
     ...

This solution does not require compiling the ‘edcg’ hook object in debug
mode or access to its source code (e.g.  to modify its expansion rules
to emit debug messages.  We could also simply use the ‘user’
pseudo-object as the monitor object:

     | ?- assertz((
              after(_, term_expansion(T,E), _) :-
                 writeq(term_expansion(T,E)), nl
          )).
     yes

     | ?- define_events(after, edcg, _, Sender, user).
     yes

Another alternative is to use a pipeline of hook objects with the
library ‘hook_pipeline/1’ and ‘write_to_stream_hook’ objects to write
the expansion results to a file.  For example, using the ‘unique.lgt’
test file from the ‘edcgs’ library directory:

     | ?- {hook_flows(loader), hook_objects(loader)}.
     ...

     | ?- open('unique_expanded.lgt', write, Stream),
          logtalk_compile(
              unique,
              [hook(hook_pipeline([edcg,write_to_stream_hook(Stream,[quoted(true)])]))]
          ),
          close(Stream).
     ...

The generated ‘unique_expanded.lgt’ file will contain the clauses
resulting from the expansion of the EDCG rules found in the ‘unique.lgt’
file by the ‘edcg’ hook object expansion.

1.17 Documenting
================

Assuming that the source_data flag is turned on, the compiler
saves all relevant documenting information collected when compiling a
source file.  The provided lgtdoc tool can access this
information by using the reflection support and generate a
documentation file for each compiled entity (object, protocol, or
category) in XML format.  Contents of the XML file include the entity
name, type, and compilation mode (static or dynamic), the entity
relations with other entities, and a description of any declared
predicates (name, compilation mode, scope, …).  The XML documentation
files can be enriched with arbitrary user-defined information, either
about an entity or about its predicates, by using the two directives
described in the next section.  The ‘lgtdoc’ tool includes POSIX and
Windows scripts for converting the XML documentation files to several
final formats (such as HTML and PDF).

1.17.1 Documenting directives
-----------------------------

Logtalk supports two documentation directives for providing arbitrary
user-defined information about an entity or a predicate.  These two
directives complement other directives that also provide important
documentation information such as the mode/2 and
meta_predicate/1 directives.

1.17.1.1 Entity directives
..........................

Arbitrary user-defined entity information can be represented using the
info/1 directive:

     :- info([
         Key1 is Value1,
         Key2 is Value2,
         ...
     ]).

In this pattern, keys should be atoms and values should be bound terms.
The following keys are predefined and may be processed specially by
Logtalk tools:

‘comment’

     Comment describing the entity purpose (an atom).  End the comment
     with a period (full stop).  As a style guideline, don’t use overly
     long comments.  If you need to provide additional details, use the
     ‘remarks’ key.

‘author’

     Entity author(s) (an atom or a compound term ‘{entity}’ where
     ‘entity’ is the name of an XML entity in a user defined
     ‘custom.ent’ file).

‘version’

     Version number (a ‘Major:Minor:Patch’ compound term) Following the
     Semantic Versioning guidelines(1) is strongly advised.

‘date’

     Date of last modification in ISO 8601 standard format
     (‘Year-Month-Day’ where ‘Year’, ‘Month’, and ‘Day’ are integers).

‘parameters’

     Parameter names and descriptions for parametric entities (a list of
     ‘Name-Description’ pairs where both names and descriptions are
     atoms).  End the ‘Description’ with a period (full stop).

‘parnames’

     Parameter names for parametric entities (a list of atoms; a simpler
     version of the previous key, used when parameter descriptions are
     deemed unnecessary).

‘copyright’

     Copyright notice for the entity source code (an atom or a compound
     term ‘{entity}’ where ‘entity’ is the name of an XML entity defined
     in a user defined ‘custom.ent’ file).

‘license’

     License terms for the entity source code; usually, just the license
     name (an atom or a compound term ‘{entity}’ where ‘entity’ is the
     name of an XML entity in a user defined ‘custom.ent’ file).
     License names should whenever possible be a license identifier as
     specified in the SPDX standard(2).

‘remarks’

     List of general remarks about the entity using ‘Topic-Text’ pairs
     where both the topic and the text must be atoms.  End the ‘Text’
     with a period (full stop).

‘see_also’

     List of related entities (using the entity identifiers, which can
     be atoms or compound terms).

For example:

     :- info([
         version is 2:1:0,
         author is 'Paulo Moura',
         date is 2000-11-20,
         comment is 'Building representation.',
         diagram is 'UML Class Diagram #312'
     ]).

Use only the keywords that make sense for your application and remember
that you are free to invent your own keywords.  All key-value pairs can
be retrieved programmatically using the reflection API and
are visible to the lgtdoc tool (which includes them in the
generated documentation).

   ---------- Footnotes ----------

   (1) https://semver.org

   (2) https://spdx.org/licenses/

1.17.1.2 Predicate directives
.............................

Arbitrary user-defined predicate information can be represented using
the info/2 directive:

     :- info(Name/Arity, [
         Key1 is Value1,
         Key2 is Value2,
         ...
     ]).

The first argument can also a grammar rule non-terminal indicator,
‘Name//Arity’.  Keys should be atoms and values should be bound terms.
The following keys are predefined and may be processed specially by
Logtalk tools:

‘comment’

     Comment describing the predicate (or non-terminal) purpose (an
     atom).  End the comment with a period (full stop).  As a style
     guideline, don’t use overly long comments.  If you need to provide
     additional details, use the ‘remarks’ key.

‘arguments’

     Names and descriptions of predicate arguments for pretty print
     output (a list of ‘Name-Description’ pairs where both names and
     descriptions are atoms).  End the ‘Description’ with a period (full
     stop).

‘argnames’

     Names of predicate arguments for pretty print output (a list of
     atoms; a simpler version of the previous key, used when argument
     descriptions are deemed unnecessary).

‘allocation’

     Objects where we should define the predicate.  Some possible values
     are ‘container’, ‘descendants’, ‘instances’, ‘classes’,
     ‘subclasses’, and ‘any’.

‘redefinition’

     Describes if predicate is expected to be redefined and, if so, in
     what way.  Some possible values are ‘never’, ‘free’, ‘specialize’,
     ‘call_super_first’, ‘call_super_last’.

‘exceptions’

     List of possible exceptions throw by the predicate using
     ‘Description-Exception’ pairs.  The description must be an atom.
     The exception term must be a ground term.

‘examples’

     List of typical predicate call examples using the format
     ‘Description-Goal-Bindings’.  The description must be an atom with
     the goal sharing variables with the bindings.  The variable
     bindings term uses the format ‘{Variable = Term, ...}’.  When there
     are no variable bindings, the success or failure of the predicate
     call should be represented by the terms ‘{true}’ or ‘{false}’,
     respectively (you can also use in alternative the terms ‘{yes}’ or
     ‘{no}’).

‘remarks’

     List of general remarks about the predicate using ‘Topic-Text’
     pairs where both the topic and the text must be atoms.  End the
     ‘Text’ with a period (full stop).

‘since’

     Version that added the predicate (‘Major:Minor:Patch’).

‘see_also’

     List of related predicates and non-terminals (using the predicate
     and non-terminal indicators).

For example:

     :- info(color/1, [
         comment is 'Table of defined colors.',
         argnames is ['Color'],
         constraint is 'Up to four visible colors allowed.',
         examples is [
            'Check that the color blue is defined' - color(blue) - {true}
         ]
     ]).

As with the ‘info/1’ directive, use only the keywords that make sense
for your application and remember that you are free to invent your own
keywords.  All key-value pairs can also be retrieved programmatically
using the reflection API and are visible to the lgtdoc
tool (which includes them in the generated documentation).

1.17.1.3 Documenting predicate exceptions
.........................................

As described above, the ‘info/2’ predicate directive supports an
‘exceptions’ key that allows us to list all exceptions that may occur
when calling a predicate.  For example:

     :- info(check_option/1, [
         comment is 'Succeeds if the option is valid. Throws an error otherwise.',
         argnames is ['Option'],
         exceptions is [
             '``Option`` is a variable' - instantiation_error,
             '``Option`` is neither a variable nor a compound term' - type_error(compound, 'Option'),
             '``Option`` is a compound term but not a valid option' - domain_error(option, 'Option')
         ]
     ]).

When possible, only standard exceptions should be used.  See e.g the
error handling methods section for a full list.  The
argument names should be the same as those provided in the ‘arguments’
or ‘argnames’ keys.  Exceptions are usually listed starting with
instantiation and uninstantiation errors, followed by type errors, and
then domain errors.  These may then be followed by permission,
existence, evaluation, representation, or resource errors.

For each exception, use of 'controlled language' as found e.g.  in the
ISO Prolog Core standard and this Handbook is advised.  Some examples:

Instantiation error when one of more arguments cannot be a variable

     ‘Argument’ is a variable

     ‘Argument1’ and ‘Argument2’ are variables

Instantiation error when a closed list with bound elements is required

     ‘Argument’ is a partial list or a list with an element ‘Element’
     which is a variable

Uninstantiation error when an argument is not a variable

     ‘Argument’ is not a variable

Type error when an argument is not a variable but also not of the expected type

     ‘Argument’ is neither a variable nor a TYPE

     ‘Argument’ is neither a partial list nor a list

Type error when an element of a list is not a variable but is not of the expected type

     An element ‘Element’ of the ‘Argument’ list is neither a variable
     nor a TYPE

Domain error when an argument is of the correct type but not in the expected domain

     ‘Argument’ is a TYPE but not a valid DOMAIN

     ‘Argument’ is an integer that is less than zero

Domain error when an element of a list is of the correct type but not in the expected domain

     An element ‘Element’ of the ‘Argument’ list is a TYPE but not a
     valid DOMAIN

Existence error when an entity of a given kind does not exist

     The KIND ‘Argument’ does not exist

Other classes of errors have a less rigid style.  In case of doubt, look
for examples in this Handbook, in the APIs documentation, and in
standard documents.

1.17.2 Processing and viewing documenting files
-----------------------------------------------

The lgtdoc tool generates an XML documenting file per
entity.  It can also generate library, directory, entity, and predicate
indexes when documenting libraries and directories.  For example,
assuming the default filename extensions, a ‘trace’ object and a
‘sort(_)’ parametric object will result in ‘trace_0.xml’ and
‘sort_1.xml’ XML files.

Each entity XML file contains references to two other files, an XML
specification file and a XSLT style-sheet file.  The XML specification
file can be either a DTD file (‘logtalk_entity.dtd’) or an XML Scheme
file (‘logtalk_entity.xsd’).  The XSLT style-sheet file is responsible
for converting the XML files to some desired format such as HTML or PDF.
The default names for the XML specification file and the XSL style-sheet
file are defined by the lgtdoc tool but can be overridden by
passing a list of options to the tool predicates.  The ‘lgtdoc/xml’
sub-directory in the Logtalk installation directory contains the XML
specification files described above, along with several sample XSL
style-sheet files and sample scripts for converting XML documenting
files to several formats (e.g.  reStructuredText, Markdown, HTML, and
PDF). For example, assume that you want to generate the API
documentation for the ‘types’ library:

     | ?- {types(loader)}.
     ....

     | ?- {lgtdoc(loader)}.
     ....

     | ?- lgtdoc::library(types).
     ...

The above queries will result in the creation of a ‘xml_docs’ in your
current directory by default.  Assuming that we want to generate
Sphinx-based documentation and that we are using a POSIX
operating-system, the next steps would be:

     $ cd xml_docs
     $ lgt2rst -s -m

The ‘lgt2rst’ script will ask a few questions (project name, author,
version, …).  After its completion, the generated HTML files will be
found in the ‘_build/html’ directory by default:

     $ open _build/html/index.html

For Windows operating-systems, PowerShell (recommended) and JScript
(legacy) scripts are available.  For example, assuming that we want to
generate HTML documentation, we could run in a PowerShell window:

     cd xml_docs
     lgt2html.ps1 -p saxon

When using the legacy JScript scripts, you can also use the ‘.bat’
script alternatives:

     cd xml_docs
     lgt2html /p:saxon

After completion, the generated HTML files will be found in the
‘xml_docs’ directory by default.

See the ‘NOTES’ file in the tool directory for details, specially on the
XSLT processor dependencies.  You may use the supplied sample files as a
starting point for generating the documentation of your Logtalk
applications.

The Logtalk DTD file, ‘logtalk_entity.dtd’, contains a reference to a
user-customizable file, ‘custom.ent’, which declares XML entities for
source code author names, license terms, and copyright string.  After
editing the ‘custom.ent’ file to reflect your personal data, you may use
the XML entities on ‘info/1’ documenting directives.  For example,
assuming that the XML entities are named 'author', 'license', and
'copyright' we may write:

     :- info([
         version is 1:1:0,
         author is {author},
         license is {license},
         copyright is {copyright}
     ]).

The entity references are replaced by the value of the corresponding XML
entity when the XML documenting files are processed ('not' when they are
generated; this notation is just a shortcut to take advantage of XML
entities).

The lgtdoc tool supports a set of options that can be used
to control the generation of the XML documentation files.  See the tool
documentation for details.  There is also a doclet tool that
allows automating the steps required to generate the documentation for
an application.

1.17.3 Inline formatting in comments text
-----------------------------------------

Inline formatting in comments text can be accomplished by using Markdown
or reStructuredText syntax and converting XML documenting files to
Markdown or reStructuredText files (and these, if required, to e.g.
HTML, ePub, or PDF formats).  Note that Markdown and reStructuredText
common syntax elements are enough for most API documentation:

     Mark *italic text* with one asterisk.
     Mark **bold text** with two asterisks.
     Mark ``monospaced text`` with two backquotes.

Rendering this block as markup gives:

     Mark 'italic text' with one asterisk.  Mark 'bold text' with two
     asterisks.  Mark ‘monospaced text’ with two backquotes.

As single backquotes have different purposes in Markdown (monospaced
text) and reStructuredText (domain- or application-dependent meaning),
never use them.  This also avoids doubts if there’s an inline formatting
typo in text meant to be rendered as monospaced text (usually inline
code fragments).

1.17.4 Diagrams
---------------

The diagrams tool supports a wide range of diagrams that can
also help in documenting an application.  The generated diagrams can
include URL links to both source code and API documentation.  They can
also be linked, connecting for example high level diagrams to detail
diagrams.  These features allow diagrams to be an effective solution for
navigating and understanding the structure and implementation of an
application.  This tool uses the same reflection API as the
‘lgtdoc’ tool and thus have access to the same source data.  See the
tool documentation for details.

1.18 Debugging
==============

The Logtalk distribution includes a command-line debugger.
tool implemented as a Logtalk application.  It can be loaded by typing:

     | ?- logtalk_load(debugger(loader)).

It can also be loaded automatically at startup time by using a
settings file.  This tool implements debugging features similar to
those found on most Prolog systems.  There are some differences,
however, between the usual implementation of Prolog debuggers and the
current implementation of the Logtalk debugger that you should be aware.
First, unlike most Prolog debuggers, the Logtalk debugger is not a
built-in feature but a regular Logtalk application using documented
debugging hook predicates.  This translates to a different, although
similar, set of debugging features when compared with some of the more
sophisticated Prolog debuggers.  Second, debugging is only possible for
entities compiled in debug mode.  When compiling an entity in debug
mode, Logtalk decorates clauses with source information to allow tracing
of the goal execution.  Third, the implementation of spy points allows
the user to specify the execution context for entering the
debugger.  This feature is a consequence of the encapsulation of
predicates inside objects.

1.18.1 Compiling source files in debug mode
-------------------------------------------

Compilation of source files in debug mode is controlled by the
debug compiler flag.  The default value for this flag, usually
‘off’, is defined in the adapter files.  Its default value may be
changed globally at runtime by calling:

     | ?- set_logtalk_flag(debug, on).

Implicitly, this goal also turns off the ‘optimize’ flag.  In
alternative, if we want to compile only some source files in debug mode,
we may instead write:

     | ?- logtalk_load([file1, file2, ...], [debug(on)]).

The logtalk_make/1 built-in predicate can also be used to
recompile all loaded files (that were compiled without using explicit
values for the debug and optimize compiler flags
in a ‘logtalk_load/2’ call or in a loader file file, if used)
in debug mode:

     | ?- logtalk_make(debug).

With most backend Prolog compilers, the ‘{+d}’ top-level
shortcut can also be used.  After debugging, the files can be recompiled
in normal or optimized mode using, respectively, the ‘{+n}’ or ‘{+o}’
top-level shortcuts.

     Warning: The clean compiler flag should be turned on
     whenever the debug flag is turned on at runtime.  This
     is necessary because debug code would not be generated for files
     previously compiled in normal or optimized mode if there are no
     changes to the source files.

After loading the debugger, we may check (or enumerate by backtracking),
all loaded entities compiled in debug mode as follows:

     | ?- debugger::debugging(Entity).

To compile only a specific entity in debug mode, use the
set_logtalk_flag/2 directive inside the entity.  To compile all
entities in a source file in debug mode, use the
set_logtalk_flag/2 directive at the beginning of the file.

1.18.2 Procedure box model
--------------------------

Logtalk uses a 'procedure box model' similar to those found on most
Prolog systems.  The traditional Prolog procedure box model defines four
ports ('call', 'exit', 'redo', and 'fail') for describing control flow
when calling a predicate:

     ‘call’
          predicate call
     ‘exit’
          success of a predicate call
     ‘redo’
          backtracking into a predicate
     ‘fail’
          failure of a predicate call

Logtalk, as found on some recent Prolog systems, adds a port for dealing
with exceptions thrown when calling a predicate:

     ‘exception’
          predicate call throws an exception

In addition to the ports described above, Logtalk adds two more ports,
‘fact’ and ‘rule’, which show the result of the unification of a goal
with, respectively, a fact and a rule head:

     ‘fact’
          unification success between a goal and a fact
     ‘rule’
          unification success between a goal and a rule head

Following Prolog tradition, the user may define for which ports the
debugger should pause for user interaction by specifying a list of
'leashed' ports.  Unleashed ports are just printed with no pause for
user interaction.  For example:

     | ?- debugger::leash([call, exit, fail]).

Alternatively, the user may use an atom abbreviation for a pre-defined
set of ports.  For example:

     | ?- debugger::leash(loose).

The abbreviations defined in Logtalk are similar to those defined on
some Prolog compilers:

     ‘none’
          ‘[]’
     ‘loose’
          ‘[fact, rule, call]’
     ‘half’
          ‘[fact, rule, call, redo]’
     ‘tight’
          ‘[fact, rule, call, redo, fail, exception]’
     ‘full’
          ‘[fact, rule, call, exit, redo, fail, exception]’

By default, the debugger pauses at every port for user interaction.

1.18.3 Activating the debugger
------------------------------

The debuggerp::trace/0(1) and debuggerp::debug/0(2) predicates
implicitly select the ‘debugger’ tool as the active debug handler.  If
you have additional debug handlers loaded (e.g.  the ‘ports_profiler’
tool), those would no longer be active (there can be only one active
debug handler at any given time).  The debuggerp::nodebug/0(3) predicate
implicitly deselects the ‘debugger’ tool as the active debug handler.

   ---------- Footnotes ----------

   (1) ../../docs/debuggerp_0.html#debuggerp-0-trace-0

   (2) ../../docs/debuggerp_0.html#debuggerp-0-debug-0

   (3) ../../docs/debuggerp_0.html#debuggerp-0-nodebug-0

1.18.4 Defining spy points
--------------------------

Logtalk spy points can be defined by simply stating which predicates
should be spied (as in most Prolog debuggers), by stating which
predicate clauses to spy given their source file line numbers, or by
specifying the execution context for activating a spy point.  In the
case of line number spy points (also known as breakpoints), the line
number must correspond to the first line of an entity clause.  To
simplify the definition of line number spy points, these are specified
using the entity identifier instead of the file name (as all entities
share a single namespace, an entity can only be defined in a single
file).

1.18.4.1 Defining line number and predicate spy points
......................................................

Line number and predicate spy points are specified using the debugger
‘spy/1’ predicate.  The argument can be a breakpoint (expressed as a
‘Entity-Line’ pair), a predicate indicator (‘Name/Arity’), a
non-terminal indicator (‘Name//Arity’), or a list of spy points.  For
example:

     | ?- debugger::spy(person-42).

     Spy points set.
     yes

     | ?- debugger::spy(foo/2).

     Spy points set.
     yes

     | ?- debugger::spy([foo/4, bar//1, agent-99]).

     Spy points set.
     yes

Note that setting a line number spy point will remove any existing log
point for the same location.

Line numbers and predicate spy points can be removed by using the
debugger ‘nospy/1’ predicate.  The argument can be a spy point, a list
of spy points, or a non-instantiated variable in which case all spy
points will be removed.  For example:

     | ?- debugger::nospy(_).

     All matching predicate spy points removed.
     yes

In breakpoints, the line number must for the first line of a clause that
we want to spy.  But note that only some Prolog backends provide
accurate source file term line numbers.  Check the debugger.
tool documentation for details.

1.18.4.2 Defining context spy points
....................................

A context spy point is a tuple describing a message execution context
and a goal:

     (Sender, This, Self, Goal)

The debugger is evoked whenever the spy point goal and the specified
execution context subsumes the goal currently being executed and its
execution context.  The user may establish any number of context spy
points as necessary.  For example, in order to call the debugger
whenever a predicate defined on an object named ‘foo’ is called we may
define the following spy point:

     | ?- debugger::spy(_, foo, _, _).

     Spy point set.
     yes

For example, we can spy all calls to a ‘foo/2’ predicate with a ‘bar’
atom in the second argument by setting the condition:

     | ?- debugger::spy(_, _, _, foo(_, bar)).

     Spy point set.
     yes

The debugger ‘nospy/4’ predicate may be used to remove all matching spy
points.  For example, the call:

     | ?- debugger::nospy(_, _, foo, _).

     All matching context spy points removed.
     yes

will remove all context spy points where the value of self.
matches the atom ‘foo’.

1.18.4.3 Removing all spy points
................................

We may remove all line number, predicate, and context spy points by
using the debugger ‘nospyall/0’ predicate:

     | ?- debugger::nospyall.

     All line number spy points removed.
     All predicate spy points removed.
     All context spy points removed.
     yes

There’s also a ‘reset/0’ predicate that can be used to reset the
debugger to its default settings.

1.18.5 Defining log points
--------------------------

Logtalk log points are similar to line number spy points and thus the
line number must correspond to the first line of an entity clause.  When
the debugger reaches a log point, it prints the corresponding
unification port data followed, optionally, by a log message and
continues without halting execution for taking a port command.  The log
message must be a valid atom when quoted.  The debugger prints a ‘@’
character at the beginning of the line for easy recognition of log
points output.  Log points are defined using the ‘log/3’ predicate.  For
example:

     | ?- debugger::log(agent, 99, 'At the secret headquarter!').
          Log point added.
     yes

Predicates ‘logging/3’ and ‘nolog/3’ can be used to, respectively, query
and remove log points.  There’s also a ‘nologall/0’ predicate that
removes all log points.

Note that setting a log point will remove any existing line number spy
point for the same location.

1.18.6 Tracing program execution
--------------------------------

Logtalk allows tracing of execution for all objects compiled in debug
mode.  To start the debugger in trace mode, write:

     | ?- debugger::trace.

     yes

Next, type the query to be debugged.  For examples, using the ‘family’
example in the Logtalk distribution compiled for debugging:

     | ?- addams::sister(Sister, Sibling).
          Call: (1) sister(_1082,_1104) ?
          Rule: (1) sister(_1082,_1104) ?
          Call: (2) ::female(_1082) ?
          Call: (3) female(_1082) ?
          Fact: (3) female(morticia) ?
         *Exit: (3) female(morticia) ?
         *Exit: (2) ::female(morticia) ?
         ...

While tracing, the debugger will pause for user input at each leashed
port, printing an informative message.  Each trace line starts with the
port, followed by the goal invocation number, followed by the goal.  The
invocation numbers are unique and allows us to correlate the ports used
for a goal.  In the output above, you can see for example that the goal
‘::female(_1082)’ succeeds with the answer ‘::female(morticia)’.  The
debugger also provides determinism information by prefixing the ‘exit’
port with a ‘*’ character when a call succeeds with choice-points
pending, thus indicating that there might be alternative solutions for
the goal.

Note that, when tracing, spy points will be ignored.  Before the port
number, when a spy point is set for the current clause or goal, the
debugger will print a ‘#’ character for line number spy points, a ‘+’
character for predicate spy points, and a ‘*’ character for context spy
points.  For example:

     | ?- debugger::spy(female/2).

     yes

     | ?- addams::sister(Sister, Sibling).
          Call: (1) sister(_1078,_1100) ?
          Rule: (1) sister(_1078,_1100) ?
          Call: (2) ::female(_1078) ?
       +  Call: (3) female(_1078) ?

To stop tracing (but still allowing the debugger to stop at defined spy
points), write:

     | ?- debugger::notrace.

     yes

1.18.7 Debugging using spy points
---------------------------------

Tracing a program execution may generate large amounts of debugging
data.  Debugging using spy points allows the user to concentrate in
specific points of the code.  To start a debugging session using spy
points, write:

     | ?- debugger::debug.

     yes

For example, assuming the spy point we set in the previous section on
the ‘female/1’ predicate:

     | ?- addams::sister(Sister, Sibling).
       +  Call: (3) female(_1078) ?

To stop the debugger, write:

     | ?- debugger::nodebug.

     yes

Note that stopping the debugger does not remove any defined spy points.

1.18.8 Debugging commands
-------------------------

The debugger pauses at leashed ports when tracing or when finding a spy
point for user interaction.  The commands available are as follows:

‘c’ — creep

     go on; you may use the spacebar, return, or enter keys in
     alternative

‘l’ — leap

     continues execution until the next spy point is found

‘s’ — skip

     skips debugging for the current goal; valid at call, redo, and
     unification ports

‘S’ - Skip

     similar to skip but displaying all intermediate ports unleashed

‘q’ — quasi-skip

     skips debugging until returning to the current goal or reaching a
     spy point; valid at call and redo ports

‘r’ — retry

     retries the current goal but side-effects are not undone; valid at
     the fail port

‘j’ — jump

     reads invocation number and continues execution until a port is
     reached for that number

‘z’ — zap

     reads either a port name and continues execution until that port is
     reached or a negated port name and continues execution until a port
     other than the negated port is reached

‘i’ — ignore

     ignores goal, assumes that it succeeded; valid at call and redo
     ports

‘f’ — fail

     forces backtracking; may also be used to convert an exception into
     a failure

‘n’ — nodebug

     turns off debugging

‘N’ — notrace

     turns off tracing

‘@’ — command; ‘!’ can be used in alternative

     reads and executes a query

‘b’ — break

     suspends execution and starts new interpreter; type ‘end_of_file’
     to terminate

‘a’ — abort

     returns to top level interpreter

‘Q’ — quit

     quits Logtalk

‘p’ — print

     writes current goal using the ‘print/1’ predicate if available

‘d’ — display

     writes current goal without using operator notation

‘w’ — write

     writes current goal quoting atoms if necessary

‘$’ — dollar

     outputs the compiled form of the current goal (for low-level
     debugging)

‘x’ — context

     prints execution context

‘.’ — file

     prints file, entity, predicate, and line number information at an
     unification port

‘e’ — exception

     prints exception term thrown by the current goal

‘E’ — raise exception

     reads and throws an exception term

‘=’ — debugging

     prints debugging information

‘<’ — write depth

     sets the write term depth (set to ‘0’ to reset)

‘*’ — add

     adds a context spy point for the current goal

‘/’ — remove

     removes a context spy point for the current goal

‘+’ — add

     adds a predicate spy point for the current goal

‘-’ — remove

     removes a predicate spy point for the current goal

‘#’ — add

     adds a line number spy point for the current clause

‘|’ — remove

     removes a line number spy point for the current clause

‘h’ — condensed help

     prints list of command options

‘?’ — extended help

     prints list of command options

1.18.9 Customizing term writing
-------------------------------

Debugging complex applications often requires customizing term writing.
The available options are limiting the writing depth of large compound
terms and defining the traditional ‘portray/1’ to define how a term
should be printed when using the ‘p’ command at a leashed port.

1.18.9.1 Term write depth
.........................

The terms written by the debugger can be quite large depending on the
application being debugged.  As described in the previous section, the
debugger accepts the ‘<’ command to set the maximum write term depth for
compound terms.  This commmand requires that the used backend Prolog compiler
supports the non-standard but common ‘max_depth/1’
option for the ‘write_term/3’ predicate.  When the compound term being
written is deeply nested, the sub-terms are only written up to the
specified depth with the omitted sub-terms replaced usually by ‘...’.
For example:

     | ?- write_term([0,1,2,3,4,5,6,7,8,9], [max_depth(5)]).

     [0,1,2,3,4|...]
     yes

The default maximum depth depends on the backend.  To print compound
terms without a depth limit, set it explicitly to zero if necessary.

1.18.9.2 Custom term writing
............................

The implicit use of the traditional ‘print/1’ predicate (using the ‘p’
command) and the ‘portray/1’ user-defined hook predicate requires
backend Prolog compiler support for these predicates.  See the
documentation of the backend you intend to use for details.  As an
example, assuming the following ‘portray/1’ definition:

     portray(e(V1,V2)) :-
         format('~q ---> ~q~n', [V1,V2]).

Calling the ‘print/1’ predicate with e.g.  a ‘e(x1,x7)’ compound term
argument will output:

     | ?- print(e(x1,x7)).

     x1 ---> x7
     yes

1.18.10 Context-switching calls
-------------------------------

Logtalk provides a control construct, see (<<)/2, which allows the
execution of a query within the context of an object.  Common debugging
uses include checking an object local predicates (e.g.  predicates
representing internal dynamic state) and sending a message from within
an object.  This control construct may also be used to write unit tests.

Consider the following toy example:

     :- object(broken).

         :- public(a/1).

         a(A) :- b(A, B), c(B).
         b(1, 2). b(2, 4). b(3, 6).
         c(3).

     :- end_object.

Something is wrong when we try the object public predicate, ‘a/1’:

     | ?- broken::a(A).

     no

For helping diagnosing the problem, instead of compiling the object in
debug mode and doing a 'trace' of the query to check the clauses for the
non-public predicates, we can instead simply type:

     | ?- broken << c(C).

     C = 3
     yes

The ‘(<<)/2’ control construct works by switching the execution context
to the object in the first argument and then compiling and executing the
second argument within that context:

     | ?- broken << (self(Self), sender(Sender), this(This)).

     Self = broken
     Sender = broken
     This = broken

     yes

As exemplified above, the ‘(<<)/2’ control construct allows you to call
an object local and private predicates.  However, it is important to
stress that we are not bypassing or defeating an object predicate scope
directives.  The calls take place within the context of the specified
object, not within the context of the object making the ‘(<<)/2’ call.
Thus, the ‘(<<)/2’ control construct implements a form of
'execution-context switching'.

The availability of the ‘(<<)/2’ control construct is controlled by the
context_switching_calls compiler flag (its default value is
defined in the adapter files of the backend Prolog compilers).

1.18.11 Debugging messages
--------------------------

Calls to the logtalk;;print_message/3 predicate where the
message kind is either ‘debug’ or ‘debug(Group)’ are only printed, by
default, when the debug flag is turned on.  Moreover, these
calls are suppressed by the compiler when the optimize flag
is turned on.  Note that actual printing of debug messages does not
require compiling the code in debug mode, only turning on the ‘debug’
flag.

1.18.11.1 Meta-messages
.......................

To avoid having to define message_tokens//2 grammar rules
for translating each and every debug message, Logtalk provides default
tokenization for seven 'meta-messages' that cover the most common cases:

‘@Message’

     By default, the message is printed as passed to the ‘write/1’
     predicate followed by a newline.

‘Key-Value’

     By default, the message is printed as ‘Key: Value’ followed by a
     newline.  The key is printed as passed to the ‘write/1’ predicate
     while the value is printed as passed to the ‘writeq/1’ predicate.

‘Format+Arguments’

     By default, the message is printed as passed to the ‘format/2’
     predicate.

‘List’

     By default, the list items are printed indented one per line.  The
     items are preceded by a dash and can be ‘@Message’, ‘Key-Value’, or
     ‘Format+Arguments’ messages.  If that is not the case, the item is
     printed as passed to the ‘writeq/1’ predicate.

‘Title::List’

     By default, the title is printed followed by a newline and the
     indented list items, one per line.  The items are printed as in the
     ‘List’ meta message.

‘[Stream,Prefix]>>Goal’

     By default, call user-defined printing ‘Goal’ in the context of
     ‘user’.  The use of a lambda expression allows passing the message
     stream and prefix.  Printing the prefix is delegated to the goal.

‘[Stream]>>Goal’

     By default, call user-defined printing ‘Goal’ in the context of
     ‘user’.  The use of a lambda expression allows passing the message
     stream.

Some simple examples of using these meta-messages:

     | ?- logtalk::print_message(debug, core, @'Phase 1 completed').
     yes

     | ?- logtalk::print_message(debug, core, [Stream]>>write(Stream,foo)).
     yes

     | ?- set_logtalk_flag(debug, on).
     yes

     | ?- logtalk::print_message(debug, core, [Stream]>>write(Stream,foo)).
     foo
     yes

     | ?- logtalk::print_message(debug, core, @'Phase 1 completed').
     >>> Phase 1 completed
     yes

     | ?- logtalk::print_message(debug, core, answer-42).
     >>> answer: 42
     yes

     | ?- logtalk::print_message(debug, core, 'Position: <~d,~d>'+[42,23]).
     >>> Position: <42,23>
     yes

     | ?- logtalk::print_message(debug, core, [arthur,ford,marvin]).
     >>> - arthur
     >>> - ford
     >>> - marvin
     yes

     | ?- logtalk::print_message(debug, core, names::[arthur,ford,marvin]).
     >>> names:
     >>> - arthur
     >>> - ford
     >>> - marvin
     yes

The ‘>>>’ prefix is the default message prefix for ‘debug’ messages.  It
can be redefined using the logtalk;;message_prefix_stream/4.
hook predicate.  For example:

     :- multifile(logtalk::message_prefix_stream/4).
     :- dynamic(logtalk::message_prefix_stream/4).

     logtalk::message_prefix_stream(debug, core, '(dbg) ', user_error).

1.18.11.2 Selective printing of debug messages
..............................................

By default, all debug messages are either printed or skipped, depending
on the debug and optimize flags.  When the code
is not compiled in optimal mode, the debug_messages tool
allows selectively enabling of debug messages per component.
and per debug group.  For example, to enable all ‘debug’ and
‘debug(Group)’ messages for the ‘parser’ component:

     % upon loading the tool, all messages are disabled by default:
     | ?- logtalk_load(debug_messages(loader)).
     ...

     % enable both debug and debug(_) messages:
     | ?- debug_messages::enable(parser).
     yes

To enable only ‘debug(tokenization)’ messages for the ‘parser’
component:

     % first disable any and all enabled messages:
     | ?- debug_messages::disable(parser).
     yes

     % enable only debug(tokenization) messages:
     | ?- debug_messages::enable(parser, tokenization).
     yes

See the tool documentation for more details.

1.18.12 Using the term-expansion mechanism for debugging
--------------------------------------------------------

Debugging messages only output information by default.  These messages
can, however, be intercepted to perform other actions.  An alternative
is to use instead the term-expansion mechanism for
conditional compilation of debugging goals.  For example, the
hook_objects library provides a print_goal_hook(1) object that
simplifies printing entity goals before or after calling them by simply
prefixing them with an operator.  See the library and hook object
documentation for details.  You can also define your own specialized
hook objects for custom debugging tasks.

   ---------- Footnotes ----------

   (1) ../../docs/print_goal_hook_0.html#print-goal-hook-0

1.18.13 Ports profiling
-----------------------

The Logtalk distribution includes a ports_profiler tool
based on the same procedure box model described above.  This tool is
specially useful for debugging performance issues (e.g.  due to lack of
determinism or unexpected backtracking).  See the tool documentation for
details.

1.18.14 Debug and trace events
------------------------------

The debugging API defines two multifile predicates,
logtalk::trace_event/2(1) and logtalk::debug_handler/3(2) for handiling
trace and debug events.  It also provides a logtalk::debug_handler/1(3)
multifile predicate that allows an object (or a category) to declare
itself as a debug handler provider.  The Logtalk ‘debugger’ and
‘ports_profiler’ tools are regular applications thar are implemented
using this API, which can also be used to implement alternative or new
debugging related tools.  See the API documentation for details and the
source code of the ‘debugger’ and ‘ports_profiler’ tools for usage
examples.

To define a new debug handler provider, add (to an object or category)
clauses for the ‘debug_handler/1’ and ‘debug_handler/3’ predicates.  For
example:

     % declare my_debug_handler as a debug handler provider
     :- multifile(logtalk::debug_handler/1).
     logtalk::debug_handler(my_debug_handler).

     % handle debug events
     :- multifile(logtalk::debug_handler/3).
     logtalk::debug_handler(my_debug_handler, Event, ExCtx) :-
         debug_handler(Event, ExCtx).

     debug_handler(fact(Entity,Fact,Clause,File,Line), ExCtx) :-
         ...
     debug_handler(rule(Entity,Head,Clause,File,Line), ExCtx) :-
         ...
     debug_handler(top_goal(Goal, TGoal), ExCtx) :-
         ...
     debug_handler(goal(Goal, TGoal), ExCtx) :-
         ...

Your debug handler provider should also either automatically call the
logtalk::activate_debug_handler/1(4) and
logtalk::deactivate_debug_handler/0(5) predicate or provide public
predicates to simplify calling these predicates.  For example:

     :- public(start/0).
     start :-
        logtalk::activate_debug_handler(my_debug_handler).

     :- public(stop/0).
     stop :-
        logtalk::deactivate_debug_handler.

If you only need to define a trace event handler, then simply define
clauses for the logtalk::trace_event/2(6) multifile predicate:

     :- multifile(logtalk::trace_event/2).
     :- dynamic(logtalk::trace_event/2).

     % the Logtalk runtime calls all defined logtalk::trace_event/2 hooks using
     % a failure-driven loop; thus we don't have to worry about handling all
     % events or failing after handling an event to give other hooks a chance
     logtalk::trace_event(fact(Entity, Fact, N, _, _), _) :-
         ...
     logtalk::trace_event(rule(Entity, Head, N, _, _), _) :-
         ...

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0-trace-event-2

   (2) ../../docs/logtalk_0.html#logtalk-0-debug-handler-3

   (3) ../../docs/logtalk_0.html#logtalk-0-debug-handler-1

   (4) ../../docs/logtalk_0.html#logtalk-0-activate-debug-handler-1

   (5) ../../docs/logtalk_0.html#logtalk-0-deactivate-debug-handler-0

   (6) ../../docs/logtalk_0.html#logtalk-0-trace-event-2

1.19 Performance
================

Logtalk is implemented as a 'trans-compiler' to Prolog.  When compiling
predicates, it preserves in the generated Prolog code all cases of
first-argument indexing and tail-recursion.  In practice, this mean that
if you know how to write efficient Prolog predicates, you already know
the basics of how to write efficient Logtalk predicates.

The Logtalk compiler appends a single argument to the compiled form of
all entity predicate clauses.  This hidden argument is used to pass the
execution-context when proving a query.  In the common case
where a predicate makes no calls to the execution-context predicates
and message-sending control constructs and
is neither a meta-predicate nor a coinductive predicate, the
execution-context is simply passed between goals.  In this case, with
most backend Prolog virtual machines, the cost of this extra argument is
null or negligible.  When the execution-context needs to be accessed
(e.g. to fetch the value of self for a (;;)/1.
call) there may be a small inherent overhead due to the access to the
individual arguments of the compound term used to represent the
execution-context.

1.19.1 Source code compilation modes
------------------------------------

Source code can be compiled in 'optimal', 'normal', or 'debug' mode,
depending on the optimize and debug compiler
flags.  Optimal mode is used when deploying an application while normal
and debug modes are used when developing an application.  Compiling code
in optimal mode enables several optimizations, notably use of
static binding whenever enough information is available at compile
time.  In debug mode, most optimizations are turned off and the code is
instrumented to generate debug events that enable developer
tools such as the command-line debugger and the ports profiler.

1.19.2 Local predicate calls
----------------------------

Local calls to object (or category) predicates have zero overhead in
terms of number of inferences, as expected, compared with local Prolog
calls.

1.19.3 Calls to imported or inherited predicates
------------------------------------------------

Assuming the optimize flag is turned on and a static
predicate, see (^^)/1 calls have zero overhead in terms of number
of inferences.

1.19.4 Calls to module predicates
---------------------------------

Local calls from an object (or category) to a module predicate have zero
overhead (assuming both the module and the predicate are bound at
compile time).

1.19.5 Messages
---------------

Logtalk implements static binding and dynamic binding
for message sending calls.  For dynamic binding, a caching mechanism
is used by the runtime.  It’s useful to measure the performance overhead
in 'number of logic inferences' compared with plain Prolog and Prolog
modules.  Note that the number of logic inferences is a metric
independent of the chosen backend Prolog compiler.  The results for
Logtalk 3.17.0 and later versions are:

   - Static binding: 0

   - Dynamic binding (object bound at compile time): +1

   - Dynamic binding (object bound at runtime): +2

Static binding is the common case with libraries and most application
code; it requires compiling code with the optimize flag
turned on.  Dynamic binding numbers are after the first call (i.e.
after the generalization of the query is cached).  All numbers with the
events flag set to ‘deny’ (setting this flag to ‘allow’ adds
an overhead of +5 inferences to the results above; note that this flag
can be defined in a per-object basis as needed instead of globally and
thus minimizing the performance impact).

The dynamic binding caches assume the used backend Prolog compiler
does indexing of dynamic predicates.  This is a common
feature of modern Prolog systems but the actual details vary from system
to system and may have an impact on dynamic binding performance.

Note that messages to 'self' (see (;;)/1 calls) and messages to
an object (see (;;)/2 calls) from the top-level interpreter
always use dynamic binding as the object that receives the message is
only know at runtime.

Messages sent from Prolog modules may use static binding depending on
the used backend Prolog compiler native support for goal-expansion.
Consult the Prolog compiler documentation and adapter file notes for
details.

     Warning: Some Prolog systems provide a ‘time/1’ predicate that also
     reports the number of inferences.  But the reported numbers are
     often misleading when the predicate is called from the top-level.
     Besides common top-level bookkeeping operations (e.g.  keeping
     track of goal history or applying goal-expansion) that may
     influence the inference counting, the Logtalk runtime code for a
     ‘::/2’ top-level goal is necessarily different from the code
     generated for a ‘::/2’ goal from a compiled object as it requires
     'runtime' compilation of the goal into the same low-level
     message-sending primitive (assuming dynamic-binding is also
     required for the compiled object goal).

1.19.6 Automatic expansion of built-in meta-predicates
------------------------------------------------------

The compiler always expands calls to the forall/2,
once/1, and ignore/1 meta-predicates into equivalent
definitions using the negation and conditional control constructs.  It
also expands calls to the call/1-N, phrase/2, and
phrase/3 meta-predicates when the first argument is bound.
These expansions are performed independently of the ‘optimize’ flag
value.

1.19.7 Inlining
---------------

When the optimize flag is turned on, the Logtalk compiler
performs 'inlining' of predicate calls whenever possible.  This includes
calls to Prolog predicates that are either built-in, foreign, or defined
in a module (including ‘user’).  Inlining notably allows wrapping module
or foreign predicates using an object without introducing any overhead.
In the specific case of the execution-context predicates,
calls are inlined independently of the ‘optimize’ flag value.

1.19.8 Generated code simplification and optimizations
------------------------------------------------------

When the optimize flag is turned on, the Logtalk compiler
simplifies and optimizes generated clauses (including those resulting
from the compilation of grammar rules), by flattening conjunctions,
folding left unifications (e.g. generated as a by-product of the
compilation of grammar rules), and removing redundant calls to ‘true/0’.

1.19.9 Size of the generated code
---------------------------------

The size of the intermediate Prolog code generated by the compiler is
proportional to the size of the source code.  Assuming that the
term-expansion mechanism is not used, each predicate clause in the
source code is compiled into a single predicate clause.  But the Logtalk
compiler also generates internal tables for the defined entities, for
the entity relations, and for the declared and defined predicates.
These tables enable support for fundamental features such as
inheritance and reflection.  The size of these tables is
proportional to the number of entities, entity relations, and predicate
declarations and definitions.  When the source_data is turned
on (the default when 'developing' an application), the generated code
also includes additional data about the source code such as entity and
predicates positions in a source file.  This data enables advanced
developer tool functionality but it is usually not required when
'deploying' an application.  Thus, turning this flag off is a common
setting for minimizing an application footprint.

1.19.10 Debug mode overhead
---------------------------

Code compiled in debug mode runs slower, as expected, when compared with
normal or optimized mode.  The overhead depends on the number of 'debug
events' generated when running the application.  A debug event is simply
a pass on a call or unification port of the procedure box model.
These debug events can be intercepted by defined clauses for the
logtalk::trace_event/2(1) and logtalk::debug_handler/3(2) multifile
predicates.  With no application (such as a debugger or a port profiler)
loaded defining clauses for these predicates, each goal have an overhead
of four extra inferences due to the runtime checking for a definition of
the hook predicates and a meta-call of the user goal.  The clause head
unification events results in one or more inferences per goal (depending
on the number of clauses whose head unify with the goal and
backtracking).  In practice, this overhead translates to code compiled
in debug mode running typically ~2x to ~7x slower than code compiled in
normal or optimized mode depending on the application (the exact
overhead is proportional to the number of passes on the call and
unification ports; deterministic code often results in a relatively
larger overhead when compared with code performing significant
backtracking).

   ---------- Footnotes ----------

   (1) ../../docs/logtalk_0.html#logtalk-0-trace-event-2

   (2) ../../docs/logtalk_0.html#logtalk-0-debug-handler-3

1.19.11 Other considerations
----------------------------

One aspect of performance, that affects both Logtalk and Prolog code, is
the characteristics of the Prolog VM. The Logtalk distribution includes
two examples, bench(1) and benchmarks(2), to help evaluate performance
with specific backend Prolog systems.  A table with benchmark results(3)
for a subset of the supported systems is also available at the Logtalk
website.  But note that multiple factors affect the performance of an
application and the benchmark examples and their results only provide a
partial assessment.

   ---------- Footnotes ----------

   (1)
https://github.com/LogtalkDotOrg/logtalk3/tree/master/examples/bench

   (2)
https://github.com/LogtalkDotOrg/logtalk3/tree/master/examples/benchmarks

   (3) https://logtalk.org/performance.html

1.20 Installing Logtalk
=======================

This page provides an overview of Logtalk installation requirements and
instructions and a description of the files contained on the Logtalk
distribution.  For detailed, up-to-date installation and configuration
instructions, please see the ‘README.md’, ‘INSTALL.md’, and
‘CUSTOMIZE.md’ files distributed with Logtalk.  The broad compatibility
of Logtalk, both with Prolog compilers and operating-systems, together
with all the possible user scenarios, means that installation can vary
from very simple by running an installer or a couple of scripts to the
need of patching both Logtalk and Prolog compilers to workaround the
lack of strong Prolog standards or to cope with the requirements of less
common operating-systems.

The preferred installation scenario is to have Logtalk installed in a
system-wide location, thus available for all users, and a local copy of
user-modifiable files on each user home directory (even when you are the
single user of your computer).  This scenario allows each user to
independently customize Logtalk and to freely modify the provided
libraries and programming examples.  Logtalk installers, installation
shell scripts, and Prolog integration scripts favor this installation
scenario, although alternative installation scenarios are always
possible.  The installers set two environment variables, ‘LOGTALKHOME’
and ‘LOGTALKUSER’, pointing, respectively, to the Logtalk installation
folder and to the Logtalk user folder.

User applications should preferable be kept outside of the Logtalk user
folder created by the installation process, however, as updating Logtalk
often results in updating the contents of this folder.  If your
applications depend on customizations to the distribution files, backup
those changes before updating Logtalk.

1.20.1 Hardware and software requirements
-----------------------------------------

1.20.1.1 Computer and operating system
......................................

Logtalk is compatible with almost any computer/operating-system with a
modern, standards compliant, Prolog compiler available.

1.20.1.2 Prolog compiler
........................

Logtalk requires a backend Prolog compiler supporting
official and de facto standards.  Capabilities needed by Logtalk that
are not defined in the official ISO Prolog Core standard include:

   - access to predicate properties

   - operating-system access predicates

   - de facto standard predicates not (yet) specified in the official
     standard

Logtalk needs access to the predicate property ‘built_in’ to properly
compile objects and categories that contain Prolog built-in predicates
calls.  In addition, some Logtalk built-ins need to know the
dynamic/static status of predicates to ensure correct application.  The
ISO standard for Prolog modules defines a ‘predicate_property/2’
predicate that is already implemented by most Prolog compilers.  Note
that if these capabilities are not built-in the user cannot easily
define them.

For optimal performance, Logtalk requires that the Prolog compiler
supports 'first-argument indexing' for both static and dynamic code
(most modern compilers support this feature).

Since most Prolog compilers are moving closer to the ISO Prolog standard
[ISO95], it is advisable that you try to use the most recent
version of your favorite Prolog compiler.

1.20.2 Logtalk installers
-------------------------

Logtalk installers are available for macOS, Linux, and Microsoft
Windows.  Depending on the chosen installer, some tasks (e.g.  setting
environment variables or integrating Logtalk with some Prolog compilers)
may need to be performed manually.

1.20.3 Source distribution
--------------------------

Logtalk sources are available in a ‘tar’ archive compressed with
‘bzip2’, ‘lgt3xxx.tar.bz2’.  You may expand the archive by using a
decompressing utility or by typing the following commands at the
command-line:

     % tar -jxvf lgt3xxx.tar.bz2

This will create a sub-directory named ‘lgt3xxx’ in your current
directory.  Almost all files in the Logtalk distribution are text files.
Different operating-systems use different end-of-line codes for text
files.  Ensure that your decompressing utility converts the end-of-lines
of all text files to match your operating system.

1.20.4 Distribution overview
----------------------------

In the Logtalk installation directory, you will find the following files
and directories:

‘ACKNOWLEDGMENTS.md’ - List of authors, contributors, sponsors, and open
source credits

‘BIBLIOGRAPHY.bib’ – Logtalk bibliography in BibTeX format

‘CITATION.cff’ - Information on how to cite Logtalk

‘CODE_OF_CONDUCT.md’ - Code of conduct for contributors and users
posting on support forums

‘CUSTOMIZE.md’ – Logtalk end-user customization instructions

‘INSTALL.md’ – Logtalk installation instructions

‘LICENSE.txt’ – Logtalk user license

‘NOTICE.txt’ – Logtalk copyright notice

‘QUICK_START.md’ – Quick start instructions for those that do not like
to read manuals

‘README.md’ – several useful information

‘RELEASE_NOTES.md’ – release notes for this version

‘UPGRADING.md’ – instructions on how to upgrade your programs to the
current Logtalk version

‘VERSION.txt’ – file containing the current Logtalk version number (used
for compatibility checking when upgrading Logtalk)

‘loader-sample.lgt’ – sample loader file for user applications

‘settings-sample.lgt’ – sample file for user-defined Logtalk settings

‘tester-sample.lgt’ – sample file for helping to automate running user
application unit tests

‘adapters’

     ‘NOTES.md’ – notes on the provided adapter files ‘template.pl’ –
     template adapter file ‘...’ – specific adapter files

‘coding’

     ‘NOTES.md’ – notes on syntax highlighter and text editor support
     files providing syntax coloring for publishing and editing Logtalk
     source code ‘...’ – syntax coloring support files

‘contributions’

     ‘NOTES.md’ – notes on the user-contributed code ‘...’ –
     user-contributed code files

‘core’

     ‘NOTES.md’ – notes on the current status of the compiler and
     runtime ‘...’ – core source files

‘docs’

     ‘NOTES.md’ – notes on the provided documentation for core, library,
     tools, and contributions entities ‘index.html’ – root document for
     all entities documentation ‘...’ – other entity documentation files

‘examples’

     ‘NOTES.md’ – short description of the provided examples

     ‘bricks’

          ‘NOTES.md’ – example description and other notes ‘SCRIPT.txt’
          – step by step example tutorial ‘loader.lgt’ – loader utility
          file for the example objects ‘...’ – bricks example source
          files

     ‘...’ – other examples

‘integration’

     ‘NOTES.md’ – notes on scripts for Logtalk integration with Prolog
     compilers ‘...’ – Prolog integration scripts

‘library’

     ‘NOTES.md’ – short description of the library contents
     ‘all_loader.lgt’ – loader utility file for all library entities
     ‘...’ – library source files

‘man’

     ‘...’ – POSIX man pages for the shell scripts

‘manuals’

     ‘NOTES.md’ – notes on the provided documentation
     ‘bibliography.html’ – bibliography ‘glossary.html’ – glossary
     ‘index.html’ – root document for all documentation ‘...’ – other
     documentation files

‘paths’

     ‘NOTES.md’ – description on how to setup library and examples paths
     ‘paths.pl’ – default library and example paths

‘ports’

     ‘NOTES.md’ – description of included ports of third-party software
     ‘...’ – ports

‘scratch’

     ‘NOTES.md’ – notes on the scratch directory

‘scripts’

     ‘NOTES.md’ – notes on scripts for Logtalk user setup, packaging,
     and installation ‘...’ – packaging, installation, and setup scripts

‘tests’

     ‘NOTES.md’ – notes on the current status of the unit tests ‘...’ –
     unit tests for built-in features

‘tools’

     ‘NOTES.md’ – notes on the provided programming tools ‘...’ –
     programming tools

1.20.4.1 Adapter files
......................

Adapter files provide the glue code between the Logtalk compiler/runtime
and a Prolog compiler.  Each adapter file contains two sets of
predicates: ISO Prolog standard predicates and directives not built-in
in the target Prolog compiler and Logtalk specific predicates.

Logtalk already includes ready to use adapter files for most academic
and commercial Prolog compilers.  If an adapter file is not available
for the compiler that you intend to use, then you need to build a new
one, starting from the included ‘template.pl’ file.  Start by making a
copy of the template file.  Carefully check (or complete if needed) each
listed definition.  If your Prolog compiler conforms to the ISO
standard, this task should only take you a few minutes.  In most cases,
you can borrow code from the predefined adapter files.  If you are
unsure that your Prolog compiler provides all the ISO predicates needed
by Logtalk, try to run the system by setting the unknown predicate error
handler to report as an error any call to a missing predicate.  Better
yet, switch to a modern, ISO compliant, Prolog compiler.  If you send me
your adapter file, with a reference to the target Prolog compiler, maybe
I can include it in the next release of Logtalk.

The adapter files specify 'default' values for most of the Logtalk
compiler flags.  They also specify values for read-only flags that
are used to describe Prolog backend specific features.

1.20.4.2 Compiler and runtime
.............................

The ‘core’ sub-directory contains the Prolog and Logtalk source files
that implement the Logtalk compiler and the Logtalk runtime.  The
compiler and the runtime may be split in two (or more) separate files or
combined in a single file, depending on the Logtalk release that you are
installing.

1.20.4.3 Library
................

The Logtalk distribution includes a standard library of useful objects,
categories, and protocols.  Read the corresponding ‘NOTES.md’ file for
details about the library contents.

1.20.4.4 Examples
.................

The Logtalk distribution includes a large number of programing examples.
The sources of each one of these examples can be found included in a
subdirectory with the same name, inside the directory examples.  The
majority of these examples include tests and a file named ‘SCRIPT.txt’
with sample calls.  Some examples may depend on other examples and
library objects to work properly.  Read the corresponding ‘NOTES.md’
file for details before running an example.

1.20.4.5 Logtalk source files
.............................

Logtalk source files are text files containing one or more entity
definitions (objects, categories, or protocols).  The Logtalk source
files may also contain plain Prolog code.  The extension ‘.lgt’ is
normally used.  Logtalk compiles these files to plain Prolog by
appending to the file name a suffix derived from the extension and by
replacing the ‘.lgt’ extension with ‘.pl’ (‘.pl’ is the default Prolog
extension; if your Prolog compiler expects the Prolog source filenames
to end with a specific, different extension, you can set it in the
corresponding adapter file).

1.21 Prolog integration and migration
=====================================

This section provides suggestions for integrating and migrating plain
Prolog code and Prolog module code to Logtalk.  Detailed instructions
are provided for encapsulating plain Prolog code in objects, converting
Prolog modules into objects, and compiling and reusing Prolog modules as
objects from inside Logtalk.  An interesting application of the
techniques described in this section is a solution for running a Prolog
application which uses modules on a Prolog compiler with no module
system.  The ‘wrapper’ tool can be used to help in migrating Prolog
code.

1.21.1 Source files with both Prolog code and Logtalk code
----------------------------------------------------------

Logtalk source files may contain plain Prolog code intermixed with
Logtalk code.  The Logtalk compiler simply copies the plain Prolog code
as-is to the generated Prolog file.  With Prolog modules, it is assumed
that the module code starts with a ‘module/1-2’ directive and ends at
the end of the file.  There is no module ending directive which would
allowed us to define more than one module per file.  In fact, most if
not all Prolog module systems always define a single module per file.
Some of them mandate that the ‘module/1-2’ directive be the first term
on a source file.  As such, when the Logtalk compiler finds a
‘module/1-2’ directive, it assumes that all code that follows until the
end of the file belongs to the module.

1.21.2 Encapsulating plain Prolog code in objects
-------------------------------------------------

Most applications consist of several plain Prolog source files, each one
defining a few top-level predicates and auxiliary predicates that are
not meant to be directly called by the user.  Encapsulating plain Prolog
code in objects allows us to make clear the different roles of each
predicate, to hide implementation details, to prevent auxiliary
predicates from being called outside the object, and to take advantage
of Logtalk advanced code encapsulating and reusing features.  It also
simplifies using its developer tools.

Encapsulating Prolog code using Logtalk objects is simple.  First, for
each source file, add an opening object directive, see object/1-5,
to the beginning of the file and an ending object directive, see
end_object/0, to the end of the file.  Choose an object name that
reflects the purpose of source file code (this is a good opportunity for
code refactoring if necessary).  Second, add public/1.
predicate directives for the top-level predicates that are used directly
by the user or called from other source files.  Third, we need to be
able to call from inside an object predicates defined in other source
files/objects.  The easiest solution, which has the advantage of not
requiring any changes to the predicate definitions, is to use the
uses/2 directive.  If your Prolog compiler supports
cross-referencing tools, you may use them to help you make sure that all
calls to predicates on other source files/objects are listed in the
uses/2 directives.  The Logtalk ‘wrapper’ tool can also help
in detecting cross predicate calls.  Compiling the resulting objects
with the Logtalk unknown_predicates and portability
flags set to ‘warning’ will help you identify calls to predicates
defined on other converted source files and possible portability issues.

1.21.2.1 Prolog multifile predicates
....................................

Prolog 'multifile' predicates are used when clauses for the same
predicate are spread among several source files.  When encapsulating
plain Prolog code that uses multifile predicates, is often the case that
the clauses of the multifile predicates get spread between different
objects and categories but conversion is straight-forward.  In the
Logtalk object (or category) holding the multifile predicate
primary declaration, add a predicate scope directive.
and a multifile/1 directive.  In all other objects (or
categories) defining clauses for the multifile predicate, add a
‘multifile/1’ directive and predicate clauses using the format:

     :- multifile(Entity::Name/Arity).

     Entity::Functor(...) :-
         ...

See the section on the ‘multifile/1’ predicate directive for
more information.  An alternative solution is to simply keep the clauses
for the multifile predicates as plain Prolog code and define, if
necessary, a parametric object to encapsulate all predicates working
with the multifile predicate clauses.  For example, assume the following
‘multifile/1’ directive:

     % city(Name, District, Population, Neighbors)
     :- multifile(city/4).

We can define a parametric object with ‘city/4’ as its identifier:

     :- object(city(_Name, _District, _Population, _Neighbors)).

         % predicates for working with city/4 clauses

     :- end_object.

This solution is preferred when the multifile predicates are used to
represent large tables of data.  See the section on Parametric objects
for more details.

1.21.3 Converting Prolog modules into objects
---------------------------------------------

Converting Prolog modules into objects may allow an application to run
on a wider range of Prolog compilers, overcoming portability problems.
Some Prolog compilers don’t support a module system.  Among those Prolog
compilers which support a module system, the lack of standardization
leads to several issues, specially with semantics, operators, and
meta-predicates.  In addition, the conversion allows you to take
advantage of Logtalk more powerful abstraction and reuse mechanisms such
as separation between interface from implementation, inheritance,
parametric objects, and categories.  It also allows you to take full
advantage of Logtalk developer tools for improved productivity.

Converting a Prolog module into an object is simplified when the
directives used in the module are supported by Logtalk (see the listing
in the next section).  Assuming that this is the case, apply the
following steps:

  1. Convert the module ‘module/1’ directive into an object/1.
     opening object directive, using the module name as the object name.
     For ‘module/2’ directives apply the same conversion and convert the
     list of exported predicates into public/1 predicate
     directives.  Add a closing object directive, see end_object/0,
     at the end of the source code.

  2. Convert any ‘export/1’ directives into ‘public/1’ predicate
     directives.

  3. Convert any ‘use_module/1’ directives for modules that will not be
     converted to objects into ‘use_module/2’ directives (see next
     section), replacing the file spec in the first argument with the
     module name.

  4. Convert any ‘use_module/1-2’ directives referencing other modules
     also being converted to objects into Logtalk uses/2.
     directives.

  5. Convert each ‘reexport/1’ directive into a uses/2.
     directive and ‘public/1’ predicate directives (see next section).

  6. Convert any ‘meta_predicate/1’ directives into Logtalk
     meta_predicate/1 directives by replacing the module
     meta-argument indicator, ‘:’, with the Logtalk meta-argument
     indicator ‘0’ for goal meta-arguments.  For closure meta-arguments,
     use an integer denoting the number of additional arguments that
     will be appended to construct a goal.  Arguments which are not
     meta-arguments are represented by the ‘*’ character.  Do not use
     argument mode indicators such as ‘?’, or ‘+’, or ‘-’ as Logtalk
     supports mode directives.

  7. Convert any explicit qualified calls to module predicates to
     messages by replacing the ‘(:)/2’ operator with the (;;)/2
     message sending operator when the referenced modules are also
     being converted into objects.  Calls in the pseudo-module ‘user’
     can be encapsulated using the {}/1. Logtalk external
     call control construct.  You can also use instead a uses/2
     directive where the first argument would be the atom ‘user’ and
     the second argument a list of all external predicates.  This
     alternative has the advantages of not requiring changes to the code
     making the predicate calls and of better visibility for the
     documenting and diagramming tools.

  8. If your module uses the database built-in predicates to implement
     module local mutable state using dynamic predicates, add both
     private/1 and dynamic/1 directives for each
     dynamic predicate.

  9. If your module declares or defines clauses for multifile module
     predicates, replace the ‘(:)/2’ functor by ‘(::)/2’ in the
     ‘multifile/1’ directives and in the clause heads for all modules
     defining the multifile predicates that are also being converted
     into objects; if that is not the case, just keep the ‘multifile/1’
     directives and the clause heads as-is).

  10. Compile the resulting objects with the Logtalk
     unknown_predicates, and portability flags set to
     ‘warning’ to help you locate possible issues and calls to
     proprietary Prolog built-in predicates and to predicates defined on
     other converted modules.  In order to improve code portability,
     check the Logtalk library for possible alternatives to the use of
     proprietary Prolog built-in predicates.

Before converting your modules to objects, you may try to compile them
first as objects (using the logtalk_compile/1. Logtalk
built-in predicates) to help identify any issues that must be dealt with
when doing the conversion to objects.  Note that Logtalk supports
compiling Prolog files as Logtalk source code without requiring changes
to the file name extensions.

1.21.4 Compiling Prolog modules as objects
------------------------------------------

A possible alternative to port Prolog code to Logtalk is to compile the
Prolog source files using the ‘logtalk_load/1-2’ and
‘logtalk_compile/1-2’ predicates.  The Logtalk compiler provides partial
support for compiling Prolog modules as Logtalk objects.  This support
may allow using modules from a backend Prolog system in a different
backend Prolog system although its main purpose is to help in porting
existing Prolog code to Logtalk in order to benefit from its extended
language features and its developer tools.  Why partial support?
Although there is a ISO Prolog standard for modules, it is (rightfully)
ignored by most implementers and vendors (due to its flaws and deviation
from common practice).  In addition, there is no de facto standard for
module systems, despite otherwise frequent misleading claims.  Key
system differences include the set of implemented module directives, the
directive semantics, the handling of operators, the locality of flags,
and on the integration of term-expansion mechanisms (when provided).
Another potential issue is that, when compiling modules as objects,
Logtalk assumes that any referenced module (e.g.  using ‘use_module/1-2’
directives) is also being compiled as an object.  If that’s not the
case, the compiled module calls being compiled as message sending goals
will still work for normal predicates but will not work for
meta-predicates called using implicit module qualification.  The reason
is that, unlike in Logtalk, calls to implicitly and explicitly qualified
module meta-predicates have different semantics.  Follows a discussion
of other limitations of this approach that you should be aware.

1.21.4.1 Supported module directives
....................................

Currently, Logtalk supports the following module directives:

‘module/1’

     The module name becomes the object name.

‘module/2’

     The module name becomes the object name.  The exported predicates
     become public object predicates.  The exported grammar rule
     non-terminals become public grammar rule non-terminals.  The
     exported operators become public object operators but are not
     active elsewhere when loading the code.

‘use_module/2’

     This directive is compiled as a Logtalk uses/2 directive
     in order to ensure correct compilation of the module predicate
     clauses.  The first argument of this directive must be the module
     'name' (an atom), not a module file specification (the adapter
     files attempt to use the Prolog dialect level term-expansion
     mechanism to find the module name from the module file
     specification).  Note that the module is not automatically loaded
     by Logtalk (as it would be when compiling the directive using
     Prolog instead of Logtalk; the programmer may also want the
     specified module to be compiled as an object).  The second argument
     must be a predicate indicator (‘Name/Arity’), a grammar rule
     non-terminal indicator (‘Name//Arity’), a operator declaration, or
     a list of predicate indicators, grammar rule non-terminal
     indicators, and operator declarations.  Predicate aliases can be
     declared using the notation ‘Name/Arity as Alias/Arity’ or, in
     alternative, the notation ‘Name/Arity:Alias/Arity’.  Similar for
     non-terminal aliases.

‘export/1’

     Exported predicates are compiled as public object predicates.  The
     argument must be a predicate indicator (‘Name/Arity’), a grammar
     rule non-terminal indicator (‘Name//Arity’), an operator
     declaration, or a list of predicate indicators, grammar rule
     non-terminal indicators, and operator declarations.

‘reexport/2’

     Reexported predicates are compiled as public object predicates.
     The first argument is the module name.  The second argument must be
     a predicate indicator (‘Name/Arity’), a grammar rule non-terminal
     indicator (‘Name//Arity’), an operator declaration, or a list of
     predicate indicators, grammar rule non-terminal indicators, and
     operator declarations.  Predicate aliases can be declared using the
     notation ‘Name/Arity as Alias/Arity’ or, in alternative, the
     notation ‘Name/Arity:Alias/Arity’.  Similar for non-terminal
     aliases.

‘meta_predicate/1’

     Module meta-predicates become object meta-predicates.  Only
     predicate arguments marked as goals or closures. (using
     an integer) are interpreted as meta-arguments.  In addition, Prolog
     module meta-predicates and Logtalk meta-predicates don’t share the
     same explicit-qualification calling semantics: in Logtalk,
     meta-arguments are always called in the context of the 'sender'.
     Logtalk expects ‘meta-predicate/1’ directives for all
     meta-predicates as it is not based on the predicate-prefixing
     mechanism common to most Prolog module systems.

A common issue when compiling modules as objects is the use of the atoms
‘dynamic’, ‘discontiguous’, and ‘multifile’ as operators in directives.
For better portability avoid this usage.  For example, write:

     :- dynamic([foo/1, bar/2]).

instead of:

     :- dynamic foo/1, bar/2.

Another common issue is missing ‘meta_predicate/1’, ‘dynamic/1’,
‘discontiguous/1’, and ‘multifile/1’ predicate directives.  The Logtalk
compiler supports detection of missing directives (by setting its
missing_directives flag to ‘warning’).

When compiling modules as objects, you probably don’t need event support
turned on.  You may use the events compiler flag to ‘deny’
with the Logtalk compiling and loading built-in methods for a small
performance gain for the compiled code.

1.21.4.2 Unsupported module directives
......................................

The ‘reexport/1’ and ‘use_module/1’ directives are not directly
supported by the Logtalk compiler.  But most Prolog adapter files
provide support for compiling these directives using Logtalk first stage
of its term-expansion mechanism.  Nevertheless, these
directives can be converted, respectively, into a sequence of ‘:-
use_module/2’ and ‘export/1’ directives and ‘use_module/2’ directives by
finding which predicates exported by the specified modules are
reexported or imported into the module containing the directive.  For
‘use_module/1’ directives, finding the names of the imported predicates
that are actually used is easy.  First, comment out the directive and
compile the file (making sure that the unknown_predicates.
compiler flag is set to ‘warning’).  Logtalk will print a warning with a
list of predicates that are called but never defined.  Second, use this
list to replace the ‘use_module/1’ directives by ‘use_module/2’
directives.  You should then be able to compile the modified Prolog
module as an object.

1.21.4.3 Modules using a term-expansion mechanism
.................................................

Although Logtalk supports term and goal expansion mechanisms,
the usage semantics are different from similar mechanisms found in some
Prolog compilers.  In particular, Logtalk does not support defining term
and goal expansions clauses in a source file for expanding the source
file itself.  Logtalk forces a clean separation between expansions
clauses and the source files that will be subject to source-to-source
expansions by using hook objects.  But hook objects also
provide a working solution here when the expansion code is separated
from the code to be expanded.  Logtalk supports using a module as a hook
object as long as its name doesn’t coincide with the name of an object
and that the module uses ‘term_expansion/2’ and ‘goal_expansion/2’
predicates.  Assuming that’s the case, before attempting to compile the
modules as objects, set the default hook object is to the module
containing the expansion code.  For example, if the expansions stored in
a ‘system’ module:

     | ?- set_logtalk_flag(hook, system).
     ...

This, however, may not be enough as expansions may be stored in multiple
modules.  A common example is to use a module named ‘prolog’ for system
expansions and to store the user-defined expansions in ‘user’.  The
Logtalk library provides a solution for these scenarios.  Using the
‘hook_flows’ library we can select multiple hook objects or hook
modules.  For example, assuming expansions stored on both ‘user’ and
‘system’ modules:

     | ?- logtalk_load(hook_flows(loader)).
     ...

     | ?- set_logtalk_flag(hook, hook_set([user, system])).
     ...

After these queries, we can try to compile the modules and look for
other porting or portability issues.  A well know issue is Prolog module
term-expansions calling predicates such as ‘prolog_load_context/2’,
which will always fail when it’s the Logtalk compiler instead of the
Prolog compiler loading a source file.  In some of these cases, it may
be possible to rewrite the expansion rules to use the
logtalk_load_context/2 predicate instead.

1.21.4.4 File search paths
..........................

Some Prolog systems provide a mechanism for defining file search paths
(this mechanism works differently from Logtalk own suport for defining
library path aliases).  When porting Prolog code that defines file
search paths, e.g.  for finding module libraries, it often helps to load
the pristine Prolog application before attempting to compile its source
files as Logtalk source files.  Depending on the Prolog backend, this
may allow the file search paths to be used when compiling modules as
objects that use file directives such as ‘use_module/2’.

1.21.5 Dealing with proprietary Prolog directives and predicates
----------------------------------------------------------------

Most Prolog compilers define proprietary, non-standard, directives and
predicates that may be used in both plain code and module code.
Non-standard Prolog built-in predicates are usually not problematic, as
Logtalk is usually able to identify and compile them correctly (but see
the notes on built-in meta-predicates for possible caveats).  However,
Logtalk will generate compilation errors on source files containing
proprietary directives unless you first specify how the directives
should be handled.  Several actions are possible on a per-directive
basis: ignoring the directive (i.e.  do not copy the directive, although
a goal can be proved as a consequence), rewriting and copy the directive
to the generated Prolog files, or rewriting and recompiling the
resulting directive.  To specify these actions, the adapter files
contain clauses for the internal ‘'$lgt_prolog_term_expansion'/2’
predicate.  For example, assume that a given Prolog compiler defines a
‘comment/2’ directive for predicates using the format:

     :- comment(foo/2, "Brief description of the predicate").

We can rewrite this predicate into a Logtalk ‘info/2’ directive by
defining a suitable clause for the ‘'$lgt_prolog_term_expansion'/2’
predicate:

     '$lgt_prolog_term_expansion'(
             (:- comment(F/A, String)),
             (:- info(F/A, [comment is Atom]))
     ) :-
         atom_codes(Atom, String).

This Logtalk feature can be used to allow compilation of legacy Prolog
code without the need of changing the sources.  When used, is advisable
to set the portability compiler flag to ‘warning’ in order
to more easily identify source files that are likely non-portable across
Prolog compilers.

A second example, where a proprietary Prolog directive is discarded
after triggering a side effect:

     '$lgt_prolog_term_expansion'(
             (:- load_foreign_files(Files,Libs,InitRoutine)),
             []
     ) :-
         load_foreign_files(Files,Libs,InitRoutine).

In this case, although the directive is not copied to the generated
Prolog file, the foreign library files are loaded as a side effect of
the Logtalk compiler calling the ‘'$lgt_prolog_term_expansion'/2’ hook
predicate.

1.21.6 Calling Prolog module predicates
---------------------------------------

Prolog module predicates can be called from within objects or categories
by simply using explicit module qualification, i.e.  by writing
‘Module:Goal’ or ‘Goal@Module’ (depending on the module system).
Logtalk also supports the use of ‘use_module/2’ directives in object and
categories (with the restriction that the first argument of the
directive must be the actual module name and not the module file name or
the module file path).  In this case, these directives are parsed in a
similar way to Logtalk uses/2 directives, with calls to the
specified module predicates being automatically translated to
‘Module:Goal’ calls.

As a general rule, the Prolog modules should be loaded (e.g.  in the
auxiliary Logtalk loader files) 'before' compiling objects that make use
of module predicates.  Moreover, the Logtalk compiler does not generate
code for the automatic loading of modules referenced in ‘use_module/1-2’
directives.  This is a consequence of the lack of standardization of
these directives, whose first argument can be a module name, a straight
file name, or a file name using some kind of library notation, depending
on the backend Prolog compiler.  Worse, modules are sometimes
defined in files with names different from the module names requiring
finding, opening, and reading the file in order to find the actual
module name.

Logtalk supports the declaration of predicate aliases and
predicate shorthands in ‘use_module/2’ directives used
within object and categories.  For example, the ECLiPSe IC Constraint
Solvers define a ‘(::)/2’ variable domain operator that clashes with the
Logtalk ‘(::)/2’ message sending operator.  We can solve the conflict by
writing:

     :- use_module(ic, [(::)/2 as ins/2]).

With this directive, calls to the ‘ins/2’ predicate alias will be
automatically compiled by Logtalk to calls to the ‘(::)/2’ predicate in
the ‘ic’ module.

Logtalk allows you to send a message to a module in order to call one of
its predicates.  This is usually not advised as it implies a performance
penalty when compared to just using the ‘Module:Call’ notation.
Moreover, this works only if there is no object with the same name as
the module you are targeting.  This feature is necessary, however, in
order to properly support compilation of modules containing
‘use_module/2’ directives as objects.  If the modules specified in the
‘use_module/2’ directives are not compiled as objects but are instead
loaded as-is by Prolog, the exported predicates would need to be called
using the ‘Module:Call’ notation but the converted module will be
calling them through message sending.  Thus, this feature ensures that,
on a module compiled as an object, any predicate calling other module
predicates will work as expected either these other modules are loaded
as-is or also compiled as objects.

For more details, see the Calling Prolog predicates section.

1.21.7 Loading converted Prolog applications
--------------------------------------------

Logtalk strongly favors and advises users to provide a main loader file
for applications that explicitly load any required libraries
and the application source files.  In contrast, Prolog applications
often either scatter loading of source files from multiple files or use
implicit loading of source files via ‘use_module/1-2’ directives.  Due
to this frequent ad-hoc approach, it’s common to find Prolog
applications with duplicated loading directives and are loading order
ignores the dependencies between source files.  These issues are easily
exposed by the Logtalk linter when compiling Prolog files as Logtalk
files.  Also common are Prolog files with multiple circular
dependencies.  While this should not affect the 'semantics' of the
ported code, it may cause some performance penalties as it prevents the
Logtalk compiler of optimizing the message sending goals using
static-binding.  It also makes the application architecture more
difficult to understand.  The definition of explicit loader files
provides a good opportunity of sorting out loading order and circular
dependencies, with the linter warnings providing hints for possible code
refactoring to eliminate these issues.  The diagrams tool
supports directory and file loading and dependency diagrams that are
also useful in understanding applications architecture.

2 Reference Manual
******************

2.1 Grammar
===========

The Logtalk grammar is here described using W3C-style Extended
Backus-Naur Form syntax.  Non-terminal symbols not defined here can be
found in the ISO Prolog Core standard.  Terminal symbols are represented
between double-quotes.

2.1.1 Entities
--------------

     entity ::=
        object
        | category
        | protocol

2.1.2 Object definition
-----------------------

     object ::=
        begin_object_directive ( object_term )* end_object_directive

     begin_object_directive ::=
        ":- object(" object_identifier ( "," object_relations )? ")."

     end_object_directive ::=
        ":- end_object."

     object_relations ::=
        prototype_relations
        | non_prototype_relations

     prototype_relations ::=
        prototype_relation
        | prototype_relation "," prototype_relations

     prototype_relation ::=
        implements_protocols
        | imports_categories
        | extends_objects

     non_prototype_relations ::=
        non_prototype_relation
        | non_prototype_relation "," non_prototype_relations

     non_prototype_relation ::=
        implements_protocols
        | imports_categories
        | instantiates_classes
        | specializes_classes

2.1.3 Category definition
-------------------------

     category ::=
        begin_category_directive  ( category_term )* end_category_directive

     begin_category_directive ::=
        ":- category(" category_identifier ( "," category_relations )? ")."

     end_category_directive ::=
        ":- end_category."

     category_relations ::=
        category_relation
        | category_relation "," category_relations

     category_relation ::=
        implements_protocols
        | extends_categories
        | complements_objects

2.1.4 Protocol definition
-------------------------

     protocol ::=
        begin_protocol_directive  ( protocol_directive )* end_protocol_directive

     begin_protocol_directive ::=
        ":- protocol(" protocol_identifier ( "," extends_protocols)? ")."

     end_protocol_directive ::=
        ":- end_protocol."

2.1.5 Entity relations
----------------------

     extends_protocols ::=
        "extends(" extended_protocols ")"

     extends_objects ::=
        "extends(" extended_objects ")"

     extends_categories ::=
        "extends(" extended_categories ")"

     implements_protocols ::=
        "implements(" implemented_protocols ")"

     imports_categories ::=
        "imports(" imported_categories ")"

     instantiates_classes ::=
        "instantiates(" instantiated_objects ")"

     specializes_classes ::=
        "specializes(" specialized_objects ")"

     complements_objects ::=
        "complements(" complemented_objects ")"

2.1.5.1 Implemented protocols
.............................

     implemented_protocols ::=
        implemented_protocol
        | implemented_protocol_sequence
        | implemented_protocol_list

     implemented_protocol ::=
        protocol_identifier
        | scope "::" protocol_identifier

     implemented_protocol_sequence ::=
        implemented_protocol
        | implemented_protocol "," implemented_protocol_sequence

     implemented_protocol_list ::=
        "[" implemented_protocol_sequence "]"

2.1.5.2 Extended protocols
..........................

     extended_protocols ::=
        extended_protocol
        | extended_protocol_sequence
        | extended_protocol_list

     extended_protocol ::=
        protocol_identifier
        | scope "::" protocol_identifier

     extended_protocol_sequence ::=
        extended_protocol
        |extended_protocol "," extended_protocol_sequence

     extended_protocol_list ::=
        "[" extended_protocol_sequence "]"

2.1.5.3 Imported categories
...........................

     imported_categories ::=
        imported_category
        | imported_category_sequence
        | imported_category_list

     imported_category ::=
        category_identifier
        | scope "::" category_identifier

     imported_category_sequence ::=
        imported_category
        | imported_category "," imported_category_sequence

     imported_category_list ::=
        "[" imported_category_sequence "]"

2.1.5.4 Extended objects
........................

     extended_objects ::=
        extended_object
        | extended_object_sequence
        | extended_object_list

     extended_object ::=
        object_identifier
        | scope "::" object_identifier

     extended_object_sequence ::=
        extended_object
        | extended_object "," extended_object_sequence

     extended_object_list ::=
        "[" extended_object_sequence "]"

2.1.5.5 Extended categories
...........................

     extended_categories ::=
        extended_category
        | extended_category_sequence
        | extended_category_list

     extended_category ::=
        category_identifier
        | scope "::" category_identifier

     extended_category_sequence ::=
        extended_category
        | extended_category "," extended_category_sequence

     extended_category_list ::=
        "[" extended_category_sequence "]"

2.1.5.6 Instantiated objects
............................

     instantiated_objects ::=
        instantiated_object
        | instantiated_object_sequence
        | instantiated_object_list

     instantiated_object ::=
        object_identifier
        | scope "::" object_identifier

     instantiated_object_sequence ::=
        instantiated_object
        | instantiated_object "," instantiated_object_sequence

     instantiated_object_list ::=
        "[" instantiated_object_sequence "]"

2.1.5.7 Specialized objects
...........................

     specialized_objects ::=
        specialized_object
        | specialized_object_sequence
        | specialized_object_list

     specialized_object ::=
        object_identifier
        | scope "::" object_identifier

     specialized_object_sequence ::=
        specialized_object
        | specialized_object "," specialized_object_sequence

     specialized_object_list ::=
        "[" specialized_object_sequence "]"

2.1.5.8 Complemented objects
............................

     complemented_objects ::=
        object_identifier
        | complemented_object_sequence
        | complemented_object_list

     complemented_object_sequence ::=
        object_identifier
        | object_identifier "," complemented_object_sequence

     complemented_object_list ::=
        "[" complemented_object_sequence "]"

2.1.5.9 Entity and predicate scope
..................................

     scope ::=
        "public"
        | "protected"
        | "private"

2.1.6 Entity identifiers
------------------------

     entity_identifier ::=
        object_identifier
        | protocol_identifier
        | category_identifier

2.1.6.1 Object identifiers
..........................

     object_identifier ::=
        atom
        | compound

2.1.6.2 Category identifiers
............................

     category_identifier ::=
        atom
        | compound

2.1.6.3 Protocol identifiers
............................

     protocol_identifier ::=
        atom

2.1.6.4 Module identifiers
..........................

     module_identifier ::=
        atom

2.1.7 Source files
------------------

     source_file ::=
        ( source_file_content )*

     source_file_content ::=
        source_file_directive
        | clause
        | grammar_rule
        | entity

2.1.8 Source file names
-----------------------

     source_file_name ::=
        atom
        | library_source_file_name

     library_source_file_name ::=
        library_name "(" atom ")"

     library_name ::=
        atom

2.1.9 Terms
-----------

2.1.9.1 Object terms
....................

     object_term ::=
        object_directive
        | clause
        | grammar_rule

2.1.9.2 Category terms
......................

     category_term ::=
        category_directive
        | clause
        | grammar_rule

2.1.10 Directives
-----------------

2.1.10.1 Source file directives
...............................

     source_file_directive ::=
        ":- encoding(" atom ")."
        | ":- set_logtalk_flag(" atom "," nonvar ")."
        | ":- include(" source_file_name ")."
        | prolog_directive

2.1.10.2 Conditional compilation directives
...........................................

     conditional_compilation_directive ::=
        ":- if(" callable ")."
        | ":- elif(" callable ")."
        | ":- else."
        | ":- endif."

2.1.10.3 Object directives
..........................

     object_directive ::=
        ":- initialization(" callable ")."
        | ":- built_in."
        | ":- threaded."
        | ":- dynamic."
        | ":- info(" entity_info_list ")."
        | ":- set_logtalk_flag(" atom "," nonvar ")."
        | ":- include(" source_file_name ")."
        | ":- uses(" object_alias_list ")."
        | ":- use_module(" module_alias_list ")."
        | conditional_compilation_directive
        | predicate_directive

2.1.10.4 Category directives
............................

     category_directive ::=
        ":- built_in."
        | ":- dynamic."
        | ":- info(" entity_info_list ")."
        | ":- set_logtalk_flag(" atom "," nonvar ")."
        | ":- include(" source_file_name ")."
        | ":- uses(" object_alias_list ")."
        | ":- use_module(" module_alias_list ")."
        | conditional_compilation_directive
        | predicate_directive

2.1.10.5 Protocol directives
............................

     protocol_directive ::=
        ":- built_in."
        | ":- dynamic."
        | ":- info(" entity_info_list ")."
        | ":- set_logtalk_flag(" atom "," nonvar ")."
        | ":- include(" source_file_name ")."
        | conditional_compilation_directive
        | predicate_directive

2.1.10.6 Predicate directives
.............................

     predicate_directive ::=
        alias_directive
        | synchronized_directive
        | uses_directive
        | use_module_directive
        | scope_directive
        | mode_directive
        | meta_predicate_directive
        | meta_non_terminal_directive
        | info_directive
        | dynamic_directive
        | discontiguous_directive
        | multifile_directive
        | coinductive_directive
        | operator_directive

     alias_directive ::=
        ":- alias(" entity_identifier "," alias_directive_resource_list ")."

     synchronized_directive ::=
        ":- synchronized(" synchronized_directive_resource_term ")."

     uses_directive ::=
        ":- uses(" ( object_identifier | parameter_variable ) "," uses_directive_resource_list ")."

     use_module_directive ::=
        ":- use_module(" ( module_identifier | parameter_variable ) "," use_module_directive_resource_list ")."

     scope_directive ::=
        ":- public(" scope_directive_resource_term ")."
        | ":- protected(" scope_directive_resource_term ")."
        | ":- private(" scope_directive_resource_term ")."

     mode_directive ::=
        ":- mode(" ( predicate_mode_term | non_terminal_mode_term ) "," number_of_proofs ")."

     meta_predicate_directive ::=
        ":- meta_predicate(" meta_predicate_template_term ")."

     meta_non_terminal_directive ::=
        ":- meta_non_terminal(" meta_non_terminal_template_term ")."

     info_directive ::=
        ":- info(" ( predicate_indicator | non_terminal_indicator ) "," predicate_info_list ")."

     dynamic_directive ::=
        ":- dynamic(" qualified_directive_resource_term ")."

     discontiguous_directive ::=
        ":- discontiguous(" qualified_directive_resource_term ")."

     multifile_directive ::=
        ":- multifile(" qualified_directive_resource_term ")."

     coinductive_directive ::=
        ":- coinductive(" ( predicate_indicator_term | coinductive_predicate_template_term ) ")."

     parameter_variable ::=
        _variable_

     scope_directive_resource_term ::=
        scope_directive_resource
        | scope_directive_resource_sequence
        | scope_directive_resource_list

     scope_directive_resource ::=
        predicate_indicator
        | non_terminal_indicator
        | operator

     scope_directive_resource_sequence ::=
        scope_directive_resource
        | scope_directive_resource "," scope_directive_resource_sequence

     scope_directive_resource_list ::=
          "[" scope_directive_resource_sequence "]"

     entity_resources_list ::=
        predicate_indicator_list
        | operator_list

     predicate_indicator_term ::=
        predicate_indicator
        | predicate_indicator_sequence
        | predicate_indicator_list

     predicate_indicator_sequence ::=
        predicate_indicator
        | predicate_indicator "," predicate_indicator_sequence

     predicate_indicator_list ::=
        "[" predicate_indicator_sequence "]"

     alias_directive_resource_list ::=
        "[" alias_directive_resource_sequence "]"

     alias_directive_resource_sequence ::=
        alias_directive_resource
        | alias_directive_resource "," alias_directive_resource_sequence

     alias_directive_resource ::=
        predicate_indicator_alias
        | non_terminal_indicator_alias

     synchronized_directive_resource_term ::=
        synchronized_directive_resource
        | synchronized_directive_resource_sequence
        | synchronized_directive_resource_list

     synchronized_directive_resource ::=
        predicate_indicator
        | non_terminal_indicator

     synchronized_directive_resource_sequence ::=
        synchronized_directive_resource
        | synchronized_directive_resource "," synchronized_directive_resource_sequence

     synchronized_directive_resource_list ::=
        "[" synchronized_directive_resource_sequence "]"

     uses_directive_resource_list ::=
        "[" uses_directive_resource_sequence "]"

     uses_directive_resource_sequence ::=
        uses_directive_resource
        | uses_directive_resource "," uses_directive_resource_sequence

     uses_directive_resource ::=
        predicate_indicator
        | non_terminal_indicator
        | predicate_template_alias
        | operator

     use_module_directive_resource_list ::=
        "[" use_module_directive_resource_sequence "]"

     use_module_directive_resource_sequence ::=
        use_module_directive_resource
        | use_module_directive_resource "," use_module_directive_resource_sequence

     use_module_directive_resource ::=
        predicate_indicator
        | non_terminal_indicator
        | predicate_template_alias
        | operator

     qualified_directive_resource_term ::=
        qualified_directive_resource
        | qualified_directive_resource_sequence
        | qualified_directive_resource_list

     qualified_directive_resource_sequence ::=
        qualified_directive_resource
        | qualified_directive_resource "," qualified_directive_resource_sequence

     qualified_directive_resource_list ::=
        "[" qualified_directive_resource_sequence "]"

     qualified_directive_resource ::=
        predicate_indicator
        | non_terminal_indicator
        | object_identifier "::" ( predicate_indicator | non_terminal_indicator)
        | category_identifier "::" ( predicate_indicator | non_terminal_indicator)
        | module_identifier ":" ( predicate_indicator | non_terminal_indicator)

     predicate_indicator_alias ::=
        predicate_indicator "as" predicate_indicator

     predicate_template_alias ::=
        callable "as" callable

     non_terminal_indicator ::=
        functor "//" arity

     non_terminal_indicator_alias ::=
        non_terminal_indicator "as" non_terminal_indicator

     operator_sequence ::=
        operator specification
        | operator specification "," operator_sequence

     operator_list ::=
        "[" operator_sequence "]"

     coinductive_predicate_template_term ::=
        coinductive_predicate_template
        | coinductive_predicate_template_sequence
        | coinductive_predicate_template_list

     coinductive_predicate_template_sequence ::=
        coinductive_predicate_template
        | coinductive_predicate_template "," coinductive_predicate_template_sequence

     coinductive_predicate_template_list ::=
        "[" coinductive_predicate_template_sequence "]"

     coinductive_predicate_template ::=
        atom "(" coinductive_mode_terms ")"

     coinductive_mode_terms ::=
        coinductive_mode_term
        | coinductive_mode_terms "," coinductive_mode_terms

     coinductive_mode_term ::=
        "+"
        | "-"

     predicate_mode_term ::=
        atom "(" mode_terms ")"

     non_terminal_mode_term ::=
        atom "(" mode_terms ")"

     mode_terms ::=
        mode_term
        |mode_term "," mode_terms

     mode_term ::=
        "@"  type?
        | "+" type?
        | "-" type?
        | "?" type?
        | "++" type?
        | "--" type?

     type ::=
        prolog_type | logtalk_type | user_defined_type

     prolog_type ::=
        "term"
        | "nonvar"
        | "var"
        | "compound"
        | "ground"
        | "callable"
        | "list"
        | "atomic"
        | "atom"
        | "number"
        | "integer"
        | "float"

     logtalk_type ::=
        "object"
        | "category"
        | "protocol"
        | "event"

     user_defined_type ::=
        atom
        | compound

     number_of_proofs ::=
        "zero"
        | "zero_or_one"
        | "zero_or_more"
        | "one"
        | "one_or_more"
        | "zero_or_error"
        | "one_or_error"
        | "zero_or_one_or_error"
        | "zero_or_more_or_error"
        | "one_or_more_or_error"
        | "error"

     meta_predicate_template_term ::=
        meta_predicate_template
        | meta_predicate_template_sequence
        | meta_predicate_template_list

     meta_predicate_template_sequence ::=
        meta_predicate_template
        | meta_predicate_template "," meta_predicate_template_sequence

     meta_predicate_template_list ::=
        "[" meta_predicate_template_sequence "]"

     meta_predicate_template ::=
        object_identifier "::" atom "(" meta_predicate_specifiers ")"
        | category_identifier "::" atom "(" meta_predicate_specifiers ")"
        | module_identifier ":" atom "(" meta_predicate_specifiers ")"
        | atom "(" meta_predicate_specifiers ")"

     meta_predicate_specifiers ::=
        meta_predicate_specifier
        | meta_predicate_specifier "," meta_predicate_specifiers

     meta_predicate_specifier ::=
        non_negative_integer
        | "::"
        | "^"
        | "*"

     meta_non_terminal_template_term ::=
        meta_predicate_template_term

     entity_info_list ::=
        "[" entity_info_sequence? "]"

     entity_info_sequence ::=
        entity_info_item "is" nonvar
        | entity_info_item "is" nonvar "," entity_info_sequence

     entity_info_item ::=
        "comment"
        | "remarks"
        | "author"
        | "version"
        | "date"
        | "copyright"
        | "license"
        | "parameters"
        | "parnames"
        | "see_also"
        | atom

     predicate_info_list ::=
        "[" predicate_info_sequence? "]"

     predicate_info_sequence ::=
        predicate_info_item "is" nonvar
        | predicate_info_item "is" nonvar "," predicate_info_sequence

     predicate_info_item ::=
        "comment"
        | "remarks"
        | "arguments"
        | "argnames"
        | "redefinition"
        | "allocation"
        | "examples"
        | "exceptions"
        | "see_also"
        | atom

     object_alias_list ::=
        "[" object_alias_sequence "]"

     object_alias_sequence ::=
        object_alias
        | object_alias "," object_alias_sequence

     object_alias ::=
        object_identifier "as" object_identifier

     module_alias_list ::=
        "[" module_alias_sequence "]"

     module_alias_sequence ::=
        module_alias
        | module_alias "," module_alias_sequence

     module_alias ::=
        module_identifier "as" module_identifier

2.1.11 Clauses and goals
------------------------

     clause ::=
        object_identifier "::" head ":-" body
        | module_identifier ":" head ":-" body
        | head ":-" body
        | object_identifier "::" fact
        | module_identifier ":" fact
        | fact

     goal ::=
        message_sending
        | super_call
        | external_call
        | context_switching_call
        | callable

     message_sending ::=
        message_to_object
        | message_delegation
        | message_to_self

     message_to_object ::=
        receiver "::" messages

     message_delegation ::=
        "[" message_to_object "]"

     message_to_self ::=
        "::" messages

     super_call ::=
        "^^" message

     messages ::=
        message
        | "(" message "," messages ")"
        | "(" message ";" messages ")"
        | "(" message "->" messages ")"

     message ::=
        callable
        | variable

     receiver ::=
        "{" callable "}"
        | object_identifier
        | variable

     external_call ::=
        "{" callable "}"

     context_switching_call ::=
        object_identifier "<<" callable

2.1.12 Lambda expressions
-------------------------

     lambda_expression ::=
        lambda_free_variables "/" lambda_parameters ">>" callable
        | lambda_free_variables "/" callable
        | lambda_parameters ">>" callable

     lambda_free_variables ::=
        "{" variables? "}"

     lambda_parameters ::=
        "[" terms? "]"

     variables ::=
        variable
        | variable "," variables

     terms ::=
        term
        | term "," terms

2.1.13 Entity properties
------------------------

     category_property ::=
        "static"
        | "dynamic"
        | "built_in"
        | "file(" atom ")"
        | "file(" atom "," atom ")"
        | "lines(" integer "," integer ")"
        | "directive(" integer "," integer ")"
        | "events"
        | "source_data"
        | "public(" entity_resources_list ")"
        | "protected(" entity_resources_list ")"
        | "private(" entity_resources_list ")"
        | "declares(" predicate_indicator "," predicate_declaration_property_list ")"
        | "defines(" predicate_indicator "," predicate_definition_property_list ")"
        | "includes(" predicate_indicator "," ( object_identifier | category_identifier ) "," predicate_definition_property_list ")"
        | "provides(" predicate_indicator "," ( object_identifier | category_identifier ) "," predicate_definition_property_list ")"
        | "alias(" ( object_identifier | module_identifier ) "," entity_alias_property_list ")"
        | "alias(" predicate_indicator "," predicate_alias_property_list ")"
        | "calls(" predicate "," predicate_call_update_property_list ")"
        | "updates(" predicate "," predicate_call_update_property_list ")"
        | "number_of_clauses(" integer ")"
        | "number_of_rules(" integer ")"
        | "number_of_user_clauses(" integer ")"
        | "number_of_user_rules(" integer ")"
        | "debugging"

     object_property ::=
        "static"
        | "dynamic"
        | "built_in"
        | "threaded"
        | "file(" atom ")"
        | "file(" atom "," atom ")"
        | "lines(" integer "," integer ")"
        | "directive(" integer "," integer ")"
        | "context_switching_calls"
        | "dynamic_declarations"
        | "events"
        | "source_data"
        | "complements(" ( "allow" | "restrict" ) ")"
        | "complements"
        | "public(" entity_resources_list ")"
        | "protected(" entity_resources_list ")"
        | "private(" entity_resources_list ")"
        | "declares(" predicate_indicator "," predicate_declaration_property_list ")"
        | "defines(" predicate_indicator "," predicate_definition_property_list ")"
        | "includes(" predicate_indicator "," ( object_identifier | category_identifier ) "," predicate_definition_property_list ")"
        | "provides(" predicate_indicator "," ( object_identifier | category_identifier ) "," predicate_definition_property_list ")"
        | "alias(" ( object_identifier | module_identifier ) "," entity_alias_property_list ")"
        | "alias(" predicate_indicator "," predicate_alias_property_list ")"
        | "calls(" predicate "," predicate_call_update_property_list ")"
        | "updates(" predicate "," predicate_call_update_property_list ")"
        | "number_of_clauses(" integer ")"
        | "number_of_rules(" integer ")"
        | "number_of_user_clauses(" integer ")"
        | "number_of_user_rules(" integer ")"
        | "module"
        | "debugging"

     protocol_property ::=
        "static"
        | "dynamic"
        | "built_in"
        | "source_data"
        | "file(" atom ")"
        | "file(" atom "," atom ")"
        | "lines(" integer "," integer ")"
        | "directive(" integer "," integer ")"
        | "public(" entity_resources_list ")"
        | "protected(" entity_resources_list ")"
        | "private(" entity_resources_list ")"
        | "declares(" predicate_indicator "," predicate_declaration_property_list ")"
        | "alias(" predicate_indicator "," predicate_alias_property_list ")"
        | "debugging"

     predicate_declaration_property_list ::=
        "[" predicate_declaration_property_sequence "]"

     predicate_declaration_property_sequence ::=
        predicate_declaration_property
        | predicate_declaration_property "," predicate_declaration_property_sequence

     predicate_declaration_property ::=
        "static"
        | "dynamic"
        | "scope(" scope ")"
        | "private"
        | "protected"
        | "public"
        | "coinductive"
        | "multifile"
        | "synchronized"
        | "meta_predicate(" meta_predicate_template ")"
        | "coinductive(" coinductive_predicate_template ")"
        | "non_terminal(" non_terminal_indicator ")"
        | "include(" atom ")"
        | "lines(" integer  "," integer ")"
        | "line_count(" integer ")"
        | "mode(" ( predicate_mode_term | non_terminal_mode_term ) "," number_of_proofs ")"
        | "info(" list ")"

     predicate_definition_property_list ::=
        "[" predicate_definition_property_sequence "]"

     predicate_definition_property_sequence ::=
        predicate_definition_property
        | predicate_definition_property "," predicate_definition_property_sequence

     predicate_definition_property ::=
        "inline"
        | "auxiliary"
        | "non_terminal(" non_terminal_indicator ")"
        | "include(" atom ")"
        | "lines(" integer  "," integer ")"
        | "line_count(" integer ")"
        | "number_of_clauses(" integer ")"
        | "number_of_rules(" integer ")"

     entity_alias_property_list ::=
        "[" entity_alias_property_sequence "]"

     entity_alias_property_sequence ::=
        entity_alias_property
        | entity_alias_property "," entity_alias_property_sequence

     entity_alias_property ::=
        "object"
        | "module"
        | "for(" ( object_identifier | module_identifier ) ")"
        | "include(" atom ")"
        | "lines(" integer  "," integer ")"
        | "line_count(" integer ")"

     predicate_alias_property_list ::=
        "[" predicate_alias_property_sequence "]"

     predicate_alias_property_sequence ::=
        predicate_alias_property
        | predicate_alias_property "," predicate_alias_property_sequence

     predicate_alias_property ::=
        "predicate"
        | "for(" predicate_indicator ")"
        | "from(" entity_identifier ")"
        | "non_terminal(" non_terminal_indicator ")"
        | "include(" atom ")"
        | "lines(" integer  "," integer ")"
        | "line_count(" integer ")"

     predicate ::=
        predicate_indicator
        | "^^" predicate_indicator
        | "::" predicate_indicator
        | ( variable | object_identifier ) "::" predicate_indicator
        | ( variable | module_identifier ) ":" predicate_indicator

     predicate_call_update_property_list ::=
        "[" predicate_call_update_property_sequence "]"

     predicate_call_update_property_sequence ::=
        predicate_call_update_property
        | predicate_call_update_property "," predicate_call_update_property_sequence

     predicate_call_update_property ::=
        "caller(" predicate_indicator ")"
        | "include(" atom ")"
        | "lines(" integer  "," integer ")"
        | "line_count(" integer ")"
        | "alias(" predicate_indicator ")"
        | "non_terminal(" non_terminal_indicator ")"

2.1.14 Predicate properties
---------------------------

     predicate_property ::=
        "static"
        | "dynamic"
        | "scope(" scope ")"
        | "private"
        | "protected"
        | "public"
        | "logtalk"
        | "prolog"
        | "foreign"
        | "coinductive(" coinductive_predicate_template ")"
        | "multifile"
        | "synchronized"
        | "built_in"
        | "inline"
        | "recursive"
        | "declared_in(" entity_identifier ")"
        | "defined_in(" ( object_identifier | category_identifier ) ")"
        | "redefined_from(" ( object_identifier | category_identifier ) ")"
        | "meta_predicate(" meta_predicate_template ")"
        | "alias_of(" callable ")"
        | "alias_declared_in(" entity_identifier ")"
        | "non_terminal(" non_terminal_indicator ")"
        | "mode(" ( predicate_mode_term | non_terminal_mode_term ) "," number_of_proofs ")"
        | "info(" list ")"
        | "number_of_clauses(" integer ")"
        | "number_of_rules(" integer ")"
        | "declared_in(" entity_identifier "," line_count ")"
        | "defined_in(" ( object_identifier | category_identifier ) "," line_count ")"
        | "redefined_from(" ( object_identifier | category_identifier ) "," line_count ")"
        | "alias_declared_in(" entity_identifier "," line_count ")"

     line_count ::=
        integer

2.1.15 Compiler flags
---------------------

     compiler_flag ::=
        flag "(" flag_value ")"

2.2 Control constructs
======================

2.2.1 Message sending
---------------------

'control construct'

2.2.1.1 ‘(::)/2’
................

2.2.1.2 Description
...................

     Object::Message
     {Proxy}::Message

Sends a message to an object.  The message argument must match a
public predicate of the receiver object.  When the message
corresponds to a protected or private predicate,
the call is only valid if the sender matches the
predicate scope container.  When the predicate is declared but not
defined, the message simply fails (as per the closed-world assumption).

The ‘{Proxy}::Message’ syntax allows simplified access to
parametric object proxies.  Its operational semantics is equivalent
to the conjunction ‘(call(Proxy), Proxy::Message)’.  I.e.  ‘Proxy’ is
proved within the context of the pseudo-object user and, if
successful, the ‘Proxy’ term is used as an object identifier.
Exceptions thrown when proving ‘Proxy’ are handled by the ‘(::)/2’
control construct.  This construct construct supports backtracking over
the ‘{Proxy}’ goal.

The lookups for the message declaration and the corresponding method are
performed using a depth-first strategy.  Depending on the value of the
optimize flag, these lookups are performed at compile time
whenever sufficient information is available.  When the lookups are
performed at runtime, a caching mechanism is used to improve performance
in subsequent messages.  See the User Manual section on
performance for details.

2.2.1.3 Modes and number of proofs
..................................

     +object_identifier::+callable - zero_or_more
     {+object_identifier}::+callable - zero_or_more

2.2.1.4 Errors
..............

     Either ‘Object’ or ‘Message’ is a variable:
          ‘instantiation_error’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Message’ is neither a variable nor a callable term:
          ‘type_error(callable, Message)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is declared private:
          ‘permission_error(access, private_predicate, Name/Arity)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is declared protected:
          ‘permission_error(access, protected_predicate, Name/Arity)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’
     ‘Object’ does not exist:
          ‘existence_error(object, Object)’

     ‘Proxy’ is a variable:
          ‘instantiation_error’
     ‘Proxy’ is neither a variable nor a callable term:
          ‘type_error(callable, Proxy)’
     ‘Proxy’, with predicate indicator ‘Name/Arity’, does not exist in the ‘user’ pseudo-object:
          ‘existence_error(procedure, Name/Arity)’

2.2.1.5 Examples
................

     | ?- list::member(X, [1, 2, 3]).

     X = 1 ;
     X = 2 ;
     X = 3
     yes

See also
........

See (;;)/1, (^^)/1, []/1.

'control construct'

2.2.1.6 ‘(::)/1’
................

2.2.1.7 Description
...................

     ::Message

Sends a message to self.  Can only used in the body of a
predicate definition.  The argument should match a public or
protected predicate of 'self'.  It may also match a
private predicate if the predicate is within the scope of the
object where the method making the call is defined, if imported from a
category, if used from within a category, or when using private
inheritance.  When the predicate is declared but not defined, the
message simply fails (as per the closed-world assumption).

The lookups for the message declaration and the corresponding method are
performed using a depth-first strategy.  A message to 'self' necessarily
implies the use of dynamic binding but a caching mechanism is
used to improve performance in subsequent messages.  See the User Manual
section on performance for details.

2.2.1.8 Modes and number of proofs
..................................

     ::+callable - zero_or_more

2.2.1.9 Errors
..............

     ‘Message’ is a variable:
          ‘instantiation_error’
     ‘Message’ is neither a variable nor a callable term:
          ‘type_error(callable, Message)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is declared private:
          ‘permission_error(access, private_predicate, Name/Arity)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.2.1.10 Examples
.................

     area(Area) :-
         ::width(Width),
         ::height(Height),
         Area is Width * Height.

See also
........

See (;;)/2, (^^)/1, []/1.

2.2.2 Message delegation
------------------------

'control construct'

2.2.2.1 ‘[]/1’
..............

2.2.2.2 Description
...................

     [Object::Message]
     [{Proxy}::Message]

This control construct allows the programmer to send a message to an
object while preserving the original sender.  It is mainly used in the
definition of object handlers for unknown messages.  This functionality
is usually known as 'delegation' but be aware that this is an overloaded
word that can mean different things in different object-oriented
programming languages.

To prevent using of this control construct to break object
encapsulation, an attempt to delegate a message to the original sender
results in an error.  The remaining error conditions are the same as the
(;;)/2 control construct.

Note that, despite the correct functor for this control construct being
(traditionally) ‘'.'/2’, we refer to it as ‘[]/1’ simply to emphasize
that the syntax is a list with a single element.

2.2.2.3 Modes and number of proofs
..................................

     [+object_identifier::+callable] - zero_or_more
     [{+object_identifier}::+callable] - zero_or_more

2.2.2.4 Errors
..............

     ‘Object’ is a variable:
          ‘instantiation_error’
     Object is neither a variable nor an object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Object’ does not exist:
          ‘existence_error(object, Object)’
     ‘Object’ and the original 'sender' are the same object:
          ‘permission_error(access, object, Sender)’

     ‘Proxy’ is a variable:
          ‘instantiation_error’
     ‘Proxy’ is neither a variable nor an object identifier:
          ‘type_error(object_identifier, Proxy)’
     ‘Proxy’, with predicate indicator ‘Name/Arity’, does not exist in the ‘user’ pseudo-object:
          ‘existence_error(procedure, Name/Arity)’

     ‘Message’ is a variable:
          ‘instantiation_error’
     ‘Message’ is neither a variable nor a callable term:
          ‘type_error(callable, Message)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is declared private:
          ‘permission_error(access, private_predicate, Name/Arity)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is declared protected:
          ‘permission_error(access, protected_predicate, Name/Arity)’
     ‘Message’, with predicate indicator ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.2.2.5 Examples
................

     % delegate unknown messages to the "backup" object:
     forward(Message) :-
         [backup::Message].

See also
........

See (;;)/2, (;;)/1, (^^)/1, forward/1.

2.2.3 Calling imported and inherited predicates
-----------------------------------------------

'control construct'

2.2.3.1 ‘(^^)/1’
................

2.2.3.2 Description
...................

     ^^Predicate

Calls an imported or inherited predicate definition.  The call fails if
the predicate is declared but there is no imported or inherited
predicate definition (as per the closed-world assumption).
This control construct may be used within objects or categories in the
body of a predicate definition.

This control construct preserves the implicit execution context
self and sender arguments (plus the meta-call context
and coinduction stack when applicable) when calling the inherited (or
imported) predicate definition.

The lookups for the predicate declaration and the predicate definition
are performed using a depth-first strategy.  Depending on the value of
the optimize flag, these lookups are performed at compile
time when the predicate is static and sufficient information is
available.  When the lookups are performed at runtime, a caching
mechanism is used to improve performance in subsequent calls.  See the
User Manual section on performance for details.

When the call is made from within an object, the lookup for the
predicate definition starts at the imported categories, if any.  If an
imported predicate definition is not found, the lookup proceeds to the
ancestor objects.  Calls from predicates defined in complementing
categories lookup inherited definitions as if the calls were made from
the complemented object, thus allowing more comprehensive object
patching.  For other categories, the predicate definition lookup is
restricted to the extended categories.

The called predicate should be declared public or
protected.  It may also be declared private if within
the scope of the entity where the method making the call is defined.

This control construct is a generalization of the Smalltalk 'super'
keyword to take into account Logtalk support for prototypes and
categories besides classes.

2.2.3.3 Modes and number of proofs
..................................

     ^^+callable - zero_or_more

2.2.3.4 Errors
..............

     ‘Predicate’ is a variable:
          ‘instantiation_error’
     ‘Predicate’ is neither a variable nor a callable term:
          ‘type_error(callable, Predicate)’
     ‘Predicate’, with predicate indicator ‘Name/Arity’, is declared private:
          ‘permission_error(access, private_predicate, Name/Arity)’
     ‘Predicate’, with predicate indicator ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.2.3.5 Examples
................

     % specialize the inherited definition
     % of the init/0 predicate:
     init :-
         assertz(counter(0)),
         ^^init.

See also
........

See (;;)/2, (;;)/1, []/1.

2.2.4 Calling predicates in 'this'
----------------------------------

'control construct'

2.2.4.1 ‘(@)/1’
...............

2.2.4.2 Description
...................

     @Predicate

Calls a predicate definition in this.  The argument must be a
callable term at compile time.  The predicate must be declared (by a
scope directive).  This control construct provides access to predicate
definitions in 'this' from categories.  For example, it allows
overriding a predicate definition from a complementing category with a
new definition that calls goals before and after calling the overriden
definition (the overriding definition is sometimes described in other
programming languages as an 'around method').  When used within an
object, it’s the same as calling its argument.

2.2.4.3 Modes and number of proofs
..................................

     @ +callable - zero_or_more

2.2.4.4 Errors
..............

     ‘Predicate’, with predicate indicator ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.2.4.5 Examples
................

Assuming an object declaring a ‘make_sound/0’ predicate, define an
'around method' in a complementing category:

     make_sound :-
         write('Started making sound...'), nl,
         @make_sound,
         write('... finished making sound.'), nl.

See also
........

See (;;)/2, (;;)/1, []/1.

2.2.5 Calling external predicates
---------------------------------

'control construct'

2.2.5.1 ‘{}/1’
..............

2.2.5.2 Description
...................

     {Goal}
     {Closure}
     {Term}

This control construct allows the programmer to bypass the Logtalk
compiler (including its linter but not its optimizer) in multiple
contexts:

   - Calling a goal as-is (from within an object or category) in the
     context of the user pseudo-object.

   - Extending a closure as-is with the remaining arguments of a
     call/2-N call in order to construct a goal that will be
     called within the context of the ‘user’ pseudo-object.

   - Wrapping a source file term (either a clause or a directive) or a
     source file goal to bypass the term-expansion mechanism.

   - Using it in place of an object identifier when sending a message.
     In this case, its argument is proved as a goal within the context
     of the ‘user’ pseudo-object with the resulting term being used as
     an object identifier in the message sending goal.  This feature is
     mainly used with parametric objects when their
     identifiers correspond to predicates defined in ‘user’.

   - Using it as a message to an object.  This is mainly useful when the
     message is e.g.  a conjunction of messages, some of which
     being calls to Prolog built-in predicates.

     Note: This control construct is opaque to cuts when used to wrap a
     goal (thus ensuring the same semantics independently of the
     argument being bound at compile time or at runtime).

2.2.5.3 Modes and number of proofs
..................................

     {+callable} - zero_or_more

2.2.5.4 Errors
..............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

     ‘Closure’ is a variable:
          ‘instantiation_error’
     ‘Closure’ is neither a variable nor a callable term:
          ‘type_error(callable, Closure)’

     ‘Term’ is a variable:
          ‘instantiation_error’
     ‘Term’ is neither a variable nor a callable term:
          ‘type_error(callable, Term)’

2.2.5.5 Examples
................

     % overload the standard (<)/2 operator by
     % calling its standard built-in definition:
     N1/D1 < N2/D2 :-
         {N1*D2 < N2*D1}.

     % call a closure in the context of "user":
     call_in_user(F, X, Y, Z) :-
         call({F}, X, Y, Z).

     % bypass the compiler for a proprietary backend directive:
     {:- load_foreign_resource(file)}.

     % use parametric object proxies:
     | ?- {circle(Id, Radius, Color)}::area(Area).
     ...

     % use Prolog built-in predicates as messages:
     | ?- logtalk::{write('hello world!'), nl}.
     hello world!
     yes

2.2.6 Context switching calls
-----------------------------

'control construct'

2.2.6.1 ‘(<<)/2’
................

2.2.6.2 Description
...................

     Object<<Goal
     {Proxy}<<Goal

Debugging control construct.  Calls a goal within the context of the
specified object.  The goal is called with the execution context (see
sender, this, and self) set to the object.
The goal may need to be written between parenthesis to avoid parsing
errors due to operator conflicts.  This control construct should only be
used for debugging or for writing unit tests.  This control construct
can only be used for objects compiled with the
context_switching_calls compiler flag set to ‘allow’.  Set this
compiler flag to ‘deny’ to disable this control construct and thus
preventing using it to break encapsulation.

The ‘{Proxy}<<Goal’ syntax allows simplified access to parametric object proxies.
Its operational semantics is equivalent to the
goal conjunction ‘(call(Proxy), Proxy<<Goal)’.  I.e.  ‘Proxy’ is proved
within the context of the pseudo-object user and, if
successful, the goal term is used as a parametric object identifier.
Exceptions thrown when proving ‘Proxy’ are handled by the ‘(<<)/2’
control construct.  This syntax construct supports backtracking over the
‘{Proxy}’ goal.

Caveat: although the goal argument is fully compiled before calling,
some necessary information for the second compiler pass may not be
available at runtime.

2.2.6.3 Modes and number of proofs
..................................

     +object_identifier<<+callable - zero_or_more
     {+object_identifier}<<+callable - zero_or_more

2.2.6.4 Errors
..............

     ‘Object’ is a variable:
          ‘instantiation_error’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Object’ does not contain a local definition for the Goal predicate:
          ‘existence_error(procedure, Goal)’
     ‘Object’ does not exist:
          ‘existence_error(object, Object)’
     ‘Object’ was created/compiled with support for context switching calls turned off:
          ‘permission_error(access, database, Goal)’

     ‘Proxy’ is a variable:
          ‘instantiation_error’
     ‘Proxy’ is neither a variable nor an object identifier:
          ‘type_error(object_identifier, Proxy)’
     The predicate ‘Proxy’ does not exist in the ‘user’ pseudo-object:
          ‘existence_error(procedure, ProxyFunctor/ProxyArity)’

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

2.2.6.5 Examples
................

     % call the member/2 predicate in the
     % context of the "list" object:
     test(member) :-
         list << member(1, [1]).

2.3 Directives
==============

2.3.1 Source file directives
----------------------------

'directive'

2.3.1.1 ‘encoding/1’
....................

2.3.1.2 Description
...................

     encoding(Encoding)

Declares the source file text encoding.  Requires a backend Prolog compiler
supporting the chosen encoding.  When used, this directive
must be the first term in the source file in the first line.  This
directive is also supported in files included in a main file or in a
dynamically created entity using include/1 directives.

The encoding used in a source file (and, in the case of a Unicode
encoding, any BOM present) will be used for the intermediate Prolog file
generated by the compiler.  Logtalk uses the encoding names specified by
IANA(1). In those cases where a preferred MIME name alias is specified,
the alias is used instead.  Examples includes ‘'US-ASCII'’,
‘'ISO-8859-1'’, ‘'ISO-8859-2'’, ‘'ISO-8859-15'’, ‘'UCS-2'’, ‘'UCS-2LE'’,
‘'UCS-2BE'’, ‘'UTF-8'’, ‘'UTF-16'’, ‘'UTF-16LE'’, ‘'UTF-16BE'’,
‘'UTF-32'’, ‘'UTF-32LE'’, ‘'UTF-32BE'’, ‘'Shift_JIS'’, and ‘'EUC-JP'’.
When writing portable code that cannot be expressed using ASCII,
‘'UTF-8'’ is the most commonly supported Unicode encoding.

The backend Prolog compiler adapter files define a table that translates
between the Logtalk and Prolog specific atoms that represent each
supported encoding.  The encoding_directive read-only flag
can be used to find if a backend supports this directive and how.

   ---------- Footnotes ----------

   (1)
http://www.iana.org/assignments/character-sets/character-sets.xhtml

2.3.1.3 Template and modes
..........................

     encoding(+atom)

2.3.1.4 Examples
................

     :- encoding('UTF-8').

'directive'

2.3.1.5 ‘include/1’
...................

2.3.1.6 Description
...................

     include(File)

Includes a file contents, which must be valid terms, at the place of
occurrence of the directive.  The file can be specified as a relative
path, an absolute path, or using library notation and is
expanded as a source file name.  Relative paths are interpreted as
relative to the path of the file containing the directive.  The file
extension is optional (the recognized Logtalk and Prolog file extensions
are defined in the backend adapter files).

When using the reflection API, predicates from an included
file can be distinguished from predicates from the main file by looking
for the ‘include/1’ predicate declaration property or the ‘include/1’
predicate definition property.  For the included predicates, the
‘line_count/1’ property stores the term line number in the included
file.

This directive can be used as either a source file directive or an
entity directive.  As an entity directive, it can be used both in
entities defined in source files and with the entity creation built-in
predicates.  In the latter case, the file should be specified using an
absolute path or using library notation (which expands to a full path)
to avoid a fragile dependency on the current working directory.

Included files may contain an encoding/1 directive, which
may specify the same encoding of the main file or a different encoding.

     Warning: When using this directive as an argument in calls to the
     create_object/4 and create_category/4.
     predicates, the objects and categories will not be recreated or
     redefined when the included file(s) are modified and the
     logtalk_make/0 predicate or the logtalk_make/1.
     (with target ‘all’) predicates are called.

2.3.1.7 Template and modes
..........................

     include(@source_file_name)

2.3.1.8 Examples
................

     % include the "raw_1.txt" text file found
     % on the "data" library directory:
     :- include(data('raw_1.txt')).

     % include a "factbase.pl" file in the same directory
     % of the source file containing the directive:
     :- include('factbase.pl').

     % include a file given its absolute path:
     :- include('/home/me/databases/countries.pl').

     % create a wrapper object for a Prolog file using
     % library notation to define the file path:
     | ?- create_object(cities, [], [public(city/4), include(geo('cities.pl'))], []).

'directive'

2.3.1.9 ‘initialization/1’
..........................

2.3.1.10 Description
....................

     initialization(Goal)

When used within an object, this directive defines a goal to be called
after the object has been successfully loaded into memory.  When used at
a global level within a source file, this directive defines a goal to be
called after the source file is successfully compiled and loaded into
memory.

The loading context can be accessed from this directive by calling the
logtalk_load_context/2 predicate.  Note that the usable
loading context keys depends on the directive scope (file or object).

Multiple initialization directives can be used in a source file or in an
object.  Their goals will be called in the same order as the directives
at loading time.

     Note: Arbitrary goals cannot be used as directives in source files.
     Any goal that should be automatically called when a source file is
     loaded must be wrapped using this directive.

     Categories and protocols cannot contain ‘initialization/1’
     directives as the initialization goals would lack a complete
     execution context that is only available for objects.

Although technically a global ‘initialization/1’ directive in a source
file is a Prolog directive, calls to Logtalk built-in predicates from it
are usually compiled to improve portability, improve performance, and
provide better support for embedded applications.

     Warning: Some backend Prolog compilers declare the atom
     ‘initialization’ as an operator for a lighter syntax.  But this
     makes the code non-portable and is therefore a practice best
     avoided.

2.3.1.11 Template and modes
...........................

     initialization(@callable)

2.3.1.12 Examples
.................

     % call the init/0 predicate after loading the
     % source file containing the directive

     :- initialization(init).

     % print a debug message after loading a
     % source file defining an object

     :- object(log).

         :- initialization(start_date).

         start_date :-
             os::date_time(Year, Month, Day, _, _, _, _),
             logtalk::print_message(debug, my_app, 'Starting date: ~d-~d-~d~n'+[Year,Month,Day]).

     :- end_object.

See also
........

See logtalk_load_context/2.

'directive'

2.3.1.13 ‘op/3’
...............

2.3.1.14 Description
....................

     op(Precedence, Associativity, Operator)
     op(Precedence, Associativity, [Operator, ...])

Declares operators.  Global operators can be declared inside a source
file by writing the respective directives before the entity opening
directives.  Operators declared inside entities have local scope.  Calls
to the standard term input and output predicates take into account any
locally defined operators.

2.3.1.15 Template and modes
...........................

     op(+integer, +associativity, +atom_or_atom_list)

2.3.1.16 Examples
.................

Some of the predicate argument instantiation mode operators used by
Logtalk:

     :- op(200, fy, +).
     :- op(200, fy, ?).
     :- op(200, fy, @).
     :- op(200, fy, -).

An example of using entity local operators.  Consider the following
‘ops.lgt’ file:

     :- initialization((write(<=>(1,2)), nl)).

     :- object(ops).

         :- op(700, xfx, <=>).

         :- public(w/1).
         w(Term) :-
             write(Term), nl.

         :- public(r/1).
         r(Term) :-
             read(Term).

     :- end_object.

Loading the file automatically calls the initialization goal.  Compare
its output with the output of the ‘ops::w/1’ predicate.  Compare also
reading a term from within the ‘ops’ object versus reading from ‘user’.

     | ?- {ops}.
     <=>(1,2)
     true.

     | ?- ops::w(<=>(1,2)).
     1<=>2
     true.

     | ?- ops::r(T).
     |: 3<=>4.

     T = <=>(3, 4).

     | ?- read(T).
     |: 5<=>6.

     SYNTAX ERROR: operator expected

See also
........

See current_op/3.

'directive'

2.3.1.17 ‘set_logtalk_flag/2’
.............................

2.3.1.18 Description
....................

     set_logtalk_flag(Flag, Value)

Sets local flag values.  The scope of this directive is the entity or
the source file containing it.  For global scope, use the corresponding
set_logtalk_flag/2 built-in predicate called from an
initialization/1 directive.  For a description of the predefined
compiler flags, consult the Compiler flags section in the
User Manual.  The directive affects the compilation of all terms that
follow it within scope of the directive.

2.3.1.19 Template and modes
...........................

     set_logtalk_flag(+atom, +nonvar)

2.3.1.20 Errors
...............

     Flag is a variable:
          ‘instantiation_error’
     Value is a variable:
          ‘instantiation_error’
     Flag is not an atom:
          ‘type_error(atom, Flag)’
     Flag is neither a variable nor a valid flag:
          ‘domain_error(flag, Flag)’
     Value is not a valid value for flag Flag:
          ‘domain_error(flag_value, Flag + Value)’
     Flag is a read-only flag:
          ‘permission_error(modify, flag, Flag)’

2.3.1.21 Examples
.................

     % turn off the compiler unknown entity warnings
     % during the compilation of this source file:
     :- set_logtalk_flag(unknown_entities, silent).

     :- object(...).

         % generate events for messages sent from this object:
         :- set_logtalk_flag(events, allow).
         ...

         % turn off suspicious call lint checks for the next predicate:
         :- set_logtalk_flag(suspicious_calls, silent).
         foo :-
             ...
         :- set_logtalk_flag(suspicious_calls, warning).
         ...

2.3.2 Conditional compilation directives
----------------------------------------

'directive'

2.3.2.1 ‘if/1’
..............

2.3.2.2 Description
...................

     if(Goal)

Starts an 'if-then' branch when performing conditional compilation.  The
code following the directive is compiled iff ‘Goal’ is true.  If ‘Goal’
throws an error instead of either succeeding or failing, the error is
reported by the compiler and compilation of the enclosing source file or
entity is aborted.  The goal is subjected to goal expansion.
when the directive occurs in a source file.  Conditional compilation
directives can be nested.

     Warning: Conditional compilation goals 'cannot' depend on predicate
     definitions contained in the same source file that contains the
     conditional compilation directives (as those predicates only become
     available after the file is successfully compiled and loaded).

2.3.2.3 Template and modes
..........................

     if(@callable)

2.3.2.4 Examples
................

A common example is checking if a built-in predicate exists and
providing a definition when the predicate is absent:

     :- if(\+ predicate_property(length(_,_), built_in)).

         length(List, Length) :-
             ...

     :- endif.

Another common example is conditionally including code for a specific
backend Prolog compiler:

     :- if(current_logtalk_flag(prolog_dialect, swi)).

         % SWI-Prolog specific code
         :- set_prolog_flag(double_quotes, codes).

     :- endif.

If necessary, test goal errors can be converted into failures using the
standard ‘catch/3’ control construct.  For example:

     :- if(catch(\+ log(7,_), _, fail)).

        % define the legacy log/2 predicate
        log(X, Y) :- Y is log(X).

     :- endif.

See also
........

See elif/1, else/0, endif/0.

'directive'

2.3.2.5 ‘elif/1’
................

2.3.2.6 Description
...................

     elif(Goal)

Supports embedded conditionals when performing conditional compilation.
The code following the directive is compiled iff ‘Goal’ is true.  If
‘Goal’ throws an error instead of either succeeding or failing, the
error is reported by the compiler and compilation of the enclosing
source file or entity is aborted.  The goal is subjected to goal expansion
when the directive occurs in a source file.

     Warning: Conditional compilation goals 'cannot' depend on predicate
     definitions contained in the same source file that contains the
     conditional compilation directives (as those predicates only become
     available after the file is successfully compiled and loaded).

2.3.2.7 Template and modes
..........................

     elif(@callable)

2.3.2.8 Examples
................

     :- if(current_prolog_flag(double_quotes, codes)).

         ...

     :- elif(current_prolog_flag(double_quotes, chars)).

         ...

     :- elif(current_prolog_flag(double_quotes, atom)).

         ...

     :- endif.

See also
........

See else/0, endif/0, if/1.

'directive'

2.3.2.9 ‘else/0’
................

2.3.2.10 Description
....................

     else

Starts an 'else' branch when performing conditional compilation.  The
code following this directive is compiled iff the goal in the matching
if/1 or elif/1 directive is false.

2.3.2.11 Template and modes
...........................

     else

2.3.2.12 Examples
.................

An example where a hypothetic application would have some limitations
that the user should be made aware when running on a backend Prolog
compiler with bounded arithmetic:

     :- if(current_prolog_flag(bounded, true)).

         :- initialization(
             logtalk::print_message(warning,app,bounded_arithmetic)
         ).

     :- else.

         :- initialization(
             logtalk::print_message(comment,app,unbounded_arithmetic)
         ).

     :- endif.

See also
........

See elif/1, endif/0, if/1.

'directive'

2.3.2.13 ‘endif/0’
..................

2.3.2.14 Description
....................

     endif

Ends conditional compilation for the matching if/1.
directive.

2.3.2.15 Template and modes
...........................

     endif

2.3.2.16 Examples
.................

     :- if(date::today(_,5,25)).

         :- initialization(write('Happy Towel Day!\n')).

     :- endif.

See also
........

See elif/1, else/0, if/1.

2.3.3 Entity directives
-----------------------

'directive'

2.3.3.1 ‘built_in/0’
....................

2.3.3.2 Description
...................

     built_in

Declares an entity as built-in.  Built-in entities must be static and
cannot be redefined once loaded.  This directive is used in the
pre-defined protocols, categories, and objects that are automatically
loaded at startup.

2.3.3.3 Template and modes
..........................

     built_in

2.3.3.4 Examples
................

     :- built_in.

'directive'

2.3.3.5 ‘category/1-4’
......................

2.3.3.6 Description
...................

     category(Category)

     category(Category,
         implements(Protocols))

     category(Category,
         extends(Categories))

     category(Category,
         complements(Objects))

     category(Category,
         implements(Protocols),
         extends(Categories))

     category(Category,
         implements(Protocols),
         complements(Objects))

     category(Category,
         extends(Categories),
         complements(Objects))

     category(Category,
         implements(Protocols),
         extends(Categories),
         complements(Objects))

Starting category directive.

2.3.3.7 Template and modes
..........................

     category(+category_identifier)

     category(+category_identifier,
         implements(+implemented_protocols))

     category(+category_identifier,
         extends(+extended_categories))

     category(+category_identifier,
         complements(+complemented_objects))

     category(+category_identifier,
         implements(+implemented_protocols),
         extends(+extended_categories))

     category(+category_identifier,
         implements(+implemented_protocols),
         complements(+complemented_objects))

     category(+category_identifier,
         extends(+extended_categories),
         complements(+complemented_objects))

     category(+category_identifier,
         implements(+implemented_protocols),
         extends(+extended_categories),
         complements(+complemented_objects))

2.3.3.8 Examples
................

     :- category(monitoring).

     :- category(monitoring,
         implements(monitoringp)).

     :- category(attributes,
         implements(protected::variables)).

     :- category(extended,
         extends(minimal)).

     :- category(logging,
         implements(monitoring),
         complements(employee)).

See also
........

See end_category/0.

'directive'

2.3.3.9 ‘dynamic/0’
...................

2.3.3.10 Description
....................

     dynamic

Declares an entity and its contents as dynamic.  Dynamic entities can be
abolished at runtime.

As entities created at runtime (using the create_object/4,
create_protocol/3, and create_category/4 built-in
predicates) are always dynamic, this directive is only necessary in the
rare cases where we want to define dynamic entities in source files.

     Warning: Some backend Prolog compilers declare the atom ‘dynamic’
     as an operator for a lighter syntax, forcing writing this atom
     between parenthesis when using this directive.

2.3.3.11 Template and modes
...........................

     dynamic

2.3.3.12 Examples
.................

     :- dynamic.

See also
........

See dynamic/1, object_property/2,
protocol_property/2, category_property/2.

'directive'

2.3.3.13 ‘end_category/0’
.........................

2.3.3.14 Description
....................

     end_category

Ending category directive.

2.3.3.15 Template and modes
...........................

     end_category

2.3.3.16 Examples
.................

     :- end_category.

See also
........

See category/1-4.

'directive'

2.3.3.17 ‘end_object/0’
.......................

2.3.3.18 Description
....................

     end_object

Ending object directive.

2.3.3.19 Template and modes
...........................

     end_object

2.3.3.20 Examples
.................

     :- end_object.

See also
........

See object/1-5.

'directive'

2.3.3.21 ‘end_protocol/0’
.........................

2.3.3.22 Description
....................

     end_protocol

Ending protocol directive.

2.3.3.23 Template and modes
...........................

     end_protocol

2.3.3.24 Examples
.................

     :- end_protocol.

See also
........

See protocol/1-2.

'directive'

2.3.3.25 ‘info/1’
.................

2.3.3.26 Description
....................

     info([Key is Value, ...])

Documentation directive for objects, protocols, and categories.  The
directive argument is a list of pairs using the format 'Key is Value'.
See the Entity directives section for a description of the
default keys.

2.3.3.27 Template and modes
...........................

     info(+entity_info_list)

2.3.3.28 Examples
.................

     :- info([
         version is 1:0:0,
         author is 'Paulo Moura',
         date is 2000-11-20,
         comment is 'List protocol.'
     ]).

See also
........

See info/2, object_property/2,
protocol_property/2, category_property/2.

'directive'

2.3.3.29 ‘object/1-5’
.....................

2.3.3.30 Description
....................

'Stand-alone objects (prototypes)'

     object(Object)

     object(Object,
         implements(Protocols))

     object(Object,
         imports(Categories))

     object(Object,
         implements(Protocols),
         imports(Categories))

'Prototype extensions'

     object(Object,
         extends(Objects))

     object(Object,
         implements(Protocols),
         extends(Objects))

     object(Object,
         imports(Categories),
         extends(Objects))

     object(Object,
         implements(Protocols),
         imports(Categories),
         extends(Objects))

'Class instances'

     object(Object,
         instantiates(Classes))

     object(Object,
         implements(Protocols),
         instantiates(Classes))

     object(Object,
         imports(Categories),
         instantiates(Classes))

     object(Object,
         implements(Protocols),
         imports(Categories),
         instantiates(Classes))

'Classes'

     object(Object,
         specializes(Classes))

     object(Object,
         implements(Protocols),
         specializes(Classes))

     object(Object,
         imports(Categories),
         specializes(Classes))

     object(Object,
         implements(Protocols),
         imports(Categories),
         specializes(Classes))

'Classes with metaclasses'

     object(Object,
         instantiates(Classes),
         specializes(Classes))

     object(Object,
         implements(Protocols),
         instantiates(Classes),
         specializes(Classes))

     object(Object,
         imports(Categories),
         instantiates(Classes),
         specializes(Classes))

     object(Object,
         implements(Protocols),
         imports(Categories),
         instantiates(Classes),
         specializes(Classes))

Starting object directive.

2.3.3.31 Template and modes
...........................

'Stand-alone objects (prototypes)'

     object(+object_identifier)

     object(+object_identifier,
         implements(+implemented_protocols))

     object(+object_identifier,
         imports(+imported_categories))

     object(+object_identifier,
         implements(+implemented_protocols),
         imports(+imported_categories))

'Prototype extensions'

     object(+object_identifier,
         extends(+extended_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         extends(+extended_objects))

     object(+object_identifier,
         imports(+imported_categories),
         extends(+extended_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         imports(+imported_categories),
         extends(+extended_objects))

'Class instances'

     object(+object_identifier,
         instantiates(+instantiated_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         instantiates(+instantiated_objects))

     object(+object_identifier,
         imports(+imported_categories),
         instantiates(+instantiated_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         imports(+imported_categories),
         instantiates(+instantiated_objects))

'Classes'

     object(+object_identifier,
         specializes(+specialized_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         specializes(+specialized_objects))

     object(+object_identifier,
         imports(+imported_categories),
         specializes(+specialized_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         imports(+imported_categories),
         specializes(+specialized_objects))

'Class with metaclasses'

     object(+object_identifier,
         instantiates(+instantiated_objects),
         specializes(+specialized_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         instantiates(+instantiated_objects),
         specializes(+specialized_objects))

     object(+object_identifier,
         imports(+imported_categories),
         instantiates(+instantiated_objects),
         specializes(+specialized_objects))

     object(+object_identifier,
         implements(+implemented_protocols),
         imports(+imported_categories),
         instantiates(+instantiated_objects),
         specializes(+specialized_objects))

2.3.3.32 Examples
.................

     :- object(list).

     :- object(list,
         implements(listp)).

     :- object(list,
         extends(compound)).

     :- object(list,
         implements(listp),
         extends(compound)).

     :- object(object,
         imports(initialization),
         instantiates(class)).

     :- object(abstract_class,
         instantiates(class),
         specializes(object)).

     :- object(agent,
         imports(private::attributes)).

See also
........

See end_object/0.

'directive'

2.3.3.33 ‘protocol/1-2’
.......................

2.3.3.34 Description
....................

     protocol(Protocol)

     protocol(Protocol,
         extends(Protocols))

Starting protocol directive.

2.3.3.35 Template and modes
...........................

     protocol(+protocol_identifier)

     protocol(+protocol_identifier,
         extends(+extended_protocols))

2.3.3.36 Examples
.................

     :- protocol(listp).

     :- protocol(listp,
         extends(compoundp)).

     :- protocol(queuep,
         extends(protected::listp)).

See also
........

See end_protocol/0.

'directive'

2.3.3.37 ‘threaded/0’
.....................

2.3.3.38 Description
....................

     threaded

Declares that an object supports threaded engines, concurrent calls, and
asynchronous messages.  Any object containing calls to the built-in
multi-threading predicates (or importing a category that contains such
calls) must include this directive.

This directive results in the automatic creation and set up of an object
message queue when the object is loaded or created at runtime.  Object
message queues are used for exchanging thread notifications and for
storing concurrent goal solutions and replies to the multi-threading
calls made within the object.  The message queue for the user.
pseudo-object is automatically created at Logtalk startup (provided that
multi-threading programming is supported and enabled for the chosen
backend Prolog compiler).

     Note: This directive requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.3.3.39 Template and modes
...........................

     threaded

2.3.3.40 Examples
.................

     :- threaded.

See also
........

See synchronized/1, object_property/2.

'directive'

2.3.3.41 ‘uses/1’
.................

2.3.3.42 Description
....................

     uses([Object as Alias, ...])

Declares object aliases.  Typically used to shorten long object names,
to simplify and consistently send messages to parameterized objects, and
to simplify using or experimenting with different object implementations
of the same protocol when using explicit message sending.  Object
aliases are local to the object (or category) where they are defined.

The objects being aliased can be parameter variables or
parametric objects where one of more parameters are parameter variables
when using the directive in a parametric object or a parametric category
defined in a source file (the common case).

Declaring multiple aliases for the same object is allowed.  But repeated
declarations of the same alias, declaring an alias for an object alias,
and redefining an alias to reference a different object are reported as
compilation errors.

To enable the use of static binding, and thus optimal message
sending performance, the objects should be loaded before compiling the
entities that call their predicates.

2.3.3.43 Template and modes
...........................

     uses(+object_alias_list)

2.3.3.44 Examples
.................

     :- object(foo(_HeapType_, _OptionsObject_)).

         :- uses([
             fast_random as rnd,
             time(utc) as time,
             heap(_HeapType_) as heap,
             _OptionsObject_ as options
         ]).

         bar :-
             ...,
             % the same as fast_random::permutation(L, P)
             rnd::permutation(L, P),
             % the same as heap(_HeapType_)::as_heap(L, H)
             heap::as_heap(L, H),
             % the same as _OptionsObject_::get(foo, X)
             options::get(foo, X),
             % the same as time(utc)::now(T)
             time::now(T),
             ...

See also
........

See uses/2, use_module/1, use_module/2.

'directive'

2.3.3.45 ‘use_module/1’
.......................

2.3.3.46 Description
....................

     use_module([Module as Alias, ...])

Declares module aliases.  Typically used to shorten long module names
and to simplify using or experimenting with different module
implementations of the same predicates when using explicitly-qualified
calls.  Module aliases are local to the object (or category) where they
are defined.

The modules being aliased can be parameter variables when
using the directive in a parametric object or a parametric category
defined in a source file (the common case).

Declaring multiple aliases for the same module is allowed.  But repeated
declarations of the same alias, declaring an alias for a module alias,
and redefining an alias to reference a different module are reported as
compilation errors.

To enable the use of static binding, and thus optimal
predicate call performance, the modules should be loaded before
compiling the entities that call their predicates.

Note that this directive semantics differs from the directive with the
same name found on some Prolog implementations where it is used to load
a module and import all its exported predicates.

2.3.3.47 Template and modes
...........................

     use_module(+module_alias_list)

2.3.3.48 Examples
.................

     :- object(foo(_DataModule_)).

         :- use_module([
             data_noise_handler as cleaner,
             _DataModule_ as data
         ]).

         bar :-
             ...,
             % the same as _DataModule_:xy(X, Y)
             data:xy(X, Y),
             % the same as data_noise_handler:filter(X, Y)
             cleaner:filter(X, Y, Z),
             ...

See also
........

See uses/1, uses/2, use_module/2.

2.3.4 Predicate directives
--------------------------

'directive'

2.3.4.1 ‘alias/2’
.................

2.3.4.2 Description
...................

     alias(Entity, [Name/Arity as Alias/Arity, ...])
     alias(Entity, [Name//Arity as Alias//Arity, ...])

Declares predicate and grammar rule non-terminal aliases.  A
predicate (non-terminal) alias is an alternative name for a predicate
(non-terminal) declared or defined in an extended protocol, an
implemented protocol, an extended category, an imported category, an
extended prototype, an instantiated class, or a specialized class.
Predicate aliases may be used to solve conflicts between imported or
inherited predicates.  It may also be used to give a predicate
(non-terminal) a name more appropriated in its usage context.  This
directive may be used in objects, protocols, and categories.

Predicate (and non-terminal) aliases are specified using (preferably)
the notation ‘Name/Arity as Alias/Arity’ or, in alternative, the
notation ‘Name/Arity::Alias/Arity’.

It is also possible to declare predicate and grammar rule non-terminal
aliases in implicit qualification directives for sending messages to
objects and calling module predicates.

2.3.4.3 Template and modes
..........................

     alias(@entity_identifier, +list(predicate_indicator_alias))
     alias(@entity_identifier, +list(non_terminal_indicator_alias))

2.3.4.4 Examples
................

     % resolve a predicate name conflict:
     :- alias(list, [member/2 as list_member/2]).
     :- alias(set,  [member/2 as set_member/2]).

     % define an alternative name for a non-terminal:
     :- alias(words, [singular//0 as peculiar//0]).

See also
........

See uses/2, use_module/2, uses/1.

'directive'

2.3.4.5 ‘coinductive/1’
.......................

2.3.4.6 Description
...................

     coinductive(Name/Arity)
     coinductive((Name/Arity, ...))
     coinductive([Name/Arity, ...])

     coinductive(Name//Arity)
     coinductive((Name//Arity, ...))
     coinductive([Name//Arity, ...])

     coinductive(Template)
     coinductive((Template1, ...))
     coinductive([Template1, ...])

This is an 'experimental' directive, used for declaring
coinductive predicates.  Requires a backend Prolog compiler
with minimal support for cyclic terms.  The current implementation
of coinduction allows the generation of only the 'basic cycles' but all
valid solutions should be recognized.  Use a predicate indicator or a
non-terminal indicator as argument when all the coinductive predicate
arguments are relevant for coinductive success.  Use a template when
only some coinductive predicate arguments (represented by a “‘+’”)
should be considered when testing for coinductive success (represent the
arguments that should be disregarded by a “‘-’“).  It’s possible to
define local coinductive_success_hook/1-2 predicates that
are automatically called with the coinductive predicate term resulting
from a successful unification with an ancestor goal as first argument.
The second argument, when present, is the coinductive hypothesis (i.e.
the ancestor goal) used.  These hook predicates can provide an
alternative to the use of tabling when defining some coinductive
predicates.  There is no overhead when these hook predicates are not
defined.

This directive must precede any calls to the declared coinductive
predicates.

2.3.4.7 Template and modes
..........................

     coinductive(+predicate_indicator_term)
     coinductive(+non_terminal_indicator_term)
     coinductive(+coinductive_predicate_template_term)

2.3.4.8 Examples
................

     :- coinductive(comember/2).
     :- coinductive(ones_and_zeros//0).
     :- coinductive(controller(+,+,+,-,-)).

See also
........

See coinductive_success_hook/1-2, predicate_property/2.

'directive'

2.3.4.9 ‘discontiguous/1’
.........................

2.3.4.10 Description
....................

     discontiguous(Name/Arity)
     discontiguous((Name/Arity, ...))
     discontiguous([Name/Arity, ...])

     discontiguous(Name//Arity)
     discontiguous((Name//Arity, ...))
     discontiguous([Name//Arity, ...])

Declares discontiguous predicates and discontiguous grammar
rule non-terminals.  The use of this directive should be avoided as not
all backend Prolog compilers support discontiguous
predicates.

     Warning: Some backend Prolog compilers declare the atom
     ‘discontiguous’ as an operator for a lighter syntax.  But this
     makes the code non-portable and is therefore a practice best
     avoided.

2.3.4.11 Template and modes
...........................

     discontiguous(+predicate_indicator_term)
     discontiguous(+non_terminal_indicator_term)

2.3.4.12 Examples
.................

     :- discontiguous(counter/1).

     :- discontiguous((lives/2, works/2)).

     :- discontiguous([db/4, key/2, file/3]).

'directive'

2.3.4.13 ‘dynamic/1’
....................

2.3.4.14 Description
....................

     dynamic(Name/Arity)
     dynamic((Name/Arity, ...))
     dynamic([Name/Arity, ...])

     dynamic(Entity::Name/Arity)
     dynamic((Entity::Name/Arity, ...))
     dynamic([Entity::Name/Arity, ...])

     dynamic(Module:Name/Arity)
     dynamic((Module:Name/Arity, ...))
     dynamic([Module:Name/Arity, ...])

     dynamic(Name//Arity)
     dynamic((Name//Arity, ...))
     dynamic([Name//Arity, ...])

     dynamic(Entity::Name//Arity)
     dynamic((Entity::Name//Arity, ...))
     dynamic([Entity::Name//Arity, ...])

     dynamic(Module:Name//Arity)
     dynamic((Module:Name//Arity, ...))
     dynamic([Module:Name//Arity, ...])

Declares dynamic predicates and dynamic grammar rule
non-terminals.  Note that an object can be static and have both static
and dynamic predicates/non-terminals.  When the dynamic predicates are
local to an object, declaring them also as private predicates
allows the Logtalk compiler to generate optimized code for
asserting and retracting predicate clauses.  Categories can also contain
dynamic predicate directives but cannot contain clauses for dynamic
predicates.

The predicate indicators (or non-terminal indicators) can be explicitly
qualified with an object, category, or module identifier when the
predicates (or non-terminals) are also declared multifile.

Note that dynamic predicates cannot be declared synchronized (when
necessary, declare the predicates updating the dynamic predicates as
synchronized).

     Warning: Some backend Prolog compilers declare the atom ‘dynamic’
     as an operator for a lighter syntax.  But this makes the code
     non-portable and is therefore a practice best avoided.

2.3.4.15 Template and modes
...........................

     dynamic(+qualified_predicate_indicator_term)
     dynamic(+qualified_non_terminal_indicator_term)

2.3.4.16 Examples
.................

     :- dynamic(counter/1).

     :- dynamic((lives/2, works/2)).

     :- dynamic([db/4, key/2, file/3]).

See also
........

See dynamic/0, predicate_property/2.

'directive'

2.3.4.17 ‘info/2’
.................

2.3.4.18 Description
....................

     info(Name/Arity, [Key is Value, ...])
     info(Name//Arity, [Key is Value, ...])

Documentation directive for predicates and grammar rule non-terminals.
The first argument is either a predicate indicator or a grammar rule
non-terminal indicator.  The second argument is a list of pairs using
the format 'Key is Value'.  See the Predicate directives.
section for a description of the default keys.

2.3.4.19 Template and modes
...........................

     info(+predicate_indicator, +predicate_info_list)
     info(+non_terminal_indicator, +predicate_info_list)

2.3.4.20 Examples
.................

     :- info(empty/1, [
         comment is 'True if the argument is an empty list.',
         argnames is ['List']
     ]).

     :- info(sentence//0, [
         comment is 'Rewrites a sentence into a noun phrase and a verb phrase.'
     ]).

See also
........

See info/1, mode/2, predicate_property/2.

'directive'

2.3.4.21 ‘meta_predicate/1’
...........................

2.3.4.22 Description
....................

     meta_predicate(Template)
     meta_predicate((Template, ...))
     meta_predicate([Template, ...])

     meta_predicate(Entity::Template)
     meta_predicate((Entity::Template, ...))
     meta_predicate([Entity::Template, ...])

     meta_predicate(Module:Template)
     meta_predicate((Module:Template, ...))
     meta_predicate([Module:Template, ...])

Declares meta-predicates, i.e., predicates that have arguments
interpreted as goals, arguments interpreted as closures used
to construct goals, or arguments with sub-terms that will be interpreted
as goals or closures.  Meta-arguments are always called in the
meta-predicate calling context, not in the meta-predicate
definition context.

Meta-arguments which are goals are represented by the integer ‘0’.
Meta-arguments which are closures are represented by a positive integer,
‘N’, representing the number of additional arguments that will be
appended to the closure in order to construct the corresponding goal
(typically by calling the call/1-N built-in method).
Meta-arguments with sub-terms that will be interpreted as goals or
closures are represented by ‘::’.  Meta-arguments that will be called
using the ‘bagof/3’ or ‘setof/3’ predicates and that can thus be
existentially-qualified are represented by the atom ‘^’.  Normal
arguments are represented by the atom ‘*’.

Logtalk allows the use of this directive to override the original
meta-predicate directive.  This is sometimes necessary when calling
Prolog built-in meta-predicates or Prolog module meta-predicates due to
the lack of standardization of the syntax of the meta-predicate
templates.

     Warning: Some backend Prolog compilers declare the atom
     ‘meta_predicate’ as an operator for a lighter syntax.  But this
     makes the code non-portable and is therefore a practice best
     avoided.

2.3.4.23 Template and modes
...........................

     meta_predicate(+meta_predicate_template_term)

     meta_predicate(+object_identifier::+meta_predicate_template_term)
     meta_predicate(+category_identifier::+meta_predicate_template_term)

     meta_predicate(+module_identifier:+meta_predicate_template_term)

2.3.4.24 Examples
.................

     % findall/3 second argument is interpreted as a goal:
     :- meta_predicate(findall(*, 0, *)).

     % both forall/2 arguments are interpreted as goals:
     :- meta_predicate(forall(0, 0)).

     % maplist/3 first argument is interpreted as a closure
     % that will be expanded to a goal by appending two
     % arguments:
     :- meta_predicate(maplist(2, *, *)).

See also
........

See meta_non_terminal/1, predicate_property/2.

'directive'

2.3.4.25 ‘meta_non_terminal/1’
..............................

2.3.4.26 Description
....................

     meta_non_terminal(Template)
     meta_non_terminal((Template, ...))
     meta_non_terminal([Template, ...])

     meta_non_terminal(Entity::Template)
     meta_non_terminal((Entity::Template, ...))
     meta_non_terminal([Entity::Template, ...])

     meta_non_terminal(Module:Template)
     meta_non_terminal((Module:Template, ...))
     meta_non_terminal([Module:Template, ...])

Declares meta-non-terminals, i.e., non-terminals that have arguments
that will be called as non-terminals (or grammar rule bodies).  An
argument may also be a closure instead of a goal if the
non-terminal uses the call//1-N built-in methods to
construct and call the actual non-terminal from the closure and the
additional arguments or the phrase//1 built-in method.

Meta-arguments which are non-terminals are represented by the integer
‘0’.  Meta-arguments which are closures are represented by a positive
integer, ‘N’, representing the number of additional arguments that will
be appended to the closure in order to construct the corresponding
meta-call.  Normal arguments are represented by the atom ‘*’.
Meta-arguments are always called in the meta-non-terminal calling
context, not in the meta-non-terminal definition context.

2.3.4.27 Template and modes
...........................

     meta_non_terminal(+meta_non_terminal_template_term)

     meta_non_terminal(+object_identifier::+meta_non_terminal_template_term)
     meta_non_terminal(+category_identifier::+meta_non_terminal_template_term)

     meta_non_terminal(+module_identifier:+meta_non_terminal_template_term)

2.3.4.28 Examples
.................

     :- meta_non_terminal(phrase(1, *)).
     phrase(X, T) --> call(X, T).

See also
........

See meta_predicate/1, predicate_property/2.

'directive'

2.3.4.29 ‘mode/2’
.................

2.3.4.30 Description
....................

     mode(Mode, NumberOfProofs)

Most predicates can be used with several instantiations modes.  This
directive enables the specification of each instantiation mode
and the corresponding number of proofs. (but not
necessarily distinct solutions).  The instantiation mode of each
argument can include type information.  You may also use this directive
for documenting grammar rule non-terminals.

2.3.4.31 Template and modes
...........................

     mode(+predicate_mode_term, +number_of_proofs)
     mode(+non_terminal_mode_term, +number_of_proofs)

2.3.4.32 Examples
.................

     :- mode(atom_concat(-atom, -atom, +atom), one_or_more).
     :- mode(atom_concat(+atom, +atom, -atom), one).

     :- mode(var(@term), zero_or_one).

     :- mode(solve(+callable, -list(atom)), zero_or_one).

See also
........

See info/2, predicate_property/2.

'directive'

2.3.4.33 ‘multifile/1’
......................

2.3.4.34 Description
....................

     multifile(Name/Arity)
     multifile((Name/Arity, ...))
     multifile([Name/Arity, ...])

     multifile(Entity::Name/Arity)
     multifile((Entity::Name/Arity, ...))
     multifile([Entity::Name/Arity, ...])

     multifile(Module:Name/Arity)
     multifile((Module:Name/Arity, ...))
     multifile([Module:Name/Arity, ...])

     multifile(Name//Arity)
     multifile((Name//Arity, ...))
     multifile([Name//Arity, ...])

     multifile(Entity::Name//Arity)
     multifile((Entity::Name//Arity, ...))
     multifile([Entity::Name//Arity, ...])

     multifile(Module:Name//Arity)
     multifile((Module:Name//Arity, ...))
     multifile([Module:Name//Arity, ...])

Declares multifile predicates and multifile grammar rule
non-terminals.  In the case of object or category multifile predicates,
the predicate (or non-terminal) must also have a scope directive
in the object or category holding its 'primary declaration' (i.e.
the declaration without the ‘Entity::’ prefix).  Entities holding
multifile predicate primary declarations must be compiled and loaded
prior to any entities contributing with clauses for the multifile
predicates (to prevent using multifile predicates to break entity
encapsulation).

Protocols cannot declare or define multifile predicates as protocols
cannot contain predicate definitions.

     Warning: Some backend Prolog compilers declare the atom ‘multifile’
     as an operator for a lighter syntax.  But this makes the code
     non-portable and is therefore a practice best avoided.

2.3.4.35 Template and modes
...........................

     multifile(+qualified_predicate_indicator_term)
     multifile(+qualified_non_terminal_indicator_term)

2.3.4.36 Examples
.................

     :- multifile(table/3).
     :- multifile(user::hook/2).

See also
........

See public/1, protected/1, private/1,
predicate_property/2.

'directive'

2.3.4.37 ‘private/1’
....................

2.3.4.38 Description
....................

     private(Name/Arity)
     private((Name/Arity, ...))
     private([Name/Arity, ...])

     private(Name//Arity)
     private((Name//Arity, ...))
     private([Name//Arity, ...])

     private(op(Precedence,Associativity,Operator))
     private((op(Precedence,Associativity,Operator), ...))
     private([op(Precedence,Associativity,Operator), ...])

Declares private predicates, private grammar rule
non-terminals, and private operators.  A private predicate can only be
called from the object containing the private directive.  A private
non-terminal can only be used in a call of the phrase/2 and
phrase/3 methods from the object containing the private
directive.

2.3.4.39 Template and modes
...........................

     private(+predicate_indicator_term)
     private(+non_terminal_indicator_term)
     private(+operator_declaration)

2.3.4.40 Examples
.................

     :- private(counter/1).

     :- private((init/1, free/1)).

     :- private([data/3, key/1, keys/1]).

See also
........

See protected/1, public/1, predicate_property/2.

'directive'

2.3.4.41 ‘protected/1’
......................

2.3.4.42 Description
....................

     protected(Name/Arity)
     protected((Name/Arity, ...))
     protected([Name/Arity, ...])

     protected(Name//Arity)
     protected((Name//Arity, ...))
     protected([Name//Arity, ...])

     protected(op(Precedence,Associativity,Operator))
     protected((op(Precedence,Associativity,Operator), ...))
     protected([op(Precedence,Associativity,Operator), ...])

Declares protected predicates, protected grammar rule
non-terminals, and protected operators.  A protected predicate can only
be called from the object containing the directive or from an object
that inherits the directive.  A protected non-terminal can only be used
as an argument in a phrase/2 and phrase/3 calls
from the object containing the directive or from an object that inherits
the directive.

     Note: Protected operators are not inherited but declaring them
     provides a reusable specification for using them in descendant
     objects (or categories).

2.3.4.43 Template and modes
...........................

     protected(+predicate_indicator_term)
     protected(+non_terminal_indicator_term)
     protected(+operator_declaration)

2.3.4.44 Examples
.................

     :- protected(init/1).

     :- protected((print/2, convert/4)).

     :- protected([load/1, save/3]).

See also
........

See private/1, public/1, predicate_property/2.

'directive'

2.3.4.45 ‘public/1’
...................

2.3.4.46 Description
....................

     public(Name/Arity)
     public((Name/Arity, ...))
     public([Name/Arity, ...])

     public(Name//Arity)
     public((Name//Arity, ...))
     public([Name//Arity, ...])

     public(op(Precedence,Associativity,Operator))
     public((op(Precedence,Associativity,Operator), ...))
     public([op(Precedence,Associativity,Operator), ...])

Declares public predicates, public grammar rule
non-terminals, and public operators.  A public predicate can be called
from any object.  A public non-terminal can be used as an argument in
phrase/2 and phrase/3 calls from any object.

     Note: Declaring a public operator does not make it global when the
     entity holding the scope directive is compiled and loaded.  But
     declaring public operators provides a reusable specification for
     using them in the entity clients (e.g.  in uses/2.
     directives).

2.3.4.47 Template and modes
...........................

     public(+predicate_indicator_term)
     public(+non_terminal_indicator_term)
     public(+operator_declaration)

2.3.4.48 Examples
.................

     :- public(ancestor/1).

     :- public((instance/1, instances/1)).

     :- public([leaf/1, leaves/1]).

See also
........

See private/1, protected/1,
predicate_property/2.

'directive'

2.3.4.49 ‘synchronized/1’
.........................

2.3.4.50 Description
....................

     synchronized(Name/Arity)
     synchronized((Name/Arity, ...))
     synchronized([Name/Arity, ...])

     synchronized(Name//Arity)
     synchronized((Name//Arity, ...))
     synchronized([Name//Arity, ...])

Declares synchronized predicates and synchronized grammar
rule non-terminals.  The most common use is for predicates that have
side effects (e.g.  asserting or retracting clauses for a dynamic
predicate) in multi-threaded applications.  A synchronized predicate (or
synchronized non-terminal) is protected by a mutex in order to allow for
thread synchronization when proving a call to the predicate (or
non-terminal).

All predicates (and non-terminals) declared in the same synchronized
directive share the same mutex.  In order to use a separate mutex for
each predicate (non-terminal) so that they are independently
synchronized, a per-predicate synchronized directive must be used.

     Warning: Declaring a predicate synchronized implicitly makes it
     'deterministic'.  When using a single-threaded backend Prolog compiler,
     calls to synchronized predicates behave as wrapped by
     the standard once/1 meta-predicate.

Note that synchronized predicates cannot be declared dynamic.
(when necessary, declare the predicates updating the dynamic predicates
as synchronized).

2.3.4.51 Template and modes
...........................

     synchronized(+predicate_indicator_term)
     synchronized(+non_terminal_indicator_term)

2.3.4.52 Examples
.................

     :- synchronized(db_update/1).

     :- synchronized((write_stream/2, read_stream/2)).

     :- synchronized([add_to_queue/2, remove_from_queue/2]).

See also
........

See predicate_property/2.

'directive'

2.3.4.53 ‘uses/2’
.................

2.3.4.54 Description
....................

     uses(Object, [Name/Arity, ...])
     uses(Object, [Name/Arity as Alias/Arity, ...])

     uses(Object, [Predicate as Alias, ...])

     uses(Object, [Name//Arity, ...])
     uses(Object, [Name//Arity as Alias//Arity, ...])

     uses(Object, [op(Precedence, Associativity, Operator), ...])

Declares that all calls made from predicates (or non-terminals) defined
in the category or object containing the directive to the specified
predicates (or non-terminals) are to be interpreted as messages to the
specified object.  Thus, this directive may be used to simplify writing
of predicate definitions by allowing the programmer to omit the
‘Object::’ prefix when using the predicates listed in the directive (as
long as the calls do not occur as arguments for non-standard Prolog
meta-predicates not declared on the adapter files).  It is also possible
to include operator declarations in the second argument.

This directive is also taken into account when compiling calls to the
database and reflection built-in methods by
looking into these methods predicate arguments if bound at compile time.

It is possible to specify a predicate alias using the notation
‘Name/Arity as Alias/Arity’ or, in alternative, the notation
‘Name/Arity::Alias/Arity’.  Aliases may be used either for avoiding
conflicts between predicates specified in ‘use_module/2’ and ‘uses/2’
directives or for giving more meaningful names considering the calling
context of the predicates.  For predicates, is also possible to define
alias shorthands using the notation ‘Predicate as Alias’ or, in
alternative, the notation ‘Predicate::Alias’, where ‘Predicate’ and
‘Alias’ are callable terms where some or all arguments may be
instantiated.

To enable the use of static binding, and thus optimal message
sending performance, the objects should be loaded before compiling the
entities that call their predicates.

The object identifier argument can also be a parameter variable
when using the directive in a parametric object or a parametric
category defined in a source file (the common case).  In this case,
see dynamic binding will be used for all listed predicates (and
non-terminals).  The parameter variable must be instantiated at runtime
when the messages are sent.

2.3.4.55 Template and modes
...........................

     uses(+object_identifier, +predicate_indicator_list)
     uses(+object_identifier, +predicate_indicator_alias_list)

     uses(+object_identifier, +predicate_template_alias_list)

     uses(+object_identifier, +non_terminal_indicator_list)
     uses(+object_identifier, +non_terminal_indicator_alias_list)

     uses(+object_identifier, +operator_list)

2.3.4.56 Examples
.................

     :- uses(list,  [append/3, member/2]).
     :- uses(store, [data/2]).
     :- uses(user,  [table/4]).

     foo :-
         ...,
         % the same as findall(X, list::member(X, L), A)
         findall(X, member(X, L), A),
         % the same as list::append(A, B, C)
         append(A, B, C),
         % the same as store::assertz(data(X, C))
         assertz(data(X, C)),
         % call the table/4 predicate in "user"
         table(X, Y, Z, T),
         ...

Another example, using the extended notation that allows us to define
predicate aliases:

     :- uses(btrees, [new/1 as new_btree/1]).
     :- uses(queues, [new/1 as new_queue/1]).

     btree_to_queue :-
         ...,
         % the same as btrees::new(Tree)
         new_btree(Tree),
         % the same as queues::new(Queue)
         new_queue(Queue),
         ...

An example of defining a predicate alias that is also a shorthand:

     :- uses(logtalk, [
         print_message(debug, my_app, Message) as dbg(Message)
     ]).

Predicate aliases can also be used to change argument order:

     :- uses(meta, [
         fold_left(Closure,Accumulator,List,Result) as foldl(Closure,List,Accumulator,Result)
     ]).

An example of using a parameter variable in place of the
object identifier to allow using the same test set for checking multiple
implementations of the same protocol:

     :- object(tests(_HeapObject_),
         extends(lgtunit)).

         :- uses(_HeapObject_, [
             as_heap/2, as_list/2, valid/1, new/1,
             insert/4, insert_all/3, delete/4, merge/3,
             empty/1, size/2, top/3, top_next/5
         ]).

See also
........

See uses/1, use_module/1, use_module/2,
alias/2.

'directive'

2.3.4.57 ‘use_module/2’
.......................

2.3.4.58 Description
....................

     use_module(Module, [Name/Arity, ...])
     use_module(Module, [Name/Arity as Alias/Arity, ...])

     use_module(Module, [Predicate as Alias, ...])

     use_module(Module, [Name//Arity, ...])
     use_module(Module, [Name//Arity as Alias//Arity, ...])

     use_module(Module, [op(Precedence,Associativity,Operator), ...])

This directive declares that all calls (made from predicates defined in
the category or object containing the directive) to the specified
predicates (or non-terminals) are to be interpreted as calls to
explicitly-qualified module predicates (or non-terminals).  Thus, this
directive may be used to simplify writing of predicate definitions by
allowing the programmer to omit the ‘Module:’ prefix when using the
predicates listed in the directive (as long as the predicate calls do
not occur as arguments for non-standard Prolog meta-predicates not
declared on the adapter files).  It is also possible to include operator
declarations in the second argument.

This directive is also taken into account when compiling calls to the
database and reflection built-in methods by
looking into these methods predicate arguments if bound at compile time.

It is possible to specify a predicate alias using the notation
‘Name/Arity as Alias/Arity’ or, in alternative, the notation
‘Name/Arity:Alias/Arity’.  Aliases may be used either for avoiding
conflicts between predicates specified in ‘use_module/2’ and
uses/2 directives or for giving more meaningful names considering
the calling context of the predicates.  For predicates, is also possible
to define alias shorthands using the notation ‘Predicate as Alias’ or,
in alternative, the notation ‘Predicate::Alias’, where ‘Predicate’ and
‘Alias’ are callable terms where some or all arguments may be
instantiated.

Note that this directive differs from the directive with the same name
found on some Prolog implementations by requiring the first argument to
be a module name (an atom) instead of a file specification.  In Logtalk,
there’s no mixing between 'loading' a resource and (declaring the)
'using' (of) a resource.  As a consequence, this directive doesn’t
automatically load the module.  Loading the module file is dependent of
the used backend Prolog compiler and must be done separately
(usually, using a source file directive such as ‘use_module/1’ or
‘use_module/2’ in the entity file or preferably in the application
loader file file).  Also, note that the name of the module may
differ from the name of the module file.

     Warning: The modules 'must' be loaded prior to the compilation of
     entities that call the module predicates.  This is required in
     general to allow the compiler to check if the called module
     predicate is a meta-predicate and retrieve its meta-predicate
     template to ensure proper call compilation.

The module identifier argument can also be a parameter variable
when using the directive in a parametric object or a parametric
category defined in a source file (the common case).  In this case,
see dynamic binding will be used for all listed predicates (and
non-terminals).  The parameter variable must be instantiated at runtime
when the calls are made.

2.3.4.59 Template and modes
...........................

     use_module(+module_identifier, +predicate_indicator_list)
     use_module(+module_identifier, +module_predicate_indicator_alias_list)

     use_module(+module_identifier, +predicate_template_alias_list)

     use_module(+module_identifier, +non_terminal_indicator_list)
     use_module(+module_identifier, +module_non_terminal_indicator_alias_list)

     use_module(+module_identifier, +operator_list)

2.3.4.60 Examples
.................

     :- use_module(lists, [append/3, member/2]).
     :- use_module(store, [data/2]).
     :- use_module(user,  [foo/1 as bar/1]).

     foo :-
         ...,
         % same as findall(X, lists:member(X, L), A)
         findall(X, member(X, L), A),
         % same as lists:append(A, B, C)
         append(A, B, C),
         % same as assertz(store:data(X, C))
         assertz(data(X, C)),
         % same as retractall(user:foo(_))
         retractall(bar(_)),
         ...

Another example, using the extended notation that allows us to define
predicate aliases:

     :- use_module(ugraphs, [transpose_ugraph/2 as transpose/2]).

     convert_graph :-
         ...,
         % the same as ugraphs:transpose_ugraph(Graph0, Graph)
         transpose(Graph0, Graph),
         ...

An example of defining a predicate alias that is also a shorthand:

     :- use_module(pairs, [
         map_list_to_pairs(length, Lists, Pairs) as length_pairs(Lists, Pairs)
     ]).

An example of using a parameter variable in place of the
module identifier to delay to runtime the actual module to use:

     :- object(bar(_OptionsModule_)).

         :- use_module(_OptionsModule_, [
             set/2, get/2, reset/0
         ]).

See also
........

See use_module/1, uses/2, uses/1,
alias/2.

2.4 Built-in predicates
=======================

2.4.1 Enumerating objects, categories and protocols
---------------------------------------------------

'built-in predicate'

2.4.1.1 ‘current_category/1’
............................

2.4.1.2 Description
...................

     current_category(Category)

Enumerates, by backtracking, all currently defined categories.  All
categories are found, either static, dynamic, or built-in.

2.4.1.3 Modes and number of proofs
..................................

     current_category(?category_identifier) - zero_or_more

2.4.1.4 Errors
..............

     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Category)’

2.4.1.5 Examples
................

     % enumerate the defined categories:
     | ?- current_category(Category).

     Category = core_messages ;
     ...

See also
........

See abolish_category/1, category_property/2,
create_category/4, complements_object/2,
extends_category/2-3, imports_category/2-3.

'built-in predicate'

2.4.1.6 ‘current_object/1’
..........................

2.4.1.7 Description
...................

     current_object(Object)

Enumerates, by backtracking, all currently defined objects.  All objects
are found, either static, dynamic or built-in.

2.4.1.8 Modes and number of proofs
..................................

     current_object(?object_identifier) - zero_or_more

2.4.1.9 Errors
..............

     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’

2.4.1.10 Examples
.................

     % enumerate the defined objects:
     | ?- current_object(Object).

     Object = user ;
     Object = logtalk ;
     ...

See also
........

See abolish_object/1, create_object/4,
object_property/2, extends_object/2-3,
instantiates_class/2-3, specializes_class/2-3,
complements_object/2.

'built-in predicate'

2.4.1.11 ‘current_protocol/1’
.............................

2.4.1.12 Description
....................

     current_protocol(Protocol)

Enumerates, by backtracking, all currently defined protocols.  All
protocols are found, either static, dynamic, or built-in.

2.4.1.13 Modes and number of proofs
...................................

     current_protocol(?protocol_identifier) - zero_or_more

2.4.1.14 Errors
...............

     ‘Protocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Protocol)’

2.4.1.15 Examples
.................

     % enumerate the defined protocols:
     | ?- current_protocol(Protocol).

     Protocol = expanding ;
     Protocol = monitoring ;
     Protocol = forwarding ;
     ...

See also
........

See abolish_protocol/1, create_protocol/3,
protocol_property/2, conforms_to_protocol/2-3,
extends_protocol/2-3, implements_protocol/2-3.

2.4.2 Enumerating objects, categories and protocols properties
--------------------------------------------------------------

'built-in predicate'

2.4.2.1 ‘category_property/2’
.............................

2.4.2.2 Description
...................

     category_property(Category, Property)

Enumerates, by backtracking, the properties associated with the defined
categories.  The valid properties are listed in the language grammar
section on entity properties and described in the User Manual
section on category properties.

2.4.2.3 Modes and number of proofs
..................................

     category_property(?category_identifier, ?category_property) - zero_or_more

2.4.2.4 Errors
..............

     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Category)’

     ‘Property’ is neither a variable nor a callable term:
          ‘type_error(callable, Property)’
     ‘Property’ is a callable term but not a valid category property:
          ‘domain_error(category_property, Property)’

2.4.2.5 Examples
................

     % enumerate the properties of the core_messages built-in category:
     | ?- category_property(core_messages, Property).

     Property = source_data ;
     Property = static ;
     Property = built_in ;
     ...

See also
........

See abolish_category/1, create_category/4,
current_category/1, complements_object/2,
extends_category/2-3, imports_category/2-3.

'built-in predicate'

2.4.2.6 ‘object_property/2’
...........................

2.4.2.7 Description
...................

     object_property(Object, Property)

Enumerates, by backtracking, the properties associated with the defined
objects.  The valid properties are listed in the language grammar
section on entity properties and described in the User Manual
section on object properties.

2.4.2.8 Modes and number of proofs
..................................

     object_property(?object_identifier, ?object_property) - zero_or_more

2.4.2.9 Errors
..............

     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Property’ is neither a variable nor a callable term:
          ‘type_error(callable, Property)’
     ‘Property’ is a callable term but not a valid object property:
          ‘domain_error(object_property, Property)’

2.4.2.10 Examples
.................

     % enumerate the properties of the logtalk built-in object:
     | ?- object_property(logtalk, Property).

     Property = context_switching_calls ;
     Property = source_data ;
     Property = threaded ;
     Property = static ;
     Property = built_in ;
     ...

See also
........

See abolish_object/1, create_object/4,
current_object/1, extends_object/2-3,
instantiates_class/2-3, specializes_class/2-3,
complements_object/2.

'built-in predicate'

2.4.2.11 ‘protocol_property/2’
..............................

2.4.2.12 Description
....................

     protocol_property(Protocol, Property)

Enumerates, by backtracking, the properties associated with the
currently defined protocols.  The valid properties are listed in the
language grammar section on entity properties and described
in the User Manual section on protocol properties.

2.4.2.13 Modes and number of proofs
...................................

     protocol_property(?protocol_identifier, ?protocol_property) - zero_or_more

2.4.2.14 Errors
...............

     ‘Protocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Protocol)’
     ‘Property’ is neither a variable nor a callable term:
          ‘type_error(callable, Property)’
     ‘Property’ is a callable term but not a valid protocol property:
          ‘domain_error(protocol_property, Property)’

2.4.2.15 Examples
.................

     % enumerate the properties of the monitoring built-in protocol:
     | ?- protocol_property(monitoring, Property).

     Property = source_data ;
     Property = static ;
     Property = built_in ;
     ...

See also
........

See abolish_protocol/1, create_protocol/3,
current_protocol/1, conforms_to_protocol/2-3,
extends_protocol/2-3, implements_protocol/2-3.

2.4.3 Creating new objects, categories and protocols
----------------------------------------------------

'built-in predicate'

2.4.3.1 ‘create_category/4’
...........................

2.4.3.2 Description
...................

     create_category(Identifier, Relations, Directives, Clauses)

Creates a new, dynamic category.  This predicate is often used as a
primitive to implement high-level category creation methods.

Note that, when opting for runtime generated category identifiers, it’s
possible to run out of identifiers when using a backend Prolog compiler
with bounded integer support.  The portable solution, when
creating a large number of dynamic category in long-running
applications, is to recycle, whenever possible, the identifiers.

When creating a new dynamic parametric category, access to the object
parameters must use the parameter/2 built-in execution
context method.

When using Logtalk multi-threading features, predicates calling this
built-in predicate may need to be declared synchronized in order to
avoid race conditions.

2.4.3.3 Modes and number of proofs
..................................

     create_category(?category_identifier, @list(category_relation), @list(category_directive), @list(clause)) - one

2.4.3.4 Errors
..............

     ‘Relations’, ‘Directives’, or ‘Clauses’ is a variable:
          ‘instantiation_error’
     ‘Identifier’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Identifier)’
     ‘Identifier’ is already in use:
          ‘permission_error(modify, category, Identifier)’
          ‘permission_error(modify, object, Identifier)’
          ‘permission_error(modify, protocol, Identifier)’
     ‘Relations’ is neither a variable nor a proper list:
          ‘type_error(list, Relations)’
     Repeated entity relation clause:
          ‘permission_error(repeat, entity_relation, implements/1)’
          ‘permission_error(repeat, entity_relation, extends/1)’
          ‘permission_error(repeat, entity_relation, complements/1)’
     ‘Directives’ is neither a variable nor a proper list:
          ‘type_error(list, Directives)’
     ‘Clauses’ is neither a variable nor a proper list:
          ‘type_error(list, Clauses)’

2.4.3.5 Examples
................

     | ?- create_category(
             tolerances,
             [implements(comparing)],
             [],
             [epsilon(1e-15), (equal(X, Y) :- epsilon(E), abs(X-Y) =< E)]
          ).

See also
........

See abolish_category/1, category_property/2,
current_category/1, complements_object/2,
extends_category/2-3, imports_category/2-3.

'built-in predicate'

2.4.3.6 ‘create_object/4’
.........................

2.4.3.7 Description
...................

     create_object(Identifier, Relations, Directives, Clauses)

Creates a new, dynamic object.  The word object is used here
as a generic term.  This predicate can be used to create new prototypes,
instances, and classes.  This predicate is often used as a primitive to
implement high-level object creation methods.

Note that, when opting for runtime generated object identifiers, it’s
possible to run out of identifiers when using a backend Prolog compiler
with bounded integer support.  The portable solution, when
creating a large number of dynamic objects in long-running applications,
is to recycle, whenever possible, the identifiers.

When creating a new dynamic parametric object, access to the object
parameters must use the parameter/2 built-in execution
context method.

Declared predicates (using scope clauses in the ‘Directives’ argument)
are implicitly declared also as dynamic.

When using Logtalk multi-threading features, predicates calling this
built-in predicate may need to be declared synchronized in order to
avoid race conditions.

2.4.3.8 Modes and number of proofs
..................................

     create_object(?object_identifier, @list(object_relation), @list(object_directive), @list(clause)) - one

2.4.3.9 Errors
..............

     ‘Relations’, ‘Directives’, or ‘Clauses’ is a variable:
          ‘instantiation_error’
     ‘Identifier’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Identifier)’
     ‘Identifier’ is already in use:
          ‘permission_error(modify, category, Identifier)’
          ‘permission_error(modify, object, Identifier)’
          ‘permission_error(modify, protocol, Identifier)’
     ‘Relations’ is neither a variable nor a proper list:
          ‘type_error(list, Relations)’
     Repeated entity relation clause:
          ‘permission_error(repeat, entity_relation, implements/1)’
          ‘permission_error(repeat, entity_relation, imports/1)’
          ‘permission_error(repeat, entity_relation, extends/1)’
          ‘permission_error(repeat, entity_relation, instantiates/1)’
          ‘permission_error(repeat, entity_relation, specializes/1)’
     ‘Directives’ is neither a variable nor a proper list:
          ‘type_error(list, Directives)’
     ‘Clauses’ is neither a variable nor a proper list:
          ‘type_error(list, Clauses)’

2.4.3.10 Examples
.................

     % create a stand-alone object (a prototype):
     | ?- create_object(
              translator,
              [],
              [public(int/2)],
              [int(0, zero)]
          ).

     % create a prototype derived from a parent prototype:
     | ?- create_object(
              mickey,
              [extends(mouse)],
              [public(alias/1)],
              [alias(mortimer)]
          ).

     % create a class instance:
     | ?- create_object(
              p1,
              [instantiates(person)],
              [],
              [name('Paulo Moura'), age(42)]
          ).

     % create a subclass:
     | ?- create_object(
              hovercraft,
              [specializes(vehicle)],
              [public([propeller/2, fan/2])],
              []
          ).

     % create an object with an initialization goal:
     | ?- create_object(
              runner,
              [instantiates(runners)],
              [initialization(::start)],
              [length(22), time(60)]
          ).

     % create an object supporting dynamic predicate declarations:
     | ?- create_object(
              database,
              [],
              [set_logtalk_flag(dynamic_declarations, allow)],
              []
          ).

See also
........

See abolish_object/1, current_object/1,
object_property/2, extends_object/2-3,
instantiates_class/2-3, specializes_class/2-3,
complements_object/2.

'built-in predicate'

2.4.3.11 ‘create_protocol/3’
............................

2.4.3.12 Description
....................

     create_protocol(Identifier, Relations, Directives)

Creates a new, dynamic, protocol.  This predicate is often used as a
primitive to implement high-level protocol creation methods.

Note that, when opting for runtime generated protocol identifiers, it’s
possible to run out of identifiers when using a backend Prolog compiler
with bounded integer support.  The portable solution, when
creating a large number of dynamic protocols in long-running
applications, is to recycle, whenever possible, the identifiers.

When using Logtalk multi-threading features, predicates calling this
built-in predicate may need to be declared synchronized in order to
avoid race conditions.

2.4.3.13 Modes and number of proofs
...................................

     create_protocol(?protocol_identifier, @list(protocol_relation), @list(protocol_directive)) - one

2.4.3.14 Errors
...............

     Either ‘Relations’ or ‘Directives’ is a variable:
          ‘instantiation_error’
     ‘Identifier’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Identifier)’
     ‘Identifier’ is already in use:
          ‘permission_error(modify, category, Identifier)’
          ‘permission_error(modify, object, Identifier)’
          ‘permission_error(modify, protocol, Identifier)’
     ‘Relations’ is neither a variable nor a proper list:
          ‘type_error(list, Relations)’
     Repeated entity relation clause:
          ‘permission_error(repeat, entity_relation, extends/1)’
     ‘Directives’ is neither a variable nor a proper list:
          ‘type_error(list, Directives)’

2.4.3.15 Examples
.................

     | ?- create_protocol(
             logging,
             [extends(monitoring)],
             [public([log_file/1, log_on/0, log_off/0])]
          ).

See also
........

See abolish_protocol/1, current_protocol/1,
protocol_property/2, conforms_to_protocol/2-3,
extends_protocol/2-3, implements_protocol/2-3.

2.4.4 Abolishing objects, categories and protocols
--------------------------------------------------

'built-in predicate'

2.4.4.1 ‘abolish_category/1’
............................

2.4.4.2 Description
...................

     abolish_category(Category)

Abolishes a dynamic category.  The category identifier can then be
reused when creating a new category.

2.4.4.3 Modes and number of proofs
..................................

     abolish_category(+category_identifier) - one

2.4.4.4 Errors
..............

     ‘Category’ is a variable:
          ‘instantiation_error’
     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Category)’
     ‘Category’ is an identifier of a static category:
          ‘permission_error(modify, static_category, Category)’
     ‘Category’ does not exist:
          ‘existence_error(category, Category)’

2.4.4.5 Examples
................

     | ?- abolish_category(monitoring).

See also
........

See category_property/2, create_category/4,
current_category/1. See complements_object/2,
extends_category/2-3, imports_category/2-3.

'built-in predicate'

2.4.4.6 ‘abolish_object/1’
..........................

2.4.4.7 Description
...................

     abolish_object(Object)

Abolishes a dynamic object.  The object identifier can then be reused
when creating a new object.

2.4.4.8 Modes and number of proofs
..................................

     abolish_object(+object_identifier) - one

2.4.4.9 Errors
..............

     ‘Object’ is a variable:
          ‘instantiation_error’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Object’ is an identifier of a static object:
          ‘permission_error(modify, static_object, Object)’
     ‘Object’ does not exist:
          ‘existence_error(object, Object)’

2.4.4.10 Examples
.................

     | ?- abolish_object(list).

See also
........

See create_object/4, current_object/1,
object_property/2, extends_object/2-3,
instantiates_class/2-3, specializes_class/2-3,
complements_object/2.

'built-in predicate'

2.4.4.11 ‘abolish_protocol/1’
.............................

2.4.4.12 Description
....................

     abolish_protocol(Protocol)

Abolishes a dynamic protocol.  The protocol identifier can then be
reused when creating a new protocol.

2.4.4.13 Modes and number of proofs
...................................

     abolish_protocol(@protocol_identifier) - one

2.4.4.14 Errors
...............

     ‘Protocol’ is a variable:
          ‘instantiation_error’
     ‘Protocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Protocol)’
     ‘Protocol’ is an identifier of a static protocol:
          ‘permission_error(modify, static_protocol, Protocol)’
     ‘Protocol’ does not exist:
          ‘existence_error(protocol, Protocol)’

2.4.4.15 Examples
.................

     | ?- abolish_protocol(listp).

See also
........

See create_protocol/3, current_protocol/1,
protocol_property/2, conforms_to_protocol/2-3,
extends_protocol/2-3, implements_protocol/2-3.

2.4.5 Objects, categories, and protocols relations
--------------------------------------------------

'built-in predicate'

2.4.5.1 ‘extends_object/2-3’
............................

2.4.5.2 Description
...................

     extends_object(Prototype, Parent)
     extends_object(Prototype, Parent, Scope)

Enumerates, by backtracking, all pairs of objects such that the first
one extends the second.  The relation scope is represented by the atoms
‘public’, ‘protected’, and ‘private’.

2.4.5.3 Modes and number of proofs
..................................

     extends_object(?object_identifier, ?object_identifier) - zero_or_more
     extends_object(?object_identifier, ?object_identifier, ?scope) - zero_or_more

2.4.5.4 Errors
..............

     ‘Prototype’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Prototype)’
     ‘Parent’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Parent)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.5 Examples
................

     % enumerate objects derived from the state_space prototype:
     | ?- extends_object(Object, state_space).

     % enumerate objects publicly derived from the list prototype:
     | ?- extends_object(Object, list, public).

See also
........

See current_object/1, instantiates_class/2-3,
specializes_class/2-3.

'built-in predicate'

2.4.5.6 ‘extends_protocol/2-3’
..............................

2.4.5.7 Description
...................

     extends_protocol(Protocol, ParentProtocol)
     extends_protocol(Protocol, ParentProtocol, Scope)

Enumerates, by backtracking, all pairs of protocols such that the first
one extends the second.  The relation scope is represented by the atoms
‘public’, ‘protected’, and ‘private’.

2.4.5.8 Modes and number of proofs
..................................

     extends_protocol(?protocol_identifier, ?protocol_identifier) - zero_or_more
     extends_protocol(?protocol_identifier, ?protocol_identifier, ?scope) - zero_or_more

2.4.5.9 Errors
..............

     ‘Protocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Protocol)’
     ‘ParentProtocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, ParentProtocol)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.10 Examples
.................

     % enumerate the protocols extended by the listp protocol:
     | ?- extends_protocol(listp, Protocol).

     % enumerate protocols that privately extend the termp protocol:
     | ?- extends_protocol(Protocol, termp, private).

See also
........

See current_protocol/1, implements_protocol/2-3,
conforms_to_protocol/2-3.

'built-in predicate'

2.4.5.11 ‘extends_category/2-3’
...............................

2.4.5.12 Description
....................

     extends_category(Category, ParentCategory)
     extends_category(Category, ParentCategory, Scope)

Enumerates, by backtracking, all pairs of categories such that the first
one extends the second.  The relation scope is represented by the atoms
‘public’, ‘protected’, and ‘private’.

2.4.5.13 Modes and number of proofs
...................................

     extends_category(?category_identifier, ?category_identifier) - zero_or_more
     extends_category(?category_identifier, ?category_identifier, ?scope) - zero_or_more

2.4.5.14 Errors
...............

     ‘Category’ is neither a variable nor a valid protocol identifier:
          ‘type_error(category_identifier, Category)’
     ‘ParentCategory’ is neither a variable nor a valid protocol identifier:
          ‘type_error(category_identifier, ParentCategory)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.15 Examples
.................

     % enumerate the categories extended by the derailleur category:
     | ?- extends_category(derailleur, Category).

     % enumerate categories that privately extend the basics category:
     | ?- extends_category(Category, basics, private).

See also
........

See current_category/1, complements_object/2,
imports_category/2-3.

'built-in predicate'

2.4.5.16 ‘implements_protocol/2-3’
..................................

2.4.5.17 Description
....................

     implements_protocol(Object, Protocol)
     implements_protocol(Category, Protocol)

     implements_protocol(Object, Protocol, Scope)
     implements_protocol(Category, Protocol, Scope)

Enumerates, by backtracking, all pairs of entities such that an object
or a category implements a protocol.  The relation scope is represented
by the atoms ‘public’, ‘protected’, and ‘private’.  This predicate only
returns direct implementation relations.  For a transitive closure, see
the conforms_to_protocol/2-3 predicate.

2.4.5.18 Modes and number of proofs
...................................

     implements_protocol(?object_identifier, ?protocol_identifier) - zero_or_more
     implements_protocol(?category_identifier, ?protocol_identifier) - zero_or_more

     implements_protocol(?object_identifier, ?protocol_identifier, ?scope) - zero_or_more
     implements_protocol(?category_identifier, ?protocol_identifier, ?scope) - zero_or_more

2.4.5.19 Errors
...............

     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Category)’
     ‘Protocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Protocol)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.20 Examples
.................

     % check that the list object implements the listp protocol:
     | ?- implements_protocol(list, listp).

     % check that the list object publicly implements the listp protocol:
     | ?- implements_protocol(list, listp, public).

     % enumerate only objects that implement the listp protocol:
     | ?- current_object(Object), implements_protocol(Object, listp).

     % enumerate only categories that implement the serialization protocol:
     | ?- current_category(Category), implements_protocol(Category, serialization).

See also
........

See current_object/1, current_protocol/1,
current_category/1, conforms_to_protocol/2-3.

'built-in predicate'

2.4.5.21 ‘conforms_to_protocol/2-3’
...................................

2.4.5.22 Description
....................

     conforms_to_protocol(Object, Protocol)
     conforms_to_protocol(Category, Protocol)

     conforms_to_protocol(Object, Protocol, Scope)
     conforms_to_protocol(Category, Protocol, Scope)

Enumerates, by backtracking, all pairs of entities such that an object
or a category conforms to a protocol.  The relation scope is represented
by the atoms ‘public’, ‘protected’, and ‘private’.  This predicate
implements a transitive closure for the protocol implementation
relation.

2.4.5.23 Modes and number of proofs
...................................

     conforms_to_protocol(?object_identifier, ?protocol_identifier) - zero_or_more
     conforms_to_protocol(?category_identifier, ?protocol_identifier) - zero_or_more

     conforms_to_protocol(?object_identifier, ?protocol_identifier, ?scope) - zero_or_more
     conforms_to_protocol(?category_identifier, ?protocol_identifier, ?scope) - zero_or_more

2.4.5.24 Errors
...............

     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Category)’
     ‘Protocol’ is neither a variable nor a valid protocol identifier:
          ‘type_error(protocol_identifier, Protocol)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.25 Examples
.................

     % enumerate objects and categories that conform to the listp protocol:
     | ?- conforms_to_protocol(Object, listp).

     % enumerate objects and categories that privately conform to the listp protocol:
     | ?- conforms_to_protocol(Object, listp, private).

     % enumerate only objects that conform to the listp protocol:
     | ?- current_object(Object), conforms_to_protocol(Object, listp).

     % enumerate only categories that conform to the serialization protocol:
     | ?- current_category(Category), conforms_to_protocol(Category, serialization).

See also
........

See current_object/1, current_protocol/1,
current_category/1, implements_protocol/2-3.

'built-in predicate'

2.4.5.26 ‘complements_object/2’
...............................

2.4.5.27 Description
....................

     complements_object(Category, Object)

Enumerates, by backtracking, all category–object pairs such that the
category explicitly complements the object.

2.4.5.28 Modes and number of proofs
...................................

     complements_object(?category_identifier, ?object_identifier) - zero_or_more

2.4.5.29 Errors
...............

     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Prototype)’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Parent)’

2.4.5.30 Examples
.................

     % check that the logging category complements the employee object:
     | ?- complements_object(logging, employee).

See also
........

See current_category/1, imports_category/2-3.

'built-in predicate'

2.4.5.31 ‘imports_category/2-3’
...............................

2.4.5.32 Description
....................

     imports_category(Object, Category)

     imports_category(Object, Category, Scope)

Enumerates, by backtracking, importation relations between objects and
categories.  The relation scope is represented by the atoms ‘public’,
‘protected’, and ‘private’.

2.4.5.33 Modes and number of proofs
...................................

     imports_category(?object_identifier, ?category_identifier) - zero_or_more
     imports_category(?object_identifier, ?category_identifier, ?scope) - zero_or_more

2.4.5.34 Errors
...............

     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Category’ is neither a variable nor a valid category identifier:
          ‘type_error(category_identifier, Category)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.35 Examples
.................

     % check that the xref_diagram object imports the diagram category:
     | ?- imports_category(xref_diagram, diagram).

     % enumerate the objects that privately import the diagram category:
     | ?- imports_category(Object, diagram, private).

See also
........

See current_category/1, complements_object/2.

'built-in predicate'

2.4.5.36 ‘instantiates_class/2-3’
.................................

2.4.5.37 Description
....................

     instantiates_class(Instance, Class)
     instantiates_class(Instance, Class, Scope)

Enumerates, by backtracking, all pairs of objects such that the first
one instantiates the second.  The relation scope is represented by the
atoms ‘public’, ‘protected’, and ‘private’.

2.4.5.38 Modes and number of proofs
...................................

     instantiates_class(?object_identifier, ?object_identifier) - zero_or_more
     instantiates_class(?object_identifier, ?object_identifier, ?scope) - zero_or_more

2.4.5.39 Errors
...............

     ‘Instance’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Instance)’
     ‘Class’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Class)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.40 Examples
.................

     % check that the water_jug is an instante of state_space:
     | ?- instantiates_class(water_jug, state_space).

     % enumerate the state_space instances where the
     % instantiation relation is public:
     | ?- instantiates_class(Space, state_space, public).

See also
........

See current_object/1, extends_object/2-3,
specializes_class/2-3.

'built-in predicate'

2.4.5.41 ‘specializes_class/2-3’
................................

2.4.5.42 Description
....................

     specializes_class(Class, Superclass)
     specializes_class(Class, Superclass, Scope)

Enumerates, by backtracking, all pairs of objects such that the first
one specializes the second.  The relation scope is represented by the
atoms ‘public’, ‘protected’, and ‘private’.

2.4.5.43 Modes and number of proofs
...................................

     specializes_class(?object_identifier, ?object_identifier) - zero_or_more
     specializes_class(?object_identifier, ?object_identifier, ?scope) - zero_or_more

2.4.5.44 Errors
...............

     ‘Class’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Class)’
     ‘Superclass’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Superclass)’
     ‘Scope’ is neither a variable nor an atom:
          ‘type_error(atom, Scope)’
     ‘Scope’ is an atom but an invalid entity scope:
          ‘domain_error(scope, Scope)’

2.4.5.45 Examples
.................

     % enumerate the state_space subclasses:
     | ?- specializes_class(Subclass, state_space).

     % enumerate the state_space subclasses where the
     % specialization relation is public:
     | ?- specializes_class(Subclass, state_space, public).

See also
........

See current_object/1, extends_object/2-3,
instantiates_class/2-3.

2.4.6 Event handling
--------------------

'built-in predicate'

2.4.6.1 ‘abolish_events/5’
..........................

2.4.6.2 Description
...................

     abolish_events(Event, Object, Message, Sender, Monitor)

Abolishes all matching events.  The two types of events are represented
by the atoms ‘before’ and ‘after’.  When the predicate is called with
the first argument unbound, both types of events are abolished.

2.4.6.3 Modes and number of proofs
..................................

     abolish_events(@term, @term, @term, @term, @term) - one

2.4.6.4 Errors
..............

     ‘Event’ is neither a variable nor a valid event identifier:
          ‘type_error(event, Event)’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Message’ is neither a variable nor a callable term:
          ‘type_error(callable, Message)’
     ‘Sender’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Sender)’
     ‘Monitor’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Monitor)’

2.4.6.5 Examples
................

     % abolish all events for messages sent to the "list"
     % object being monitored by the "debugger" object:
     | ?- abolish_events(_, list, _, _, debugger).

See also
........

See current_event/5, define_events/5, before/3,
see after/3.

'built-in predicate'

2.4.6.6 ‘current_event/5’
.........................

2.4.6.7 Description
...................

     current_event(Event, Object, Message, Sender, Monitor)

Enumerates, by backtracking, all defined events.  The two types of
events are represented by the atoms ‘before’ and ‘after’.

2.4.6.8 Modes and number of proofs
..................................

     current_event(?event, ?term, ?term, ?term, ?object_identifier) - zero_or_more

2.4.6.9 Errors
..............

     ‘Event’ is neither a variable nor a valid event identifier:
          ‘type_error(event, Event)’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Message’ is neither a variable nor a callable term:
          ‘type_error(callable, Message)’
     ‘Sender’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Sender)’
     ‘Monitor’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Monitor)’

2.4.6.10 Examples
.................

     % enumerate all events monitored by the "debugger" object:
     | ?- current_event(Event, Object, Message, Sender, debugger).

See also
........

See abolish_events/5, define_events/5, before/3,
see after/3.

'built-in predicate'

2.4.6.11 ‘define_events/5’
..........................

2.4.6.12 Description
....................

     define_events(Event, Object, Message, Sender, Monitor)

Defines a new set of events.  The two types of events are represented by
the atoms ‘before’ and ‘after’.  When the predicate is called with the
first argument unbound, both types of events are defined.  The object
‘Monitor’ must define the event handler methods required by the ‘Event’
argument.

2.4.6.13 Modes and number of proofs
...................................

     define_events(@term, @term, @term, @term, +object_identifier) - one

2.4.6.14 Errors
...............

     ‘Event’ is neither a variable nor a valid event identifier:
          ‘type_error(event, Event)’
     ‘Object’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Object)’
     ‘Message’ is neither a variable nor a callable term:
          ‘type_error(callable, Message)’
     ‘Sender’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Sender)’
     ‘Monitor’ is a variable:
          ‘instantiation_error’
     ‘Monitor’ is neither a variable nor a valid object identifier:
          ‘type_error(object_identifier, Monitor)’
     ‘Monitor’ does not define the required ‘before/3’ method:
          ‘existence_error(procedure, before/3)’
     ‘Monitor’ does not define the required ‘after/3’ method:
          ‘existence_error(procedure, after/3)’

2.4.6.15 Examples
.................

     % define "debugger" as a monitor for member/2 messages
     % sent to the "list" object:
     | ?- define_events(_, list, member(_, _), _ , debugger).

See also
........

See abolish_events/5, current_event/5, before/3,
see after/3.

2.4.7 Multi-threading
---------------------

'built-in predicate'

2.4.7.1 ‘threaded/1’
....................

2.4.7.2 Description
...................

     threaded(Conjunction)
     threaded(Disjunction)

Proves each goal in a conjunction or a disjunction of goals in its own
thread.  This meta-predicate is deterministic and opaque to cuts.  The
predicate argument is 'not' flattened.

When the argument is a conjunction of goals, a call to this predicate
blocks until either all goals succeed, one of the goals fail, or one of
the goals generate an exception; the failure of one of the goals or an
exception on the execution of one of the goals results in the
termination of the remaining threads.  The predicate call is true 'iff'
all goals are true.  The predicate call fails if all goals fails.  When
one of the goals throw an exception, the predicate call re-throws that
exception.

When the argument is a disjunction of goals, a call to this predicate
blocks until either one of the goals succeeds or all the goals fail or
throw exceptions; the success of one of the goals results in the
termination of the remaining threads.  The predicate call is true 'iff'
one of the goals is true.  The predicate call fails if all goals fails.
When no goal succeeds and one of the goals throws an exception, the
predicate call re-throws that exception.

When the predicate argument is neither a conjunction not a disjunction
of goals, no threads are used.  In this case, the predicate call is
equivalent to a ‘once/1’ predicate call.

A dedicated message queue is used per call of this predicate to collect
the individual goal results.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.3 Meta-predicate template
...............................

     threaded(0)

2.4.7.4 Modes and number of proofs
..................................

     threaded(+callable) - zero_or_one

2.4.7.5 Errors
..............

     ‘Goals’ is a variable:
          ‘instantiation_error’
     A goal in ‘Goals’ is a variable:
          ‘instantiation_error’
     ‘Goals’ is neither a variable nor a callable term:
          ‘type_error(callable, Goals)’
     A goal ‘Goal’ in ‘Goals’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

2.4.7.6 Examples
................

     Prove a conjunction of goals, each one in its own thread:
          ‘threaded((Goal, Goals))’
     Prove a disjunction of goals, each one in its own thread:
          ‘threaded((Goal; Goals))’

See also
........

See threaded_call/1-2, threaded_once/1-2,
threaded_ignore/1, synchronized/1.

'built-in predicate'

2.4.7.7 ‘threaded_call/1-2’
...........................

2.4.7.8 Description
...................

     threaded_call(Goal)
     threaded_call(Goal, Tag)

Proves ‘Goal’ asynchronously using a new thread.  The argument can be a
message sending goal.  Calls to this predicate always succeeds and
return immediately.  The results (success, failure, or exception) are
sent back to the message queue of the object containing the call (see
this) and can be retrieved by calling the threaded_exit/1
predicate.

The ‘threaded_call/2’ variant returns a threaded call identifier tag
that can be used with the threaded_exit/2 and
threaded_cancel/1 predicates.  Tags shall be regarded as opaque
terms; users shall not rely on its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.9 Meta-predicate template
...............................

     threaded_call(0)
     threaded_call(0, *)

2.4.7.10 Modes and number of proofs
...................................

     threaded_call(@callable) - one
     threaded_call(@callable, --nonvar) - one

2.4.7.11 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Tag’ is not a variable:
          ‘uninstantiation_error(Tag)’

2.4.7.12 Examples
.................

     Prove ‘Goal’ asynchronously in a new thread:
          ‘threaded_call(Goal)’
     Prove ‘::Message’ asynchronously in a new thread:
          ‘threaded_call(::Message)’
     Prove ‘Object::Message’ asynchronously in a new thread:
          ‘threaded_call(Object::Message)’

See also
........

See threaded_exit/1-2, threaded_ignore/1,
threaded_once/1-2, threaded_peek/1-2,
threaded_cancel/1, threaded/1, synchronized/1.

'built-in predicate'

2.4.7.13 ‘threaded_once/1-2’
............................

2.4.7.14 Description
....................

     threaded_once(Goal)
     threaded_once(Goal, Tag)

Proves ‘Goal’ asynchronously using a new thread.  Only the first goal
solution is found.  The argument can be a message sending goal.  This
call always succeeds.  The result (success, failure, or exception) is
sent back to the message queue of the object containing the call (see
this).

The ‘threaded_once/2’ variant returns a threaded call identifier tag
that can be used with the threaded_exit/2 and
threaded_cancel/1 predicates.  Tags shall be regarded as opaque
terms; users shall not rely on its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.15 Meta-predicate template
................................

     threaded_once(0)
     threaded_once(0, *)

2.4.7.16 Modes and number of proofs
...................................

     threaded_once(@callable) - one
     threaded_once(@callable, --nonvar) - one

2.4.7.17 Errors
...............

     Goal is a variable:
          ‘instantiation_error’
     Goal is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     Tag is not a variable:
          ‘uninstantiation_error(Tag)’

2.4.7.18 Examples
.................

     Prove ‘Goal’ asynchronously in a new thread:
          ‘threaded_once(Goal)’
     Prove ‘::Message’ asynchronously in a new thread:
          ‘threaded_once(::Message)’
     Prove ‘Object::Message’ asynchronously in a new thread:
          ‘threaded_once(Object::Message)’

See also
........

See threaded_call/1-2, threaded_exit/1-2,
threaded_ignore/1, threaded_peek/1-2,
threaded_cancel/1, threaded/1, synchronized/1.

'built-in predicate'

2.4.7.19 ‘threaded_ignore/1’
............................

2.4.7.20 Description
....................

     threaded_ignore(Goal)

Proves ‘Goal’ asynchronously using a new thread.  Only the first goal
solution is found.  The argument can be a message sending goal.  This
call always succeeds, independently of the result (success, failure, or
exception), which is simply discarded instead of being sent back to the
message queue of the object containing the call (see this).

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.21 Meta-predicate template
................................

     threaded_ignore(0)

2.4.7.22 Modes and number of proofs
...................................

     threaded_ignore(@callable) - one

2.4.7.23 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

2.4.7.24 Examples
.................

     Prove ‘Goal’ asynchronously in a new thread:
          ‘threaded_ignore(Goal)’
     Prove ‘::Message’ asynchronously in a new thread:
          ‘threaded_ignore(::Message)’
     Prove ‘Object::Message’ asynchronously in a new thread:
          ‘threaded_ignore(Object::Message)’

See also
........

See threaded_call/1-2, threaded_exit/1-2,
threaded_once/1-2, threaded_peek/1-2, threaded/1,
see synchronized/1.

'built-in predicate'

2.4.7.25 ‘threaded_exit/1-2’
............................

2.4.7.26 Description
....................

     threaded_exit(Goal)
     threaded_exit(Goal, Tag)

Retrieves the result of proving ‘Goal’ in a new thread.  This predicate
blocks execution until the reply is sent to the this message
queue by the thread executing the goal.  When there is no thread proving
the goal, the predicate generates an exception.  This predicate is
non-deterministic, providing access to any alternative solutions of its
argument.

The argument of this predicate should be a 'variant' of the argument of
the corresponding threaded_call/1 or threaded_once/1
call.  When the predicate argument is subsumed by the
‘threaded_call/1’ or ‘threaded_once/1’ call argument, the
‘threaded_exit/1’ call will succeed iff its argument is a solution of
the (more general) goal.

The ‘threaded_exit/2’ variant accepts a threaded call identifier tag
generated by the calls to the threaded_call/2 and
threaded_once/2 predicates.  Tags shall be regarded as an opaque
term; users shall not rely on its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.27 Modes and number of proofs
...................................

     threaded_exit(+callable) - zero_or_more
     threaded_exit(+callable, +nonvar) - zero_or_more

2.4.7.28 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     No thread is running for proving ‘Goal’ in the ‘Object’ calling context:
          ‘existence_error(thread, Object)’
     ‘Tag’ is a variable:
          ‘instantiation_error’

2.4.7.29 Examples
.................

     To retrieve an asynchronous goal proof result:
          ‘threaded_exit(Goal)’
     To retrieve an asynchronous message to 'self' result:
          ‘threaded_exit(::Goal)’
     To retrieve an asynchronous message result:
          ‘threaded_exit(Object::Goal)’

See also
........

See threaded_call/1-2, threaded_ignore/1,
threaded_once/1-2, threaded_peek/1-2,
threaded_cancel/1, threaded/1.

'built-in predicate'

2.4.7.30 ‘threaded_peek/1-2’
............................

2.4.7.31 Description
....................

     threaded_peek(Goal)
     threaded_peek(Goal, Tag)

Checks if the result of proving ‘Goal’ in a new thread is already
available.  This call succeeds or fails without blocking execution
waiting for a reply to be available.  When there is no thread proving
the goal, the predicate generates an exception.

The argument of this predicate should be a 'variant' of the argument of
the corresponding threaded_call/1 or threaded_once/1
call.  When the predicate argument is subsumed by the
‘threaded_call/1’ or ‘threaded_once/1’ call argument, the
‘threaded_peek/1’ call will succeed iff its argument unifies with an
already available solution of the (more general) goal.

The ‘threaded_peek/2’ variant accepts a threaded call identifier tag
generated by the calls to the threaded_call/2 and
threaded_once/2 predicates.  Tags shall be regarded as an opaque
term; users shall not rely on its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.32 Modes and number of proofs
...................................

     threaded_peek(+callable) - zero_or_one
     threaded_peek(+callable, +nonvar) - zero_or_one

2.4.7.33 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     No thread is running for proving ‘Goal’ in the ‘Object’ calling context:
          ‘existence_error(thread, Object)’
     ‘Tag’ is a variable:
          ‘instantiation_error’

2.4.7.34 Examples
.................

     To check for an asynchronous goal proof result:
          ‘threaded_peek(Goal)’
     To check for an asynchronous message to 'self' result:
          ‘threaded_peek(::Goal)’
     To check for an asynchronous message result:
          ‘threaded_peek(Object::Goal)’

See also
........

See threaded_call/1-2, threaded_exit/1-2,
threaded_ignore/1, threaded_once/1-2,
threaded_cancel/1, threaded/1.

'built-in predicate'

2.4.7.35 ‘threaded_cancel/1’
............................

2.4.7.36 Description
....................

     threaded_cancel(Tag)

Cancels a tagged threaded call.  When there is no asynchronous call with
the given tag, calling this predicate succeeds assuming the asynchronous
call have already terminated or canceled.  The threaded call identifier
tag is generated by calls to the threaded_call/2 and
threaded_once/2 predicates.  Tags shall be regarded as an opaque
term; users shall not rely on its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.37 Modes and number of proofs
...................................

     threaded_cancel(+nonvar) - one

2.4.7.38 Errors
...............

     ‘Tag’ is a variable:
          ‘instantiation_error’

2.4.7.39 Examples
.................

(none)

See also
........

See threaded_call/1-2, threaded_exit/1-2,
threaded_ignore/1, threaded_once/1-2, threaded/1.

'built-in predicate'

2.4.7.40 ‘threaded_wait/1’
..........................

2.4.7.41 Description
....................

     threaded_wait(Term)
     threaded_wait([Term| Terms])

Suspends the thread making the call until a notification is received
that unifies with ‘Term’.  The call must be made within the same object
(see this) containing the calls to the threaded_notify/1
predicate that will eventually send the notification.  The argument
may also be a list of notifications, ‘[Term| Terms]’.  In this case, the
thread making the call will suspend until all notifications in the list
are received.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.42 Modes and number of proofs
...................................

     threaded_wait(?term) - one
     threaded_wait(+list(term)) - one

2.4.7.43 Errors
...............

(none)

2.4.7.44 Examples
.................

     % wait until the "data_available" notification is received:
     ..., threaded_wait(data_available), ...

See also
........

See threaded_notify/1.

'built-in predicate'

2.4.7.45 ‘threaded_notify/1’
............................

2.4.7.46 Description
....................

     threaded_notify(Term)
     threaded_notify([Term| Terms])

Sends ‘Term’ as a notification to any thread suspended waiting for it in
order to proceed.  The call must be made within the same object (see
this) containing the calls to the threaded_wait/1.
predicate waiting for the notification.  The argument may also be a list
of notifications, ‘[Term| Terms]’.  In this case, all notifications in
the list will be sent to any threads suspended waiting for them in order
to proceed.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only threads flag is set to ‘supported’ when that
     is the case.

2.4.7.47 Modes and number of proofs
...................................

     threaded_notify(@term) - one
     threaded_notify(@list(term)) - one

2.4.7.48 Errors
...............

(none)

2.4.7.49 Examples
.................

     % send a "data_available" notification:
     ..., threaded_notify(data_available), ...

See also
........

See threaded_wait/1.

2.4.8 Multi-threading engines
-----------------------------

'built-in predicate'

2.4.8.1 ‘threaded_engine_create/3’
..................................

2.4.8.2 Description
...................

     threaded_engine_create(AnswerTemplate, Goal, Engine)

Creates a new engine for proving the given goal and defines an answer
template for retrieving the goal solution bindings.  A message queue for
passing arbitrary terms to the engine is also created.  If the name for
the engine is not given, a unique name is generated and returned.
Engine names shall be regarded as opaque terms; users shall not rely on
its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.3 Meta-predicate template
...............................

     threaded_engine_create(*, 0, *)

2.4.8.4 Modes and number of proofs
..................................

     threaded_engine_create(@term, @callable, @nonvar) - one
     threaded_engine_create(@term, @callable, --nonvar) - one

2.4.8.5 Errors
..............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Engine’ is the name of an existing engine:
          ‘permission_error(create, engine, Engine)’

2.4.8.6 Examples
................

     % create a new engine for finding members of a list:
     | ?- threaded_engine_create(X, member(X, [1,2,3]), worker_1).

See also
........

See threaded_engine_destroy/1, threaded_engine_self/1,
threaded_engine/1, threaded_engine_next/2,
threaded_engine_next_reified/2.

'built-in predicate'

2.4.8.7 ‘threaded_engine_destroy/1’
...................................

2.4.8.8 Description
...................

     threaded_engine_destroy(Engine)

Stops and destroys an engine.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.9 Modes and number of proofs
..................................

     threaded_engine_destroy(@nonvar) - one

2.4.8.10 Errors
...............

     ‘Engine’ is a variable:
          ‘instantiation_error’
     ‘Engine’ is neither a variable nor the name of an existing engine:
          ‘existence_error(engine, Engine)’

2.4.8.11 Examples
.................

     % stop the worker_1 engine:
     | ?- threaded_engine_destroy(worker_1).

     % stop all engines:
     | ?- forall(
              threaded_engine(Engine),
              threaded_engine_destroy(Engine)
          ).

See also
........

See threaded_engine_create/3, threaded_engine_self/1,
threaded_engine/1.

'built-in predicate'

2.4.8.12 ‘threaded_engine/1’
............................

2.4.8.13 Description
....................

     threaded_engine(Engine)

Enumerates, by backtracking, all existing engines.  Engine names shall
be regarded as opaque terms; users shall not rely on its type.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.14 Modes and number of proofs
...................................

     threaded_engine(?nonvar) - zero_or_more

2.4.8.15 Errors
...............

(none)

2.4.8.16 Examples
.................

     % check that the worker_1 engine exists:
     | ?- threaded_engine(worker_1).

     % write the names of all existing engines:
     | ?- forall(
              threaded_engine(Engine),
              (writeq(Engine), nl)
          ).

See also
........

See threaded_engine_create/3, threaded_engine_self/1,
threaded_engine_destroy/1.

'built-in predicate'

2.4.8.17 ‘threaded_engine_self/1’
.................................

2.4.8.18 Description
....................

     threaded_engine_self(Engine)

Queries the name of engine calling the predicate.  Fails if not called
from within an engine or if the argument doesn’t unify with the engine
name.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.19 Modes and number of proofs
...................................

     threaded_engine_self(?nonvar) - zero_or_one

2.4.8.20 Errors
...............

(none)

2.4.8.21 Examples
.................

     % find the name of the engine making the query:
     ..., threaded_engine_self(Engine), ...

     % check if the the engine making the query is worker_1:
     ..., threaded_engine_self(worker_1), ...

See also
........

See threaded_engine_create/3, threaded_engine_destroy/1,
see threaded_engine/1.

'built-in predicate'

2.4.8.22 ‘threaded_engine_next/2’
.................................

2.4.8.23 Description
....................

     threaded_engine_next(Engine, Answer)

Retrieves an answer from an engine and signals it to start computing the
next answer.  This predicate blocks until an answer becomes available.
The predicate fails when there are no more solutions to the engine goal.
If the engine goal throws an exception, calling this predicate will
re-throw the exception and subsequent calls will fail.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.24 Modes and number of proofs
...................................

     threaded_engine_next(@nonvar, ?term) - zero_or_one

2.4.8.25 Errors
...............

     ‘Engine’ is a variable:
          ‘instantiation_error’
     ‘Engine’ is neither a variable nor the name of an existing engine:
          ‘existence_error(engine, Engine)’

2.4.8.26 Examples
.................

     % get the next answer from the worker_1 engine:
     | ?- threaded_engine_next(worker_1, Answer).

See also
........

See threaded_engine_create/3,
threaded_engine_next_reified/2, threaded_engine_yield/1.

'built-in predicate'

2.4.8.27 ‘threaded_engine_next_reified/2’
.........................................

2.4.8.28 Description
....................

     threaded_engine_next_reified(Engine, Answer)

Retrieves an answer from an engine and signals it to start computing the
next answer.  This predicate always succeeds and blocks until an answer
becomes available.  Answers are returned using the terms ‘the(Answer)’,
‘no’, and ‘exception(Error)’.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.29 Modes and number of proofs
...................................

     threaded_engine_next_reified(@nonvar, ?nonvar) - one

2.4.8.30 Errors
...............

     ‘Engine’ is a variable:
          ‘instantiation_error’
     ‘Engine’ is neither a variable nor the name of an existing engine:
          ‘existence_error(engine, Engine)’

2.4.8.31 Examples
.................

     % get the next reified answer from the worker_1 engine:
     | ?- threaded_engine_next_reified(worker_1, Answer).

See also
........

See threaded_engine_create/3, threaded_engine_next/2,
threaded_engine_yield/1.

'built-in predicate'

2.4.8.32 ‘threaded_engine_yield/1’
..................................

2.4.8.33 Description
....................

     threaded_engine_yield(Answer)

Returns an answer independent of the solutions of the engine goal.
Fails if not called from within an engine.  This predicate is usually
used when the engine goal is a call to a recursive predicate processing
terms from the engine term queue.

This predicate blocks until the returned answer is consumed.

Note that this predicate should not be called as the last element of a
conjunction resulting in an engine goal solution as, in this case, an
answer will always be returned.  For example, instead of
‘(threaded_engine_yield(ready); member(X,[1,2,3]))’ use ‘(X=ready;
member(X,[1,2,3]))’.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.34 Modes and number of proofs
...................................

     threaded_engine_yield(@term) - zero_or_one

2.4.8.35 Errors
...............

(none)

2.4.8.36 Examples
.................

     % returns the atom "ready" as an engine answer:
     ..., threaded_engine_yield(ready), ...

See also
........

See threaded_engine_create/3, threaded_engine_next/2,
threaded_engine_next_reified/2.

'built-in predicate'

2.4.8.37 ‘threaded_engine_post/2’
.................................

2.4.8.38 Description
....................

     threaded_engine_post(Engine, Term)

Posts a term to the engine term queue.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.39 Modes and number of proofs
...................................

     threaded_engine_post(@nonvar, @term) - one

2.4.8.40 Errors
...............

     ‘Engine’ is a variable:
          ‘instantiation_error’
     ‘Engine’ is neither a variable nor the name of an existing engine:
          ‘existence_error(engine, Engine)’

2.4.8.41 Examples
.................

     % post the atom "ready" to the worker_1 engine queue:
     | ?- threaded_engine_post(worker_1, ready).

See also
........

See threaded_engine_fetch/1.

'built-in predicate'

2.4.8.42 ‘threaded_engine_fetch/1’
..................................

2.4.8.43 Description
....................

     threaded_engine_fetch(Term)

Fetches a term from the engine term queue.  Blocks until a term is
available.  Fails if not called from within an engine.

     Note: This predicate requires a backend Prolog compiler.
     providing compatible multi-threading primitives.  The value of the
     read-only engines flag is set to ‘supported’ when that
     is the case.

2.4.8.44 Modes and number of proofs
...................................

     threaded_engine_fetch(?term) - zero_or_one

2.4.8.45 Errors
...............

(none)

2.4.8.46 Examples
.................

     % fetch a term from the engine term queue:
     ..., threaded_engine_fetch(Term), ...

See also
........

See threaded_engine_post/2.

2.4.9 Compiling and loading source files
----------------------------------------

'built-in predicate'

2.4.9.1 ‘logtalk_compile/1’
...........................

2.4.9.2 Description
...................

     logtalk_compile(File)
     logtalk_compile(Files)

Compiles to disk a source file or a list of source files
using the default compiler flag values.  The Logtalk source file name
extension (by default, ‘.lgt’) can be omitted.  Source file paths can be
absolute, relative to the current directory, or use library notation.
This predicate can also be used to compile Prolog source
files as Logtalk source code.  When no recognized Logtalk or Prolog
extension is specified, the compiler tries first to append a Logtalk
source file extension and then a Prolog source file extension.  If that
fails, the compiler tries to use the file name as-is.  The recognized
Logtalk and Prolog file extensions are defined in the backend adapter files.

     Note: This predicate does not load into memory the compiled source
     file.  If you want to both compile and load a source file, use
     instead the logtalk_load/1 built-in predicate.

When this predicate is called from the top-level interpreter, relative
source file paths are resolved using the current working directory.
When the calls are made from a source file, relative source file paths
are resolved using the source file directory.

Note that only the errors related to problems in the predicate argument
are listed below.  This predicate fails on the first error found during
compilation of a source file.  In this case, no file with the compiled
code is written to disk.

2.4.9.3 Modes and number of proofs
..................................

     logtalk_compile(@source_file_name) - zero_or_one
     logtalk_compile(@list(source_file_name)) - zero_or_one

2.4.9.4 Errors
..............

     ‘File’ is a variable:
          ‘instantiation_error’
     ‘Files’ is a variable or a list with an element which is a variable:
          ‘instantiation_error’
     ‘File’, or an element ‘File’ of the ‘Files’ list, is neither a variable nor a source file name:
          ‘type_error(source_file_name, File)’
     ‘File’, or an element ‘File’ of the ‘Files’ list, uses library notation but the library does not exist:
          ‘existence_error(library, Library)’
     ‘File’ or an element ‘File’ of the ‘Files’ list does not exist:
          ‘existence_error(file, File)’

2.4.9.5 Examples
................

     % compile to disk the "set" source file in the
     % current directory:
     | ?- logtalk_compile(set).

     % compile to disk the "tree" source file in the
     % "types" library directory:
     | ?- logtalk_load(types(tree)).

     % compile to disk the "listp" and "list" source
     % files in the current directory:
     | ?- logtalk_compile([listp, list]).

See also
........

See logtalk_compile/2, logtalk_load/1,
logtalk_load/2, logtalk_make/0, logtalk_make/1,
see logtalk_library_path/2.

'built-in predicate'

2.4.9.6 ‘logtalk_compile/2’
...........................

2.4.9.7 Description
...................

     logtalk_compile(File, Flags)
     logtalk_compile(Files, Flags)

Compiles to disk a source file or a list of source files
using a list of compiler flags.  The Logtalk source file name extension
(by default, ‘.lgt’) can be omitted.  Source file paths can be absolute,
relative to the current directory, or use library notation.
This predicate can also be used to compile Prolog source files as
Logtalk source code.  When no recognized Logtalk or Prolog extension is
specified, the compiler tries first to append a Logtalk source file
extension and then a Prolog source file extension.  If that fails, the
compiler tries to use the file name as-is.  Compiler flags are
represented as 'flag(value)'.  For a description of the available
compiler flags, please see the Compiler flags section in the
User Manual.  The recognized Logtalk and Prolog file extensions are
defined in the backend adapter files.

     Note: This predicate does not load into memory the compiled source
     file.  If you want to both compile and load a source file, use
     instead the logtalk_load/2 built-in predicate.

When this predicate is called from the top-level interpreter, relative
source file paths are resolved using the current working directory.
When the calls are made from a source file, relative source file paths
are resolved by default using the source file directory (unless a
relative_to flag is passed).

Note that only the errors related to problems in the predicate argument
are listed below.  This predicate fails on the first error found during
compilation of a source file.  In this case, no file with the compiled
code is written to disk.

     Warning: The compiler flags specified in the second argument only
     apply to the files listed in the first argument.  Notably, if you
     are compiling a loader file, the flags only apply to the
     loader file itself.

2.4.9.8 Modes and number of proofs
..................................

     logtalk_compile(@source_file_name, @list(compiler_flag)) - zero_or_one
     logtalk_compile(@list(source_file_name), @list(compiler_flag)) - zero_or_one

2.4.9.9 Errors
..............

     ‘File’ is a variable:
          ‘instantiation_error’
     ‘Files’ is a variable or a list with an element which is a variable:
          ‘instantiation_error’
     ‘File’, or an element ‘File’ of the ‘Files’ list, is neither a variable nor a source file name:
          ‘type_error(source_file_name, File)’
     ‘File’, or an element ‘File’ of the ‘Files’ list, uses library notation but the library does not exist:
          ‘existence_error(library, Library)’
     ‘File’ or an element ‘File’ of the ‘Files’ list, does not exist:
          ‘existence_error(file, File)’

     ‘Flags’ is a variable or a list with an element which is a variable:
          ‘instantiation_error’
     ‘Flags’ is neither a variable nor a proper list:
          ‘type_error(list, Flags)’
     An element ‘Flag’ of the ‘Flags’ list is not a valid compiler flag:
          ‘type_error(compiler_flag, Flag)’
     An element ‘Flag’ of the ‘Flags’ list defines a value for a read-only compiler flag:
          ‘permission_error(modify, flag, Flag)’
     An element ‘Flag’ of the ‘Flags’ list defines an invalid value for a flag:
          ‘domain_error(flag_value, Flag+Value)’

2.4.9.10 Examples
.................

     % compile to disk the "list" source file in the
     % current directory using default compiler flags:
     | ?- logtalk_compile(list, []).

     % compile to disk the "tree" source file in the "types"
     % library directory with the source_data flag turned on:
     | ?- logtalk_compile(types(tree), [source_data(on)]).

     % compile to disk the "file_system" source file in the
     % current directory with portability warnings suppressed:
     | ?- logtalk_compile(file_system, [portability(silent)]).

See also
........

See logtalk_compile/1, logtalk_load/1,
logtalk_load/2, logtalk_make/0, logtalk_make/1,
see logtalk_library_path/2.

'built-in predicate'

2.4.9.11 ‘logtalk_load/1’
.........................

2.4.9.12 Description
....................

     logtalk_load(File)
     logtalk_load(Files)

Compiles to disk and then loads to memory a source file or a
list of source files using the default compiler flag values.  The
Logtalk source file name extension (by default, ‘.lgt’) can be omitted.
Source file paths can be absolute, relative to the current directory, or
use library notation.  This predicate can also be used to
compile Prolog source files as Logtalk source code.  When no recognized
Logtalk or Prolog extension is specified, the compiler tries first to
append a Logtalk source file extension and then a Prolog source file
extension.  If that fails, the compiler tries to use the file name
as-is.  The recognized Logtalk and Prolog file extensions are defined in
the backend adapter files.

When this predicate is called from the top-level interpreter, relative
source file paths are resolved using the current working directory.
When the calls are made from a source file, relative source file paths
are resolved using the source file directory.

Note that only the errors related to problems in the predicate argument
are listed below.  This predicate fails on the first error found during
compilation of a source file.  In this case, no contents of the source
file are loaded.

Depending on the backend Prolog compiler, the shortcuts
‘{File}’ or ‘{File1, File2, ...}’ may be used in alternative.  Check the
adapter files for the availability of these shortcuts as they are not
part of the language (and thus should only be used at the top-level
interpreter).

2.4.9.13 Modes and number of proofs
...................................

     logtalk_load(@source_file_name) - zero_or_one
     logtalk_load(@list(source_file_name)) - zero_or_one

2.4.9.14 Errors
...............

     ‘File’ is a variable:
          ‘instantiation_error’
     ‘Files’ is a variable or a list with an element which is a variable:
          ‘instantiation_error’
     ‘File’, or an element ‘File’ of the ‘Files’ list, is neither a variable nor a source file name:
          ‘type_error(source_file_name, File)’
     ‘File’, or an element ‘File’ of the ‘Files’ list, uses library notation but the library does not exist:
          ‘existence_error(library, Library)’
     ‘File’ or an element ‘File’ of the ‘Files’ list, does not exist:
          ‘existence_error(file, File)’

2.4.9.15 Examples
.................

     % compile and load the "set" source file in the
     % current directory:
     | ?- logtalk_load(set).

     % compile and load the "tree" source file in the
     % "types" library directory:
     | ?- logtalk_load(types(tree)).

     % compile and load the "listp" and "list" source
     % files in the current directory:
     | ?- logtalk_load([listp, list]).

See also
........

See logtalk_compile/1, logtalk_compile/2,
logtalk_load/2, logtalk_make/0, logtalk_make/1,
see logtalk_library_path/2.

'built-in predicate'

2.4.9.16 ‘logtalk_load/2’
.........................

2.4.9.17 Description
....................

     logtalk_load(File, Flags)
     logtalk_load(Files, Flags)

Compiles to disk and then loads to memory a source file or a
list of source files using a list of compiler flags.  The Logtalk source
file name extension (by default, ‘.lgt’) can be omitted.  Source file
paths can be absolute, relative to the current directory, or use
library notation.  Compiler flags are represented as 'flag(value)'.
This predicate can also be used to compile Prolog source files as
Logtalk source code.  When no recognized Logtalk or Prolog extension is
specified, the compiler tries first to append a Logtalk source file
extension and then a Prolog source file extension.  If that fails, the
compiler tries to use the file name as-is.  For a description of the
available compiler flags, please see the Compiler flags.
section in the User Manual.  The recognized Logtalk and Prolog file
extensions are defined in the backend adapter files.  The
recognized Logtalk and Prolog file extensions are defined in the
backend adapter files.

When this predicate is called from the top-level interpreter, relative
source file paths are resolved using the current working directory.
When the calls are made from a source file, relative source file paths
are resolved by default using the source file directory (unless a
relative_to flag is passed).

Note that only the errors related to problems in the predicate argument
are listed below.  This predicate fails on the first error found during
compilation of a source file.  In this case, no contents of the source
file are loaded.

     Warning: The compiler flags specified in the second argument only
     apply to the files listed in the first argument and not to any
     files that those files may load or compile.  Notably, if you are
     loading a loader file, the flags only apply to the loader
     file itself and not to the files loaded by it.

2.4.9.18 Modes and number of proofs
...................................

     logtalk_load(@source_file_name, @list(compiler_flag)) - zero_or_one
     logtalk_load(@list(source_file_name), @list(compiler_flag)) - zero_or_one

2.4.9.19 Errors
...............

     ‘File’ is a variable:
          ‘instantiation_error’
     ‘Files’ is a variable or a list with an element which is a variable:
          ‘instantiation_error’
     ‘File’, or an element ‘File’ of the ‘Files’ list, is neither a variable nor a source file name:
          ‘type_error(source_file_name, File)’
     ‘File’, or an element ‘File’ of the ‘Files’ list, uses library notation but the library does not exist:
          ‘existence_error(library, Library)’
     ‘File’ or an element ‘File’ of the ‘Files’ list, does not exist:
          ‘existence_error(file, File)’
     ‘Flags’ is a variable or a list with an element which is a variable:
          ‘instantiation_error’
     ‘Flags’ is neither a variable nor a proper list:
          ‘type_error(list, Flags)’
     An element ‘Flag’ of the ‘Flags’ list is not a valid compiler flag:
          ‘type_error(compiler_flag, Flag)’
     An element ‘Flag’ of the ‘Flags’ list defines a value for a read-only compiler flag:
          ‘permission_error(modify, flag, Flag)’
     An element ‘Flag’ of the ‘Flags’ list defines an invalid value for a flag:
          ‘domain_error(flag_value, Flag+Value)’

2.4.9.20 Examples
.................

     % compile and load the "list" source file in the
     % current directory using default compiler flags:
     | ?- logtalk_load(list, []).

     % compile and load the "tree" source file in the "types"
     % library directory with the source_data flag turned on:
     | ?- logtalk_load(types(tree)).

     % compile and load the "file_system" source file in the
     % current directory with portability warnings suppressed:
     | ?- logtalk_load(file_system, [portability(silent)]).

See also
........

See logtalk_compile/1, logtalk_compile/2,
logtalk_load/1, logtalk_make/0, logtalk_make/1,
see logtalk_library_path/2.

'built-in predicate'

2.4.9.21 ‘logtalk_make/0’
.........................

2.4.9.22 Description
....................

     logtalk_make

Reloads all Logtalk source files that have been modified since the time
they are last loaded.  Only source files loaded using the
logtalk_load/1 and logtalk_load/2 predicates are
reloaded.  Non-modified files will also be reloaded when there is a
change to the compilation mode (i.e.  when the files were loaded without
explicit debug or optimize flags and the default
values of these flags changed after loading; no check is made, however,
for other implicit compiler flags that may have changed since loading).
When an included file is modified, this predicate reloads its main file
(i.e.  the file that contains the include/1 directive).

Depending on the backend Prolog compiler, the shortcut ‘{*}’
may be used in alternative.  Check the adapter files for the
availability of the shortcut as it is not part of the language.

     Warning: Only use the ‘{*}’ shortcut at the top-level interpreter
     and never in source files.

This predicate can be extended by the user by defining clauses for the
logtalk_make_target_action/1 multifile and dynamic hook
predicate using the argument ‘all’.  The additional user defined actions
are run after the default one.

2.4.9.23 Modes and number of proofs
...................................

     logtalk_make - one

2.4.9.24 Errors
...............

(none)

2.4.9.25 Examples
.................

     % reload all files modified since last loaded:
     | ?- logtalk_make.

See also
........

See logtalk_compile/1, logtalk_compile/2,
logtalk_load/1, logtalk_load/2, logtalk_make/1,
logtalk_make_target_action/1.

'built-in predicate'

2.4.9.26 ‘logtalk_make/1’
.........................

2.4.9.27 Description
....................

     logtalk_make(Target)

Runs a make target.  Prints a warning message and fails when the target
is not valid.

Allows reloading all Logtalk source files that have been modified since
last loaded when called with the target ‘all’, deleting all intermediate
files generated by the compilation of Logtalk source files when called
with the target ‘clean’, checking for code issues when called with the
target ‘check’, listing of circular dependencies between pairs or trios
of objects when called with the target ‘circular’, generating
documentation when called with the target ‘documentation’, and deleting
the dynamic binding caches with the target ‘caches’.

There are also three variants of the ‘all’ target: ‘debug’, ‘normal’,
and ‘optimal’.  These targets change the compilation mode (by changing
the default value of the debug and optimize.
flags) and reload all affected files (i.e.  all files loaded without an
explicit ‘debug/1’ or ‘optimize/1’ compiler option).

When using the ‘all’ target, only source files loaded using the
logtalk_load/1 and logtalk_load/2 predicates are
reloaded.  Non-modified files will also be reloaded when there is a
change to the compilation mode (i.e.  when the files were loaded without
explicit debug or optimize flags and the default
values of these flags changed after loading; no check is made, however,
for other implicit compiler flags that may have changed since loading).
When an included file is modified, this target reloads its main file
(i.e.  the file that contains the include/1 directive).

When using the ‘check’ or ‘circular’ targets, be sure to compile your
source files with the source_data flag turned on for complete
and detailed reports.

The ‘check’ target scans for missing entities (objects, protocols,
categories, and modules), missing entity predicates, and duplicated
library aliases.  Predicates for messages sent to objects that implement
the forwarding(1) built-in protocol are not reported.  While this
usually avoids only false positives, it may also result in failure to
report true missing predicates in some cases.

When using the ‘circular’ target, be prepared for a lengthy computation
time for applications with a large combined number of objects and
message calls.  Only mutual and triangular dependencies are checked due
to the computational cost.  Circular dependencies occur when an object
sends a message to a second object that, in turn, sends a message to the
first object.  These circular dependencies are often a consequence of
lack of separation of concerns.  But, when they cannot be fixed, the
only practical consequence is a small performance cost as some of the
messages would be forced to use dynamic binding.

The ‘documentation’ target requires the ‘doclet’ tool and a single
doclet object to be loaded.  See the ‘doclet’ tool documentation
for more details.

Depending on the backend Prolog compiler, the following
top-level shortcuts are usually defined:

   * ‘{*}’ - ‘logtalk_make(all)’

   * ‘{!}’ - ‘logtalk_make(clean)’

   * ‘{?}’ - ‘logtalk_make(check)’

   * ‘{@}’ - ‘logtalk_make(circular)’

   * ‘{#}’ - ‘logtalk_make(documentation)’

   * ‘{$}’ - ‘logtalk_make(caches)’

   * ‘{+d}’ - ‘logtalk_make(debug)’

   * ‘{+n}’ - ‘logtalk_make(normal)’

   * ‘{+o}’ - ‘logtalk_make(optimal)’

Check the adapter files for the availability of these
shortcuts as they are not part of the language.

     Warning: Only use the shortcuts at the top-level interpreter and
     never in source files.

The target actions can be extended by defining clauses for the multifile
and dynamic hook predicate logtalk_make_target_action(Target)
where ‘Target’ is one of the targets listed above.  The additional
user defined actions are run after the default ones.

   ---------- Footnotes ----------

   (1) ../../../docs/forwarding_0.html#forwarding-0

2.4.9.28 Modes and number of proofs
...................................

     logtalk_make(+atom) - zero_or_one

2.4.9.29 Errors
...............

(none)

2.4.9.30 Examples
.................

     % reload loaded source files in debug mode:
     | ?- logtalk_make(debug).

     % check for code issues in the loaded source files:
     | ?- logtalk_make(check).

     % delete all intermediate files generated by
     % the compilation of Logtalk source files:
     | ?- logtalk_make(clean).

See also
........

See logtalk_compile/1, logtalk_compile/2,
logtalk_load/1, logtalk_load/2, logtalk_make/0,
logtalk_make_target_action/1.

'built-in predicate'

2.4.9.31 ‘logtalk_make_target_action/1’
.......................................

2.4.9.32 Description
....................

     logtalk_make_target_action(Target)

Multifile and dynamic hook predicate that allows defining user actions
for the logtalk_make/1 targets.  The user defined actions
are run after the default ones using a failure driven loop.  This loop
does not catch any exceptions thrown when calling the user-defined
actions.

2.4.9.33 Modes and number of proofs
...................................

     logtalk_make_target_action(+atom) - zero_or_more

2.4.9.34 Errors
...............

(none)

2.4.9.35 Examples
.................

     % integrate the dead_code_scanner tool with logtalk_make/1

     :- multifile(logtalk_make_target_action/1).
     :- dynamic(logtalk_make_target_action/1).

     logtalk_make_target_action(check) :-
         dead_code_scanner::all.

See also
........

See logtalk_make/1, logtalk_make/0.

'built-in predicate'

2.4.9.36 ‘logtalk_library_path/2’
.................................

2.4.9.37 Description
....................

     logtalk_library_path(Library, Path)

Dynamic and multifile user-defined predicate, allowing the declaration
of aliases to library paths.  Library aliases may also be
used on the second argument (using the notation 'alias(path)').  Paths
must always end with the path directory separator character (‘'/'’).

     Warning: Library aliases should be unique.  The
     logtalk_make/1 built-in predicate can be used to detect and
     report duplicated library aliases using the ‘check’ target.

Clauses for this predicate should preferably be facts.  Defining rules
to dynamically compute at runtime both library alias names and their
paths, although sometimes handy, can inadvertently result in endless
loops when those rules also attempt to expand library paths.  When
asserting facts for this predicate, use preferably ‘asserta/1’ instead
of ‘assertz/1’ to help ensure the facts will be used before any rules.

Relative paths (e.g.  ‘'../'’ or ‘'./'’) should only be used within the
'alias(path)') notation so that library paths can always be expanded to
absolute paths independently of the (usually unpredictable) current
directory at the time the ‘logtalk_library_path/2’ predicate is called.

When working with a relocatable application, the actual application
installation directory can be retrieved by calling the
logtalk_load_context/2 predicate with the ‘directory’ key and
using the returned value to define the ‘logtalk_library_path/2’
predicate.  On a settings file file or a loader file
file, simply use an initialization/1 directive to wrap
the call to the ‘logtalk_load_context/2’ predicate and the assert of the
‘logtalk_library_path/2’ fact.

This predicate may be used to override the default scratch directory
by defining the library alias ‘scratch_directory’ in a
backend Prolog initialization file (assumed to be loaded prior to
Logtalk loading).  This allows e.g.  Logtalk to be installed in a
read-only directory by setting this alias to the operating-system
directory for temporary files.  It also allows several Logtalk instances
to run concurrently without conflict by using a unique scratch directory
per instance (e.g.  using a process ID or a UUID generator).

This predicate may be used to override the default location used by the
packs tool to store registries and packs by defining the
‘logtalk_packs’ library alias in settings file or in a backend Prolog
initialization file (assumed to be loaded prior to Logtalk loading).

The logtalk built-in object provides an
expand_library_path/2(1) predicate that can be used to expand library
aliases and files expressed using library notation.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0-expand-library-path-2

2.4.9.38 Modes and number of proofs
...................................

     logtalk_library_path(?atom, -atom) - zero_or_more
     logtalk_library_path(?atom, -compound) - zero_or_more

2.4.9.39 Errors
...............

(none)

2.4.9.40 Examples
.................

     :- initialization((
        logtalk_load_context(directory, Directory),
        asserta(logtalk_library_path(my_application_root, Directory))
     )).

     | ?- logtalk_library_path(viewpoints, Path).

     Path = examples('viewpoints/')
     yes

     | ?- logtalk_library_path(Library, Path).

     Library = home,
     Path = '$HOME/' ;

     Library = logtalk_home,
     Path = '$LOGTALKHOME/' ;

     Library = logtalk_user
     Path = '$LOGTALKUSER/' ;

     Library = examples
     Path = logtalk_user('examples/') ;

     Library = library
     Path = logtalk_user('library/') ;

     Library = viewpoints
     Path = examples('viewpoints/')
     yes

     | ?- logtalk::expand_library_path(viewpoints, Path).

     Path = '/Users/pmoura/logtalk/examples/viewpoints/'.
     yes

     | ?- logtalk::expand_library_path(viewpoints('loader.lgt'), Path).

     Path = '/Users/pmoura/logtalk/examples/viewpoints/loader.lgt'.
     yes

See also
........

See logtalk_compile/1, logtalk_compile/2,
logtalk_load/1, logtalk_load/2.

'built-in predicate'

2.4.9.41 ‘logtalk_load_context/2’
.................................

2.4.9.42 Description
....................

     logtalk_load_context(Key, Value)

Provides access to the Logtalk compilation/loading context.  The
following keys are currently supported:

   * ‘entity_identifier’ - identifier of the entity being compiled if
     any

   * ‘entity_prefix’ - internal prefix for the entity compiled code

   * ‘entity_type’ - returns the value ‘module’ when compiling a module
     as an object

   * ‘entity_relation’ - returns the entity relations as declared in the
     entity opening directive

   * ‘source’ - full path of the source file being compiled

   * ‘file’ - the actual file being compiled, different from ‘source’
     only when processing an ‘include/1’ directive

   * ‘basename’ - source file basename

   * ‘directory’ - source file directory

   * ‘stream’ - input stream being used to read source file terms

   * ‘target’ - the full path of the intermediate Prolog file

   * ‘flags’ - the list of the explicit flags used for the compilation
     of the source file

   * ‘term’ - the source file term being compiled

   * ‘term_position’ - the position of the term being compiled
     (‘StartLine-EndLine’)

   * ‘variables’ - the variables of the term being compiled
     (‘[Variable1, ...]’)

   * ‘variable_names’ - the variable names of the term being compiled
     (‘[Name1=Variable1, ...]’)

   * ‘variable_names(Term)’ - the variable names of the term being
     compiled (‘[Name1=Variable1, ...]’)

   * ‘singletons’ - the singleton variables of the term being compiled
     (‘[Name1=Variable1, ...]’)

   * ‘singletons(Term)’ - the singleton variables of the term being
     compiled (‘[Name1=Variable1, ...]’)

   * ‘parameter_variables’ - list of parameter variable names and
     positions (‘[Name1-Position1, ...]’)

For the ‘entity_relation’ key, the possible values are:

   * ‘extends_protocol(Protocol, ParentProtocol, Scope)’

   * ‘implements_protocol(ObjectOrCategory, Protocol, Scope)’

   * ‘extends_category(Category, ParentCategory, Scope)’

   * ‘imports_category(Object, Category, Scope)’

   * ‘extends_object(Prototype, Parent, Scope)’

   * ‘instantiates_class(Instance, Class, Scope)’

   * ‘specializes_class(Class, Superclass, Scope)’

   * ‘complements_object(Category, Object)’

Calling this predicate with the ‘parameter_variables’ key only succeeds
when compiling a parametric entity containing parameter variables.

This predicate is usually called by the term_expansion/2 and
goal_expansion/2 methods.  It can also be called directly
from initialization/1 directives in a source file.  Note that
the entity keys are only available when compiling an entity term or from
an object ‘initialization/1’ directive.

     Warning: The ‘term_position’ key is only supported in backend Prolog compilers
     that provide access to the start and end
     lines of a read term.  When such support is not available, the
     value ‘-1’ is returned for both the start and the end lines.

     Variables in the values of the ‘term’, ‘variables’,
     ‘variable_names’, and ‘singletons’ keys are not shared with,
     respectively, the term and goal arguments of the ‘term_expansion/2’
     and ‘goal_expansion/2’ methods.  Use instead the
     ‘variable_names(Term)’ and ‘singletons(Term)’ keys.

2.4.9.43 Modes and number of proofs
...................................

     logtalk_load_context(?atom, -nonvar) - zero_or_more

2.4.9.44 Errors
...............

(none)

2.4.9.45 Examples
.................

     % expand source file terms only if they are entity terms
     term_expansion(Term, ExpandedTerms) :-
         logtalk_load_context(entity_identifier, _),
         ....

     % expand source file term while accessing its variable names
     term_expansion(Term, ExpandedTerms) :-
         logtalk_load_context(variable_names(Term), VariableNames),
         ....

     % define a library alias based on the source directory
     :- initialization((
         logtalk_load_context(directory, Directory),
         assertz(logtalk_library_path(my_app, Directory))
     )).

See also
........

See term_expansion/2, goal_expansion/2,
initialization/1.

2.4.10 Flags
------------

'built-in predicate'

2.4.10.1 ‘current_logtalk_flag/2’
.................................

2.4.10.2 Description
....................

     current_logtalk_flag(Flag, Value)

Enumerates, by backtracking, the current Logtalk flag values.  For a
description of the predefined compiler flags, please see the
Compiler flags section in the User Manual.

2.4.10.3 Modes and number of proofs
...................................

     current_logtalk_flag(?atom, ?atom) - zero_or_more

2.4.10.4 Errors
...............

     ‘Flag’ is neither a variable nor an atom:
          ‘type_error(atom, Flag)’
     ‘Flag’ is an atom but an invalid flag:
          ‘domain_error(flag, Value)’

2.4.10.5 Examples
.................

     % get the current value of the source_data flag:
     | ?- current_logtalk_flag(source_data, Value).

See also
........

See create_logtalk_flag/3, set_logtalk_flag/2.

'built-in predicate'

2.4.10.6 ‘set_logtalk_flag/2’
.............................

2.4.10.7 Description
....................

     set_logtalk_flag(Flag, Value)

Sets global, default, flag values.  For local flag scope, use the
corresponding set_logtalk_flag/2 directive.  To set a global
flag value when compiling and loading a source file, wrap the calls to
this built-in predicate with an initialization/1 directive.
For a description of the predefined compiler flags, please see the
Compiler flags section in the User Manual.

2.4.10.8 Modes and number of proofs
...................................

     set_logtalk_flag(+atom, +nonvar) - one

2.4.10.9 Errors
...............

     ‘Flag’ is a variable:
          ‘instantiation_error’
     ‘Value’ is a variable:
          ‘instantiation_error’
     ‘Flag’ is neither a variable nor an atom:
          ‘type_error(atom, Flag)’
     ‘Flag’ is an atom but an invalid flag:
          ‘domain_error(flag, Flag)’
     ‘Value’ is not a valid value for flag ‘Flag’:
          ‘domain_error(flag_value, Flag + Value)’
     ‘Flag’ is a read-only flag:
          ‘permission_error(modify, flag, Flag)’

2.4.10.10 Examples
..................

     % turn off globally and by default the compiler
     % unknown entities warnings:
     | ?- set_logtalk_flag(unknown_entities, silent).

See also
........

See create_logtalk_flag/3, current_logtalk_flag/2.

'built-in predicate'

2.4.10.11 ‘create_logtalk_flag/3’
.................................

2.4.10.12 Description
.....................

     create_logtalk_flag(Flag, Value, Options)

Creates a new Logtalk flag and sets its default value.  User-defined
flags can be queried and set in the same way as predefined flags by
using, respectively, the current_logtalk_flag/2 and
set_logtalk_flag/2 built-in predicates.  For a description of the
predefined compiler flags, please see the Compiler flags.
section in the User Manual.

This predicate is based on the specification of the SWI-Prolog
‘create_prolog_flag/3’ built-in predicate and supports the same options:
‘access(Access)’, where ‘Access’ can be either ‘read_write’ (the
default) or ‘read_only’; ‘keep(Keep)’, where ‘Keep’ can be either
‘false’ (the default) or ‘true’, for deciding if an existing definition
of the flag should be kept or replaced by the new one; and ‘type(Type)’
for specifying the type of the flag, which can be ‘boolean’, ‘atom’,
‘integer’, ‘float’, or ‘term’ (which only restricts the flag value to
ground terms).  When the ‘type/1’ option is not specified, the type of
the flag is inferred from its initial value.

2.4.10.13 Modes and number of proofs
....................................

     create_logtalk_flag(+atom, +ground, +list(ground)) - one

2.4.10.14 Errors
................

     ‘Flag’ is a variable:
          ‘instantiation_error’
     ‘Value’ is not a ground term:
          ‘instantiation_error’
     ‘Options’ is not a ground term:
          ‘instantiation_error’
     ‘Flag’ is neither a variable nor an atom:
          ‘type_error(atom, Flag)’
     ‘Options’ is neither a variable nor a list:
          ‘type_error(atom, Flag)’
     ‘Value’ is not a valid value for flag Flag:
          ‘domain_error(flag_value, Flag + Value)’
     ‘Flag’ is a system-defined flag:
          ‘permission_error(modify, flag, Flag)’
     An element ‘Option’ of the list ‘Options’ is not a valid option
          domain_error(flag_option,Option)
     The list ‘Options’ contains a ‘type(Type)’ option and ‘Value’ is not of type ‘Type’
          type_error(Type, Value)

2.4.10.15 Examples
..................

     % create a new boolean flag with default value set to false:
     | ?- create_logtalk_flag(pretty_print_blobs, false, []).

See also
........

See current_logtalk_flag/2, set_logtalk_flag/2.

2.4.11 Linter
-------------

'built-in predicate'

2.4.11.1 ‘logtalk_linter_hook/7’
................................

2.4.11.2 Description
....................

     logtalk_linter_hook(Goal, Flag, File, Lines, Type, Entity, Warning)

Multifile user-defined predicate, supporting the definition of custom
linter warnings.  Experimental.  The ‘Goal’ argument can be a message
sending goal, ‘Object::Message’, a call to a Prolog built-in predicate,
or a call to a module predicate, ‘Module:Predicate’.  The ‘Flag’
argument must be a supported linter flag.  The ‘Warning’ argument must
be a valid ‘core’ message term.  For a given ‘Goal’, only the first
successful call to this predicate is considered.

2.4.11.3 Modes and number of proofs
...................................

     logtalk_linter_hook(@callable, +atom, +atom, +pair(integer), +atom, @object_identifier, --callable) - zero_or_one

2.4.11.4 Errors
...............

(none)

2.4.11.5 Examples
.................

     :- multifile(user::logtalk_linter_hook/7).
     % warn about using list::append/3 to construct a list from an head and a tail
     user::logtalk_linter_hook(
         list::append(L1,L2,L), suspicious_calls,
         File, Lines, Type, Entity,
         suspicious_call(File, Lines, Type, Entity, list::append(L1,L2,L), [L=[Head|L2]])
     ) :-
         nonvar(L1),
         L1 = [Head].

2.5 Built-in methods
====================

2.5.1 Logic and control
-----------------------

'built-in method'

2.5.1.1 ‘!/0’
.............

2.5.1.2 Description
...................

     !

Always succeeds with the side-effect of discarding choice-points.  See
also the ISO Prolog standard definition.  This built-in method is
declared as a public method and can be used as a message to an object.

2.5.1.3 Modes and number of proofs
..................................

     ! - one

2.5.1.4 Errors
..............

(none)

2.5.1.5 Examples
................

(none)

See also
........

See true/0, fail/0, false/0,
repeat/0.

'built-in method'

2.5.1.6 ‘true/0’
................

2.5.1.7 Description
...................

     true

Always succeeds.  See also the ISO Prolog standard definition.  This
built-in method is declared as a public method and can be used as a
message to an object.

2.5.1.8 Modes and number of proofs
..................................

     true - one

2.5.1.9 Errors
..............

(none)

2.5.1.10 Examples
.................

(none)

See also
........

See !/0, fail/0, false/0, repeat/0.

'built-in method'

2.5.1.11 ‘fail/0’
.................

2.5.1.12 Description
....................

     fail

Always fails.  See also the ISO Prolog standard definition.  This
built-in method is declared as a public method and can be used as a
message to an object.

2.5.1.13 Modes and number of proofs
...................................

     fail - one

2.5.1.14 Errors
...............

(none)

2.5.1.15 Examples
.................

(none)

See also
........

See !/0, true/0, false/0, repeat/0.

'built-in method'

2.5.1.16 ‘false/0’
..................

2.5.1.17 Description
....................

     false

Always fails.  See also the ISO Prolog standard definition.  This
built-in method is declared as a public method and can be used as a
message to an object.

2.5.1.18 Modes and number of proofs
...................................

     false - one

2.5.1.19 Errors
...............

(none)

2.5.1.20 Examples
.................

(none)

See also
........

See !/0, true/0, fail/0, repeat/0.

'built-in method'

2.5.1.21 ‘repeat/0’
...................

2.5.1.22 Description
....................

     repeat

Always succeeds when called and when backtracking into its call with an
infinite number of choice-points.  See also the ISO Prolog standard
definition.  This built-in method is declared as a public method and can
be used as a message to an object.

2.5.1.23 Modes and number of proofs
...................................

     repeat - one_or_more

2.5.1.24 Errors
...............

(none)

2.5.1.25 Examples
.................

(none)

See also
........

See !/0, true/0, fail/0, false/0.

2.5.2 Execution context
-----------------------

'built-in method'

2.5.2.1 ‘context/1’
...................

2.5.2.2 Description
...................

     context(Context)

Returns the execution context for a predicate clause using the term
‘logtalk(Head,ExecutionContext)’ where ‘Head’ is the head of the clause
containing the call.  This private predicate is mainly used for
providing an error context when type-checking predicate arguments.  The
‘ExecutionContext’ term should be regarded as an opaque term, which can
be decoded using the logtalk::execution_context/7(1) predicate.  Calls
to this predicate are inlined at compilation time.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0-execution-context-7

2.5.2.3 Modes and number of proofs
..................................

     context(--callable) - one

2.5.2.4 Errors
..............

(none)

2.5.2.5 Examples
................

     foo(A, N) :-
         % type-check arguments
         context(Context),
         type::check(atom, A, Context),
         type::check(integer, N, Context),
         % arguments are fine; go ahead
         ... .

See also
........

See parameter/2, self/1, sender/1,
this/1.

'built-in method'

2.5.2.6 ‘parameter/2’
.....................

2.5.2.7 Description
...................

     parameter(Number, Term)

Used in parametric objects. (and parametric categories), this
private method provides runtime access to the parameter values of the
entity that contains the predicate clause whose body is being executed
by using the argument number in the entity identifier.  This predicate
is implemented as a unification between its second argument and the
corresponding implicit execution-context argument in the predicate
clause making the call.  This unification occurs at the clause head when
the second argument is not instantiated (the most common case).  When
the second argument is instantiated, the unification must be delayed to
runtime and thus occurs at the clause body.

Entity parameters can also be accessed using 'parameter variables',
which use the syntax ‘_VariableName_’.  The compiler recognizes
occurrences of these variables in directives and clauses.  Parameter
variables allows us to abstract parameter positions thus simplifying
code maintenance.

2.5.2.8 Modes and number of proofs
..................................

     parameter(+integer, ?term) - zero_or_one

2.5.2.9 Errors
..............

     ‘Number’ is a variable:
          ‘instantiation_error’
     ‘Number’ is neither a variable nor an integer value:
          ‘type_error(integer, Number)’
     ‘Number’ is smaller than one or greater than the parametric entity identifier arity:
          ‘domain_error(out_of_range, Number)’
     ‘Entity’ identifier is not a compound term:
          ‘type_error(compound, Entity)’

2.5.2.10 Examples
.................

     :- object(box(_Color, _Weight)).

         ...

         % this clause is translated into
         % a fact upon compilation
         color(Color) :-
             parameter(1, Color).

         % upon compilation, the >/2 call will be
         % the single goal in the clause body
         heavy :-
             parameter(2, Weight),
             Weight > 10.
         ...

The same example using 'parameter variables':

     :- object(box(_Color_, _Weight_)).

         ...

         color(_Color_).

         heavy :-
             _Weight_ > 10.

         ...

See also
........

See context/1, self/1, sender/1,
this/1.

'built-in method'

2.5.2.11 ‘self/1’
.................

2.5.2.12 Description
....................

     self(Self)

Unifies its argument with the object that received the message under
processing.  This private method is compiled into a unification between
its argument and the corresponding implicit context argument in the
predicate clause making the call.  This unification occurs at the clause
head when the argument is not bound at compile time (the most common
case).

2.5.2.13 Modes and number of proofs
...................................

     self(?object_identifier) - zero_or_one

2.5.2.14 Errors
...............

(none)

2.5.2.15 Examples
.................

     % upon compilation, the write/1 call will be
     % the first goal on the clause body
     test :-
         self(Self),
         write('executing a method in behalf of '),
         writeq(Self), nl.

See also
........

See context/1, parameter/2, sender/1,
this/1.

'built-in method'

2.5.2.16 ‘sender/1’
...................

2.5.2.17 Description
....................

     sender(Sender)

Unifies its argument with the object that sent the message under
processing.  This private method is translated into a unification
between its argument and the corresponding implicit context argument in
the predicate clause making the call.  This unification occurs at the
clause head when the argument is not bound at compile time (the most
common case).

2.5.2.18 Modes and number of proofs
...................................

     sender(?object_identifier) - zero_or_one

2.5.2.19 Errors
...............

(none)

2.5.2.20 Examples
.................

     % after compilation, the write/1 call will
     % be the first goal on the clause body
     test :-
         sender(Sender),
         write('executing a method to answer a message sent by '),
         writeq(Sender), nl.

See also
........

See context/1, parameter/2, self/1,
this/1.

'built-in method'

2.5.2.21 ‘this/1’
.................

2.5.2.22 Description
....................

     this(This)

Unifies its argument with the identifier of the object for which the
predicate clause whose body is being executed is defined (or the object
importing the category that contains the predicate clause).  This
private method is implemented as a unification between its argument and
the corresponding implicit execution-context argument in the predicate
clause making the call.  This unification occurs at the clause head when
the argument is not bound at compile time (the most common case).  This
method is useful for avoiding hard-coding references to an object
identifier or for retrieving all object parameters with a single call
when using parametric objects.

2.5.2.23 Modes and number of proofs
...................................

     this(?object_identifier) - zero_or_one

2.5.2.24 Errors
...............

(none)

2.5.2.25 Examples
.................

     % after compilation, the write/1 call will
     % be the first goal on the clause body
     test :-
         this(This),
         write('Using a predicate clause contained in '),
         writeq(This), nl.

See also
........

See context/1, parameter/2, self/1,
sender/1.

2.5.3 Reflection
----------------

'built-in method'

2.5.3.1 ‘current_op/3’
......................

2.5.3.2 Description
...................

     current_op(Priority, Specifier, Operator)

Enumerates, by backtracking, the visible operators declared for an
object.  Operators not declared using a scope directive are not
enumerated.

2.5.3.3 Modes and number of proofs
..................................

     current_op(?operator_priority, ?operator_specifier, ?atom) - zero_or_more

2.5.3.4 Errors
..............

     ‘Priority’ is neither a variable nor an integer:
          ‘type_error(integer, Priority)’
     ‘Priority’ is an integer but not a valid operator priority:
          ‘domain_error(operator_priority, Priority)’
     ‘Specifier’ is neither a variable nor an atom:
          ‘type_error(atom, Specifier)’
     ‘Specifier’ is an atom but not a valid operator specifier:
          ‘domain_error(operator_specifier, Specifier)’
     ‘Operator’ is neither a variable nor an atom:
          ‘type_error(atom, Operator)’

2.5.3.5 Examples
................

     To enumerate, by backtracking, the local operators or the operators visible in this.:
          ‘current_op(Priority, Specifier, Operator)’
     To enumerate, by backtracking, the public and protected operators visible in self.:
          ‘::current_op(Priority, Specifier, Operator)’
     To enumerate, by backtracking, the public operators visible for an explicit object:
          ‘Object::current_op(Priority, Specifier, Operator)’

See also
........

See current_predicate/1, predicate_property/2,
op/3.

'built-in method'

2.5.3.6 ‘current_predicate/1’
.............................

2.5.3.7 Description
...................

     current_predicate(Predicate)

Enumerates, by backtracking, visible, user-defined, object predicates.
Built-in predicates and predicates not declared using a scope directive
are not enumerated.

This predicate also succeeds for any predicates listed in uses/2
and use_module/2 directives.

When ‘Predicate’ is bound at compile time to a ‘(:)/2’ term, this
predicate enumerates module predicates (assuming that the backend Prolog compiler
supports modules).

2.5.3.8 Modes and number of proofs
..................................

     current_predicate(?predicate_indicator) - zero_or_more

2.5.3.9 Errors
..............

     ‘Predicate’ is neither a variable nor a valid predicate indicator:
          ‘type_error(predicate_indicator, Predicate)’
     ‘Predicate’ is a ‘Name/Arity’ term but ‘Functor’ is neither a variable nor an atom:
          ‘type_error(atom, Name)’
     ‘Predicate’ is a ‘Name/Arity’ term but ‘Arity’ is neither a variable nor an integer:
          ‘type_error(integer, Arity)’
     ‘Predicate’ is a ‘Name/Arity’ term but ‘Arity’ is a negative integer:
          ‘domain_error(not_less_than_zero, Arity)’

2.5.3.10 Examples
.................

     To enumerate, by backtracking, the locally visible user predicates or the user predicates visible in this.:
          ‘current_predicate(Predicate)’
     To enumerate, by backtracking, the public and protected user predicates visible in self.:
          ‘::current_predicate(Predicate)’
     To enumerate, by backtracking, the public user predicates visible for an explicit object:
          ‘Object::current_predicate(Predicate)’

An example of enumerating locally visible object predicates.  These
include predicates listed using uses/2 and
use_module/2 directives:

     :- object(foo).

         :- uses(bar, [
             baz/1, quux/2
         ]).

         :- public(pred/1).
         pred(X) :-
            current_predicate(X).

     :- end_object.

     | ?- foo::pred(X).
     X = pred/1 ;
     X = baz/1 ;
     X = quux/2 ;
     no

See also
........

See current_op/3, predicate_property/2, uses/2,
see use_module/2.

'built-in method'

2.5.3.11 ‘predicate_property/2’
...............................

2.5.3.12 Description
....................

     predicate_property(Predicate, Property)

Enumerates, by backtracking, the properties of a visible object
predicate.  Properties for predicates not declared using a scope
directive are not enumerated.  The valid predicate properties are listed
in the language grammar section on predicate properties and
described in the User Manual section on predicate properties.

When ‘Predicate’ is listed in a uses/2 or use_module/2
directive, properties are enumerated for the referenced object or
module predicate.

When ‘Predicate’ is bound at compile time to a ‘(:)/2’ term, this
predicate enumerates properties for module predicates (assuming that the
backend Prolog compiler supports modules).

2.5.3.13 Modes and number of proofs
...................................

     predicate_property(+callable, ?predicate_property) - zero_or_more

2.5.3.14 Errors
...............

     ‘Predicate’ is a variable:
          ‘instantiation_error’
     ‘Predicate’ is neither a variable nor a callable term:
          ‘type_error(callable, Predicate)’
     ‘Property’ is neither a variable nor a valid predicate property:
          ‘domain_error(predicate_property, Property)’

2.5.3.15 Examples
.................

     To enumerate, by backtracking, the properties of a locally visible user predicate or a user predicate visible in this.:
          ‘predicate_property(Predicate, Property)’
     To enumerate, by backtracking, the properties of a public or protected predicate visible in self.:
          ‘::predicate_property(Predicate, Property)’
     To enumerate, by backtracking, the properties of a public predicate visible in an explicit object:
          ‘Object::predicate_property(Predicate, Property)’

An example of enumerating properties for locally visible object
predicates.  These include predicates listed using uses/2 and
use_module/2 directives:

     :- object(foo).

         :- uses(bar, [
             baz/1, quux/2
         ]).

         :- public(pred/1).
         pred_prop(Pred, Prop) :-
            predicate_property(Pred, Prop).

     :- end_object.

     | ?- foo::pred(baz(_), Prop).
     Prop = logtalk ;
     Prop = scope(public) ;
     Prop = public ;
     Prop = declared_in(bar) ;
     ...

See also
........

See current_op/3, current_predicate/1, uses/2,
see use_module/2.

2.5.4 Database
--------------

'built-in method'

2.5.4.1 ‘abolish/1’
...................

2.5.4.2 Description
...................

     abolish(Predicate)

Abolishes a runtime declared object dynamic predicate or an object local
dynamic predicate.  Only predicates that are dynamically declared at
runtime (using a call to the asserta/1 or assertz/1
built-in methods) can be abolished.

When the predicate indicator is declared in a uses/2 or
use_module/2 directive, the predicate is abolished in the
referenced object or module.  When the backend Prolog compiler supports
a module system, the predicate argument can also be module qualified.

2.5.4.3 Modes and number of proofs
..................................

     abolish(@predicate_indicator) - one

2.5.4.4 Errors
..............

     ‘Predicate’ is a variable:
          ‘instantiation_error’
     ‘Functor’ is a variable:
          ‘instantiation_error’
     ‘Arity’ is a variable:
          ‘instantiation_error’
     ‘Predicate’ is neither a variable nor a valid predicate indicator:
          ‘type_error(predicate_indicator, Predicate)’
     ‘Functor’ is neither a variable nor an atom:
          ‘type_error(atom, Functor)’
     ‘Arity’ is neither a variable nor an integer:
          ‘type_error(integer, Arity)’
     ‘Predicate’ is statically declared:
          ‘permission_error(modify, predicate_declaration, Name/Arity)’
     ‘Predicate’ is a private predicate:
          ‘permission_error(modify, private_predicate, Name/Arity)’
     ‘Predicate’ is a protected predicate:
          ‘permission_error(modify, protected_predicate, Name/Arity)’
     ‘Predicate’ is a static predicate:
          ‘permission_error(modify, static_predicate, Name/Arity)’
     ‘Predicate’ is not declared for the object receiving the message:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.5.4.5 Examples
................

     To abolish a local dynamic predicate or a dynamic predicate in this.:
          ‘abolish(Predicate)’
     To abolish a public or protected dynamic predicate in self.:
          ‘::abolish(Predicate)’
     To abolish a public dynamic predicate in an explicit object:
          ‘Object::abolish(Predicate)’

See also
........

See asserta/1, assertz/1, clause/2,
retract/1, retractall/1. See dynamic/0,
dynamic/1, uses/2, use_module/2.

'built-in method'

2.5.4.6 ‘asserta/1’
...................

2.5.4.7 Description
...................

     asserta(Head)
     asserta((Head:-Body))

Asserts a clause as the first one for an object dynamic predicate.

When the predicate was not previously declared (using a scope
directive), a dynamic predicate declaration is added to the object.  In
this case, the predicate scope depends on how this method is called:

‘asserta(Clause)’

     The predicate is dynamically declared as private.

‘::asserta(Clause)’

     The predicate is dynamically declared as protected.

‘Object::asserta(Clause)’

     The predicate is dynamically declared as public.

Note, however, that dynamically declaring a new predicate requires
either a local assert or the dynamic_declarations compiler
flag set to ‘allow’ when the object was created or compiled.

When the predicate indicator for ‘Head’ is declared in a uses/2
or use_module/2 directive, the clause is asserted in the
referenced object or module.  When the backend Prolog compiler supports
a module system, the predicate argument can also be module qualified.

This method may be used to assert clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in this.  This allows easy initialization of
dynamically created objects when writing constructors.

2.5.4.8 Modes and number of proofs
..................................

     asserta(+clause) - one

2.5.4.9 Errors
..............

     ‘Head’ is a variable:
          ‘instantiation_error’
     ‘Head’ is a neither a variable nor a callable term:
          ‘type_error(callable, Head)’
     ‘Body’ cannot be converted to a goal:
          ‘type_error(callable, Body)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a private predicate:
          ‘permission_error(modify, private_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a protected predicate:
          ‘permission_error(modify, protected_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a static predicate:
          ‘permission_error(modify, static_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, does not match a declared predicate and the target object was created or compiled with support for dynamic declaration of predicates turned off:
          ‘permission_error(create, predicate_declaration, Name/Arity)’

2.5.4.10 Examples
.................

     To assert a clause as the first one for a local dynamic predicate or a dynamic predicate in this.:
          ‘asserta(Clause)’
     To assert a clause as the first one for any public or protected dynamic predicate in self.:
          ‘::asserta(Clause)’
     To assert a clause as the first one for any public dynamic predicate in an explicit object:
          ‘Object::asserta(Clause)’

An example of asserting clauses in this and in self
from a category:

     :- category(attributes,
         implements(attributes_protocol)).

         :- private(attr_/1).
         :- dynamic(attr_/1).

         set_in_this(A, X) :-
            asserta(attr_(A, X)).

         set_in_self(A, X) :-
            ::asserta(attr_(A, X)).

         ...

An example of asserting clauses into another object with the predicates
listed using a uses/2 directive (similar when using a
use_module/2 directive):

     :- object(reasoner(_KnowledgeBase_)).

         :- uses(_KnowledgeBase_, [
             foo/1, bar/1
         ]).

         baz(X) :-
            % compiled as _KnowledgeBase_::assertz(foo(X))
            asserta(foo(X)).

        foobar(Name, Argument) :-
           Clause =.. [Name, Argument],
           % runtime resolved to _KnowledgeBase_::assertz(Clause)
           % when Name is either foo or bar
           asserta(Clause).

         ...

See also
........

See abolish/1, assertz/1, clause/2,
retract/1, retractall/1. See dynamic/0,
dynamic/1, uses/2, use_module/2.

'built-in method'

2.5.4.11 ‘assertz/1’
....................

2.5.4.12 Description
....................

     assertz(Head)
     assertz((Head:-Body))

Asserts a clause as the last one for a dynamic predicate.

When the predicate was not previously declared (using a scope
directive), a dynamic predicate declaration is added to the object.  In
this case, the predicate scope depends on how this method is called:

‘assertz(Clause)’

     The predicate is dynamically declared as private.

‘::assertz(Clause)’

     The predicate is dynamically declared as protected.

‘Object::assertz(Clause)’

     The predicate is dynamically declared as public.

Note, however, that dynamically declaring a new predicate requires
either a local assert or the dynamic_declarations compiler
flag set to ‘allow’ when the object was created or compiled.

When the predicate indicator for ‘Head’ is declared in a uses/2
or use_module/2 directive, the clause is asserted in the
referenced object or module.  When the backend Prolog compiler supports
a module system, the predicate argument can also be module qualified.

This method may be used to assert clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in this.  This allows easy initialization of
dynamically created objects when writing constructors.

2.5.4.13 Modes and number of proofs
...................................

     assertz(+clause) - one

2.5.4.14 Errors
...............

     ‘Head’ is a variable:
          ‘instantiation_error’
     ‘Head’ is a neither a variable nor a callable term:
          ‘type_error(callable, Head)’
     ‘Body’ cannot be converted to a goal:
          ‘type_error(callable, Body)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a private predicate:
          ‘permission_error(modify, private_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a protected predicate:
          ‘permission_error(modify, protected_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a static predicate:
          ‘permission_error(modify, static_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, does not match a declared predicate and the target object was created/compiled with support for dynamic declaration of predicates turned off:
          ‘permission_error(create, predicate_declaration, Name/Arity)’

2.5.4.15 Examples
.................

     To assert a clause as the last one for a local dynamic predicate or a dynamic predicate in this.:
          ‘assertz(Clause)’
     To assert a clause as the last one for any public or protected dynamic predicate in self.:
          ‘::assertz(Clause)’
     To assert a clause as the last one for any public dynamic predicate in an explicit object:
          ‘Object::assertz(Clause)’

An example of asserting clauses in this and in self
from a category:

     :- category(attributes,
         implements(attributes_protocol)).

         :- private(attr_/1).
         :- dynamic(attr_/1).

         set_in_this(A, X) :-
            assertz(attr_(A, X)).

         set_in_self(A, X) :-
            ::assertz(attr_(A, X)).

         ...

An example of asserting clauses into another object with the predicates
listed using a uses/2 directive (similar when using a
use_module/2 directive):

     :- object(reasoner(_KnowledgeBase_)).

         :- uses(_KnowledgeBase_, [
             foo/1, bar/1
         ]).

         baz(X) :-
            % compiled as _KnowledgeBase_::assertz(foo(X))
            assertz(foo(X)).

        foobar(Name, Argument) :-
           Clause =.. [Name, Argument],
           % runtime resolved to _KnowledgeBase_::assertz(Clause)
           % when Name is either foo or bar
           assertz(Clause).

         ...

See also
........

See abolish/1, asserta/1, clause/2,
retract/1, retractall/1. See dynamic/0,
dynamic/1, uses/2, use_module/2.

'built-in method'

2.5.4.16 ‘clause/2’
...................

2.5.4.17 Description
....................

     clause(Head, Body)

Enumerates, by backtracking, the clauses of a dynamic predicate.

When the predicate indicator for ‘Head’ is declared in a uses/2
or use_module/2 directive, the predicate enumerates the
clauses in the referenced object or module.  When the backend Prolog
compiler supports a module system, the head argument can also be module
qualified.

This method may be used to enumerate clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in this.

2.5.4.18 Modes and number of proofs
...................................

     clause(+callable, ?body) - zero_or_more

2.5.4.19 Errors
...............

     ‘Head’ is a variable:
          ‘instantiation_error’
     ‘Head’ is a neither a variable nor a callable term:
          ‘type_error(callable, Head)’
     ‘Body’ is a neither a variable nor a callable term:
          ‘type_error(callable, Body)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a private predicate:
          ‘permission_error(access, private_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a protected predicate:
          ‘permission_error(access, protected_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a static predicate:
          ‘permission_error(access, static_predicate, Name/Arity)’
     ‘Head’ is not a declared predicate:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.5.4.20 Examples
.................

     To retrieve a matching clause of a local dynamic predicate or a dynamic predicate in this.:
          ‘clause(Head, Body)’
     To retrieve a matching clause of a public or protected dynamic predicate in self.:
          ‘::clause(Head, Body)’
     To retrieve a matching clause of a public dynamic predicate in an explicit object:
          ‘Object::clause(Head, Body)’

See also
........

See abolish/1, asserta/1, assertz/1,
retract/1, retractall/1. See dynamic/0,
dynamic/1, uses/2, use_module/2.

'built-in method'

2.5.4.21 ‘retract/1’
....................

2.5.4.22 Description
....................

     retract(Head)
     retract((Head:-Body))

Retracts a clause for an object dynamic predicate.  On backtracking, the
predicate retracts the next matching clause.

When the predicate indicator for ‘Head’ is declared in a uses/2
or use_module/2 directive, the clause is retracted in the
referenced object or module.  When the backend Prolog compiler supports
a module system, the predicate argument can also be module qualified.

This method may be used to retract clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in this.

2.5.4.23 Modes and number of proofs
...................................

     retract(+clause) - zero_or_more

2.5.4.24 Errors
...............

     ‘Head’ is a variable:
          ‘instantiation_error’
     ‘Head’ is neither a variable nor a callable term:
          ‘type_error(callable, Head)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a private predicate:
          ‘permission_error(modify, private_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a protected predicate:
          ‘permission_error(modify, protected_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a static predicate:
          ‘permission_error(modify, static_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.5.4.25 Examples
.................

     To retract a matching clause of a dynamic predicate in this.:
          ‘retract(Clause)’
     To retract a matching clause of a public or protected dynamic predicate in self.:
          ‘::retract(Clause)’
     To retract a matching clause of a public dynamic predicate in an explicit object:
          ‘Object::retract(Clause)’

See also
........

See abolish/1, asserta/1, assertz/1,
clause/2, retractall/1, dynamic/0,
dynamic/1, uses/2, use_module/2.

'built-in method'

2.5.4.26 ‘retractall/1’
.......................

2.5.4.27 Description
....................

     retractall(Head)

Retracts all clauses with a matching head for an object dynamic
predicate.

When the predicate indicator for ‘Head’ is declared in a uses/2
or use_module/2 directive, the clauses are retracted in
the referenced object or module.  When the backend Prolog compiler
supports a module system, the predicate argument can also be module
qualified.

This method may be used to retract clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in this.

2.5.4.28 Modes and number of proofs
...................................

     retractall(@callable) - one

2.5.4.29 Errors
...............

     ‘Head’ is a variable:
          ‘instantiation_error’
     ‘Head’ is neither a variable nor a callable term:
          ‘type_error(callable, Head)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a private predicate:
          ‘permission_error(modify, private_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a protected predicate:
          ‘permission_error(modify, protected_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is that of a static predicate:
          ‘permission_error(modify, static_predicate, Name/Arity)’
     The predicate indicator of ‘Head’, ‘Name/Arity’, is not declared:
          ‘existence_error(predicate_declaration, Name/Arity)’

2.5.4.30 Examples
.................

     To retract all clauses with a matching head of a dynamic predicate in this.:
          ‘retractall(Head)’
     To retract all clauses with a matching head of a public or protected dynamic predicate in self.:
          ‘::retractall(Head)’
     To retract all clauses with a matching head of a public dynamic predicate in an explicit object:
          ‘Object::retractall(Head)’

See also
........

See abolish/1, asserta/1, assertz/1,
clause/2, retract/1, dynamic/0,
dynamic/1, uses/2, use_module/2.

2.5.5 Meta-calls
----------------

'built-in method'

2.5.5.1 ‘call/1-N’
..................

2.5.5.2 Description
...................

     call(Goal)
     call(Closure, Arg1, ...)

Calls a goal constructed by appending additional arguments to a
closure.  The upper limit for ‘N’ depends on the upper limit for
the arity of a compound term of the backend Prolog compiler.
This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.  The ‘Closure’ argument can
also be a lambda expression or a Logtalk control construct.  When using
a backend Prolog compiler supporting a module system, calls in the
format ‘call(Module:Closure, Arg1, ...)’ may also be used.

This meta-predicate is opaque to cuts in its arguments.

2.5.5.3 Meta-predicate template
...............................

     call(0)
     call(1, *)
     call(2, *, *)
     ...

2.5.5.4 Modes and number of proofs
..................................

     call(+callable) - zero_or_more
     call(+callable, ?term) - zero_or_more
     call(+callable, ?term, ?term) - zero_or_more
     ...

2.5.5.5 Errors
..............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Closure’ is a variable:
          ‘instantiation_error’
     ‘Closure’ is neither a variable nor a callable term:
          ‘type_error(callable, Closure)’

2.5.5.6 Examples
................

     Call a goal, constructed by appending additional arguments to a closure, in the context of the object or category containing the call:
          ‘call(Closure, Arg1, Arg2, ...)’
     To send a goal, constructed by appending additional arguments to a closure, as a message to self.:
          ‘call(::Closure, Arg1, Arg2, ...)’
     To send a goal, constructed by appending additional arguments to a closure, as a message to an explicit object:
          ‘call(Object::Closure, Arg1, Arg2, ...)’

See also
........

See ignore/1, once/1, (\+)/1.

'built-in method'

2.5.5.7 ‘ignore/1’
..................

2.5.5.8 Description
...................

     ignore(Goal)

This predicate succeeds whether its argument succeeds or fails and it is
not re-executable.  This built-in meta-predicate is declared as a
private method and thus cannot be used as a message to an object.

This meta-predicate is opaque to cuts in its argument.

2.5.5.9 Meta-predicate template
...............................

     ignore(0)

2.5.5.10 Modes and number of proofs
...................................

     ignore(+callable) - one

2.5.5.11 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

2.5.5.12 Examples
.................

     Call a goal and succeeding even if it fails:
          ‘ignore(Goal)’
     To send a message succeeding even if it fails to self.:
          ‘ignore(::Goal)’
     To send a message succeeding even if it fails to an explicit object:
          ‘ignore(Object::Goal)’

See also
........

See call/1-N, once/1, (\+)/1.

'built-in method'

2.5.5.13 ‘once/1’
.................

2.5.5.14 Description
....................

     once(Goal)

This predicate behaves as ‘call(Goal)’ but it is not re-executable.
This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

This meta-predicate is opaque to cuts in its argument.

2.5.5.15 Meta-predicate template
................................

     once(0)

2.5.5.16 Modes and number of proofs
...................................

     once(+callable) - zero_or_one

2.5.5.17 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

2.5.5.18 Examples
.................

     Call a goal deterministically in the context of the object or category containing the call:
          ‘once(Goal)’
     To send a goal as a non-backtracable message to self.:
          ‘once(::Goal)’
     To send a goal as a non-backtracable message to an explicit object:
          ‘once(Object::Goal)’

See also
........

See call/1-N, ignore/1, (\+)/1.

'built-in method'

2.5.5.19 ‘(\+)/1’
.................

2.5.5.20 Description
....................

     \+ Goal

Not-provable meta-predicate.  True iff ‘call(Goal)’ is false.  This
built-in meta-predicate is declared as a private method and thus cannot
be used as a message to an object.

     Warning: The argument is always compiled (for improved
     performance).  As a consequence, when the argument is a control
     construct (e.g.  a conjunction), any meta-variables will be wrapped
     with the equivalent to the ‘call/1’ control construct.  Note that
     these semantics differ from the ISO Prolog Core standard
     specification for the ‘(\+)/1’ built-in predicate.  For example,
     assuming a conforming system:

          | ?- X = !, \+ (member(Y,[1,2,3]), X, write(Y), fail).
          1

          X = !

     But in Logtalk ‘X’ is compiled into a meta-call, which is not
     cut-transparent:

          yes
          | ?- logtalk << (X = !, \+ (member(Y,[1,2,3]), X, write(Y), fail)).
          123

          X = !

     Note that the ISO Prolog Core standard doesn’t specify a
     cut-transparent alternative to the ‘call/1’ control construct.

2.5.5.21 Meta-predicate template
................................

     \+ 0

2.5.5.22 Modes and number of proofs
...................................

     \+ +callable - zero_or_one

2.5.5.23 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’

2.5.5.24 Examples
.................

     Not-provable goal in the context of the object or category containing the call:
          ‘\+ Goal’
     Not-provable goal sent as a message to self.:
          ‘\+ ::Goal’
     Not-provable goal sent as a message to an explicit object:
          ‘\+ Object::Goal’

See also
........

See call/1-N, ignore/1, once/1.

2.5.6 Error handling
--------------------

'built-in method'

2.5.6.1 ‘catch/3’
.................

2.5.6.2 Description
...................

     catch(Goal, Catcher, Recovery)

Catches exceptions thrown by a goal.  See also the ISO Prolog standard
definition.  This built-in meta-predicate is declared as a private
method and thus cannot be used as a message to an object.

2.5.6.3 Modes and number of proofs
..................................

     catch(?callable, ?term, ?callable) - zero_or_more

2.5.6.4 Errors
..............

(none)

2.5.6.5 Examples
................

(none)

See also
........

See throw/1, context/1, instantiation_error/0,
see uninstantiation_error/1, type_error/2,
domain_error/2, existence_error/2,
permission_error/3, evaluation_error/1,
representation_error/1. See resource_error/1,
syntax_error/1, system_error/0.

'built-in method'

2.5.6.6 ‘throw/1’
.................

2.5.6.7 Description
...................

     throw(Exception)

Throws an exception.  See also the ISO Prolog standard definition.  This
built-in method is declared private and thus cannot be used as a message
to an object.

2.5.6.8 Modes and number of proofs
..................................

     throw(+nonvar) - error

2.5.6.9 Errors
..............

     ‘Exception’ is a variable:
          ‘instantiation_error’
     ‘Exception’ does not unify with the second argument of any call of catch/3.:
          ‘system_error’

2.5.6.10 Examples
.................

(none)

See also
........

See catch/3, context/1, instantiation_error/0,
see uninstantiation_error/1, type_error/2,
domain_error/2, existence_error/2,
permission_error/3, evaluation_error/1,
representation_error/1. See resource_error/1,
syntax_error/1, system_error/0.

'built-in method'

2.5.6.11 ‘instantiation_error/0’
................................

2.5.6.12 Description
....................

     instantiation_error

Throws an instantiation error.  Used when an argument or one of its
sub-arguments is a variable but a non-variable term is required.  For
example, trying to open a file with a variable for the input/output
mode.

This built-in method is declared private and thus cannot be used as a
message to an object.  Calling this predicate is equivalent to the
following sequence of calls:

     ...,
     context(Context),
     throw(error(instantiation_error, Context)).

This allows the user to generate errors in the same format used by the
runtime.

2.5.6.13 Modes and number of proofs
...................................

     instantiation_error - error

2.5.6.14 Errors
...............

     When called:
          ‘instantiation_error’

2.5.6.15 Examples
.................

     ...,
     var(Handler),
     instantiation_error.

See also
........

See catch/3, throw/1, context/1,
uninstantiation_error/1, type_error/2,
domain_error/2, existence_error/2,
permission_error/3, representation_error/1,
evaluation_error/1, resource_error/1,
syntax_error/1, system_error/0.

'built-in method'

2.5.6.16 ‘uninstantiation_error/1’
..................................

2.5.6.17 Description
....................

     uninstantiation_error(Culprit)

Throws an uninstantiation error.  Used when an argument or one of its
sub-arguments is bound but a variable is required.  For example, trying
to open a file with a stream argument bound.

This built-in method is declared private and thus cannot be used as a
message to an object.  Calling this predicate is equivalent to the
following sequence of calls:

     ...,
     context(Context),
     throw(error(uninstantiation_error(Culprit), Context)).

This allows the user to generate errors in the same format used by the
runtime.

2.5.6.18 Modes and number of proofs
...................................

     uninstantiation_error(@nonvar) - error

2.5.6.19 Errors
...............

     When called:
          ‘uninstantiation_error(Culprit)’

2.5.6.20 Examples
.................

     ...,
     var(Handler),
     uninstantiation_error(my_stream).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, type_error/2,
domain_error/2, existence_error/2,
permission_error/3, representation_error/1,
evaluation_error/1, resource_error/1,
syntax_error/1, system_error/0.

'built-in method'

2.5.6.21 ‘type_error/2’
.......................

2.5.6.22 Description
....................

     type_error(Type, Culprit)

Throws a type error.  Used when the type of an argument is incorrect.
For example, trying to use a non-callable term as a message.  This
built-in method is declared private and thus cannot be used as a message
to an object.  Calling this predicate is equivalent to the following
sequence of goals:

     ...,
     context(Context),
     throw(error(type_error(Type,Culprit), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Type’ include all the types defined by the ‘type’
library object and by other libraries such as ‘os’, ‘expecteds’, and
‘optionals’.  The value of ‘Culprit’ is the argument or one of its
sub-terms that caused the error.

2.5.6.23 Modes and number of proofs
...................................

     type_error(@nonvar, @term) - error

2.5.6.24 Errors
...............

     When called:
          ‘type_error(Type, Culprit)’

2.5.6.25 Examples
.................

     ...,
     \+ atom(Name),
     type_error(atom, Name).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
domain_error/2, existence_error/2,
permission_error/3, representation_error/1. See
evaluation_error/1, resource_error/1,
syntax_error/1, system_error/0,

'built-in method'

2.5.6.26 ‘domain_error/2’
.........................

2.5.6.27 Description
....................

     domain_error(Domain, Culprit)

Throws a domain error.  Used when an argument is of the correct type but
outside the valid domain.  For example, trying to use an atom as an
operator specifier that is not a valid specifier.  This built-in method
is declared private and thus cannot be used as a message to an object.
Calling this predicate is equivalent to the following sequence of goals:

     ...,
     context(Context),
     throw(error(domain_error(Domain,Culprit), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Domain’ include:

   - ‘character_code_list’

   - ‘close_option’

   - ‘flag_option’

   - ‘flag_value’

   - ‘compiler_flag’

   - ‘flag’

   - ‘prolog_flag’

   - ‘io_mode’

   - ‘non_empty_list’

   - ‘not_less_than_zero’

   - ‘operator_priority’

   - ‘operator_specifier’

   - ‘read_option’

   - ‘source_sink’

   - ‘stream’

   - ‘stream_option’

   - ‘stream_or_alias’

   - ‘stream_position’

   - ‘stream_property’

   - ‘write_option’

   - ‘character_code_list’

   - ‘text_encoding’

   - ‘directive’

   - ‘object_directive’

   - ‘protocol_directive’

   - ‘category_directive’

   - ‘object_relation’

   - ‘protocol_relation’

   - ‘category_relation’

   - ‘object_property’

   - ‘protocol_property’

   - ‘category_property’

   - ‘predicate_property’

   - ‘meta_argument_specifier’

   - ‘meta_directive_template’

   - ‘closure’

   - ‘allocation’

   - ‘redefinition’

   - ‘message_sending_goal’

   - ‘class’

   - ‘prototype’

   - ‘scope’

   - ‘boolean’

The value of ‘Culprit’ is the argument or one of its sub-terms that
caused the error.

2.5.6.28 Modes and number of proofs
...................................

     domain_error(+atom, @nonvar) - error

2.5.6.29 Errors
...............

     When called:
          ‘domain_error(Domain, Culprit)’

2.5.6.30 Examples
.................

     ...,
     atom(Color),
     \+ color(Color),
     domain_error(color, Color).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, existence_error/2,
permission_error/3, representation_error/1,
evaluation_error/1, resource_error/1,
syntax_error/1, system_error/0.

'built-in method'

2.5.6.31 ‘existence_error/2’
............................

2.5.6.32 Description
....................

     existence_error(Thing, Culprit)

Throws an existence error.  Used when the subject of an operation does
not exist.  This built-in method is declared private and thus cannot be
used as a message to an object.  Calling this predicate is equivalent to
the following sequence of goals:

     ...,
     context(Context),
     throw(error(existence_error(Thing,Culprit), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Thing’ include:

   - ‘predicate_declaration’

   - ‘procedure’

   - ‘source_sink’

   - ‘stream’

   - ‘object’

   - ‘protocol’

   - ‘category’

   - ‘module’

   - ‘library’

   - ‘file’

   - ‘goal_thread’

The value of ‘Culprit’ is the argument or one of its sub-terms that
caused the error.

2.5.6.33 Modes and number of proofs
...................................

     existence_error(@nonvar, @nonvar) - error

2.5.6.34 Errors
...............

     When called:
          ‘existence_error(Thing, Culprit)’

2.5.6.35 Examples
.................

     ...,
     \+ current_object(payroll),
     existence_error(object, payroll).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, evaluation_error/1,
see permission_error/3, representation_error/1,
resource_error/1, syntax_error/1,
system_error/0.

'built-in method'

2.5.6.36 ‘permission_error/3’
.............................

2.5.6.37 Description
....................

     permission_error(Operation, Permission, Culprit)

Throws an permission error.  Used when an operation is not allowed.  For
example, sending a message for a predicate that is not within the scope
of the sender.  This built-in method is declared private and thus cannot
be used as a message to an object.  Calling this predicate is equivalent
to the following sequence of goals:

     ...,
     context(Context),
     throw(error(permission_error(Operation,Permission,Culprit), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Operation’ include:

   - ‘access’

   - ‘create’

   - ‘modify’

   - ‘open’

   - ‘input’

   - ‘output’

   - ‘reposition’

   - ‘repeat’

Possible values for ‘Permission’ include:

   - ‘predicate_declaration’

   - ‘protected_predicate’

   - ‘private_predicate’

   - ‘static_predicate’

   - ‘database’

   - ‘object’

   - ‘static_object’

   - ‘static_protocol’

   - ‘static_category’

   - ‘entity_relation’

   - ‘operator’

   - ‘flag’

   - ‘engine’

   - ‘binary_stream’

   - ‘text_stream’

   - ‘source_sink’

   - ‘stream’

   - ‘past_end_of_stream’

The value of ‘Culprit’ is the argument or one of its sub-terms that
caused the error.

2.5.6.38 Modes and number of proofs
...................................

     permission_error(@nonvar, @nonvar, @nonvar) - error

2.5.6.39 Errors
...............

     When called:
          ‘permission_error(Operation, Permission, Culprit)’

2.5.6.40 Examples
.................

     ...,
     \+ writable(File),
     permission_error(modify, file, File).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see representation_error/1, evaluation_error/1,
resource_error/1, syntax_error/1,
system_error/0.

'built-in method'

2.5.6.41 ‘representation_error/1’
.................................

2.5.6.42 Description
....................

     representation_error(Flag)

Throws a representation error.  Used when some representation limit is
exceeded.  For example, trying to construct a compound term that exceeds
the maximum arity supported by the backend Prolog system.  This built-in
method is declared private and thus cannot be used as a message to an
object.  Calling this predicate is equivalent to the following sequence
of goals:

     ...,
     context(Context),
     throw(error(representation_error(Flag), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Flag’ include:

   - ‘character’

   - ‘character_code’

   - ‘in_character_code’

   - ‘max_arity’

   - ‘max_integer’

   - ‘min_integer’

   - ‘lambda_parameters’

   - ‘entity_prefix’

2.5.6.43 Modes and number of proofs
...................................

     representation_error(+atom) - error

2.5.6.44 Errors
...............

     When called:
          ‘representation_error(Flag)’

2.5.6.45 Examples
.................

     ...,
     Code > 127,
     representation_error(character_code).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see permission_error/3, evaluation_error/1,
resource_error/1, syntax_error/1, system_error/0.

'built-in method'

2.5.6.46 ‘evaluation_error/1’
.............................

2.5.6.47 Description
....................

     evaluation_error(Error)

Throws an evaluation error.  Used when evaluating an arithmetic
expression generates an exception.  This built-in method is declared
private and thus cannot be used as a message to an object.  Calling this
predicate is equivalent to the following sequence of goals:

     ...,
     context(Context),
     throw(error(evaluation_error(Error), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Error’ include:

   - ‘float_overflow’

   - ‘int_overflow’

   - ‘undefined’

   - ‘underflow’

   - ‘zero_divisor’

2.5.6.48 Modes and number of proofs
...................................

     evaluation_error(@nonvar) - error

2.5.6.49 Errors
...............

     When called:
          ‘evaluation_error(Exception)’

2.5.6.50 Examples
.................

     ...,
     Divisor =:= 0,
     evaluation_error(zero_divisor).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see permission_error/3, representation_error/1,
resource_error/1, syntax_error/1,
system_error/0.

'built-in method'

2.5.6.51 ‘resource_error/1’
...........................

2.5.6.52 Description
....................

     resource_error(Resource)

Throws a resource error.  Used when a required resource (e.g.  memory or
disk space) to complete execution is not available.  This built-in
method is declared private and thus cannot be used as a message to an
object.  Calling this predicate is equivalent to the following sequence
of goals:

     ...,
     context(Context),
     throw(error(resource_error(Resource), Context)).

This allows the user to generate errors in the same format used by the
runtime.

Possible values for ‘Resource’ include:

   - ‘threads’

   - ‘coinduction’

   - ‘soft_cut_support’

2.5.6.53 Modes and number of proofs
...................................

     resource_error(@nonvar) - error

2.5.6.54 Errors
...............

     When called:
          ‘resource_error(Resource)’

2.5.6.55 Examples
.................

     ...,
     empty(Tank),
     resource_error(gas).

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see permission_error/3, representation_error/1,
syntax_error/1, system_error/0.

'built-in method'

2.5.6.56 ‘syntax_error/1’
.........................

2.5.6.57 Description
....................

     syntax_error(Description)

Throws a syntax error.  Used when the sequence of characters being read
are not syntactically valid.  This built-in method is declared private
and thus cannot be used as a message to an object.  Calling this
predicate is equivalent to the following sequence of goals:

     ...,
     context(Context),
     throw(error(syntax_error(Description), Context)).

This allows the user to generate errors in the same format used by the
runtime.

2.5.6.58 Modes and number of proofs
...................................

     syntax_error(@nonvar) - error

2.5.6.59 Errors
...............

     When called:
          ‘syntax_error(Description)’

2.5.6.60 Examples
.................

(none)

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see permission_error/3, representation_error/1,
system_error/0. See resource_error/1.

'built-in method'

2.5.6.61 ‘system_error/0’
.........................

2.5.6.62 Description
....................

     system_error

Throws a system error.  Used when runtime execution can no longer
proceed.  For example, an exception is thrown without an active catcher.
This built-in method is declared private and thus cannot be used as a
message to an object.  Calling this predicate is equivalent to the
following sequence of goals:

     ...,
     context(Context),
     throw(error(system_error, Context)).

This allows the user to generate errors in the same format used by the
runtime.

2.5.6.63 Modes and number of proofs
...................................

     system_error - error

2.5.6.64 Errors
...............

     When called:
          ‘system_error’

2.5.6.65 Examples
.................

(none)

See also
........

See catch/3, throw/1, context/1,
instantiation_error/0, uninstantiation_error/1,
type_error/2, domain_error/2, existence_error/2,
see permission_error/3, representation_error/1.
See evaluation_error/1, resource_error/1,
syntax_error/1,

2.5.7 All solutions
-------------------

'built-in method'

2.5.7.1 ‘bagof/3’
.................

2.5.7.2 Description
...................

     bagof(Template, Goal, List)

Collects a bag of solutions for the goal for each set of instantiations
of the free variables in the goal.  The order of the elements in the bag
follows the order of the goal solutions.  The free variables in the goal
are the variables that occur in the goal but not in the template.  Free
variables can be ignored, however, by using the ‘^/2’ existential
qualifier.  For example, if ‘T’ is term containing all the free
variables that we want to ignore, we can write ‘T^Goal’.  Note that the
term ‘T’ can be written as ‘V1^V2^...’.

When there are free variables, this method is re-executable on
backtracking.  This method fails when there are no solutions, never
returning an empty list.

This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

2.5.7.3 Meta-predicate template
...............................

     bagof(*, ^, *)

2.5.7.4 Modes and number of proofs
..................................

     bagof(@term, +callable, -list) - zero_or_more

2.5.7.5 Errors
..............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Goal’ is a call to a non-existing predicate:
          ‘existence_error(procedure, Predicate)’

2.5.7.6 Examples
................

     To find a bag of solutions in the context of the object or category containing the call:
          ‘bagof(Template, Goal, List)’
     To find a bag of solutions of sending a message to self.:
          ‘bagof(Template, ::Message, List)’
     To find a bag of solutions of sending a message to an explicit object:
          ‘bagof(Template, Object::Message, List)’

See also
........

See findall/3, findall/4, forall/2,
setof/3.

'built-in method'

2.5.7.7 ‘findall/3’
...................

2.5.7.8 Description
...................

     findall(Template, Goal, List)

Collects a list of solutions for the goal.  The order of the elements in
the list follows the order of the goal solutions.  It succeeds returning
an empty list when the goal has no solutions.

This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

2.5.7.9 Meta-predicate template
...............................

     findall(*, 0, *)

2.5.7.10 Modes and number of proofs
...................................

     findall(?term, +callable, ?list) - zero_or_one

2.5.7.11 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Goal’ is a call to a non-existing predicate:
          ‘existence_error(procedure, Predicate)’

2.5.7.12 Examples
.................

     To find all solutions in the context of the object or category containing the call:
          ‘findall(Template, Goal, List)’
     To find all solutions of sending a message to self.:
          ‘findall(Template, ::Message, List)’
     To find all solutions of sending a message to an explicit object:
          ‘findall(Template, Object::Message, List)’

See also
........

See bagof/3, findall/4, forall/2,
setof/3.

'built-in method'

2.5.7.13 ‘findall/4’
....................

2.5.7.14 Description
....................

     findall(Template, Goal, List, Tail)

Variant of the findall/3 method that allows passing the tail
of the results list.  It succeeds returning the tail argument when the
goal has no solutions.

This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

2.5.7.15 Meta-predicate template
................................

     findall(*, 0, *, *)

2.5.7.16 Modes and number of proofs
...................................

     findall(?term, +callable, ?list, ?term) - zero_or_one

2.5.7.17 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Goal’ is a call to a non-existing predicate:
          ‘existence_error(procedure, Predicate)’

2.5.7.18 Examples
.................

     To find all solutions in the context of the object or category containing the call:
          ‘findall(Template, Goal, List, Tail)’
     To find all solutions of sending a message to self.:
          ‘findall(Template, ::Message, List, Tail)’
     To find all solutions of sending a message to an explicit object:
          ‘findall(Template, Object::Message, List, Tail)’

See also
........

See bagof/3, findall/3, forall/2,
setof/3.

'built-in method'

2.5.7.19 ‘forall/2’
...................

2.5.7.20 Description
....................

     forall(Generator, Test)

For all solutions of ‘Generator’, ‘Test’ is true.  This meta-predicate
implements a 'generate-and-test' loop using a definition equivalent to
‘\+ (Generator, \+ Test)’.  As a consequence, no variables in the
arguments are bound by a call to this predicate.  This predicate often
provides a better alternative to a 'failure-driven loop' as an
unexpected ‘Test’ failure will not be ignored as it will make the
‘forall/2’ call fail.

This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

2.5.7.21 Meta-predicate template
................................

     forall(0, 0)

2.5.7.22 Modes and number of proofs
...................................

     forall(@callable, @callable) - zero_or_one

2.5.7.23 Errors
...............

     Either ‘Generator’ or ‘Test’ is a variable:
          ‘instantiation_error’
     ‘Generator’ is neither a variable nor a callable term:
          ‘type_error(callable, Generator)’
     ‘Test’ is neither a variable nor a callable term:
          ‘type_error(callable, Test)’

2.5.7.24 Examples
.................

     To call both goals in the context of the object or category containing the call:
          ‘forall(Generator, Test)’
     To send both goals as messages to self.:
          ‘forall(::Generator, ::Test)’
     To send both goals as messages to explicit objects:
          ‘forall(Object1::Generator, Object2::Test)’

See also
........

See bagof/3, findall/3, findall/4,
setof/3.

'built-in method'

2.5.7.25 ‘setof/3’
..................

2.5.7.26 Description
....................

     setof(Template, Goal, List)

Collects a set of solutions for the goal for each set of instantiations
of the free variables in the goal.  The solutions are sorted using
standard term order.  The free variables in the goal are the variables
that occur in the goal but not in the template.  Free variables can be
ignored, however, by using the ‘^/2’ existential qualifier.  For
example, if ‘T’ is term containing all the free variables that we want
to ignore, we can write ‘T^Goal’.  Note that the term ‘T’ can be written
as ‘V1^V2^...’.

When there are free variables, this method is re-executable on
backtracking.  This method fails when there are no solutions, never
returning an empty list.

This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

2.5.7.27 Meta-predicate template
................................

     setof(*, ^, *)

2.5.7.28 Modes and number of proofs
...................................

     setof(@term, +callable, -list) - zero_or_more

2.5.7.29 Errors
...............

     ‘Goal’ is a variable:
          ‘instantiation_error’
     ‘Goal’ is neither a variable nor a callable term:
          ‘type_error(callable, Goal)’
     ‘Goal’ is a call to a non-existing predicate:
          ‘existence_error(procedure, Predicate)’

2.5.7.30 Examples
.................

     To find a set of solutions in the context of the object or category containing the call:
          ‘setof(Template, Goal, List)’
     To find a set of solutions of sending a message to self.:
          ‘setof(Template, ::Message, List)’
     To find a set of solutions of sending a message to an explicit object:
          ‘setof(Template, Object::Message, List)’

See also
........

See bagof/3, findall/3, findall/4,
forall/2.

2.5.8 Event handling
--------------------

'built-in method'

2.5.8.1 ‘before/3’
..................

2.5.8.2 Description
...................

     before(Object, Message, Sender)

User-defined method for handling before events.  This method
is declared in the monitoring(1) built-in protocol as a public predicate
and automatically called by the runtime for messages sent using the
(;;)/2 control construct from within objects compiled with
the events flag set to ‘allow’.

Note that you can make this predicate scope protected or private by
using, respectively, see protected or private implementation of
the ‘monitoring’ protocol.

   ---------- Footnotes ----------

   (1) ../../../docs/monitoring_0.html#monitoring-0

2.5.8.3 Modes and number of proofs
..................................

     before(?object_identifier, ?callable, ?object_identifier) - zero_or_more

2.5.8.4 Errors
..............

(none)

2.5.8.5 Examples
................

     :- object(...,
         implements(monitoring),
         ...).

         % write a log message when a message is sent:
         before(Object, Message, Sender) :-
             writeq(Object), write('::'), writeq(Message),
             write(' from '), writeq(Sender), nl.

See also
........

See after/3, abolish_events/5, current_event/5,
see define_events/5.

'built-in method'

2.5.8.6 ‘after/3’
.................

2.5.8.7 Description
...................

     after(Object, Message, Sender)

User-defined method for handling after events.  This method
is declared in the monitoring(1) built-in protocol as a public predicate
and automatically called by the runtime for messages sent using the
(;;)/2 control construct from within objects compiled with
the events flag set to ‘allow’.

Note that you can make this predicate scope protected or private by
using, respectively, see protected or private implementation of
the ‘monitoring’ protocol.

   ---------- Footnotes ----------

   (1) ../../../docs/monitoring_0.html#monitoring-0

2.5.8.8 Modes and number of proofs
..................................

     after(?object_identifier, ?callable, ?object_identifier) - zero_or_more

2.5.8.9 Errors
..............

(none)

2.5.8.10 Examples
.................

     :- object(...,
         implements(monitoring),
         ...).

         % write a log message when a message is successful:
         after(Object, Message, Sender) :-
             writeq(Object), write('::'), writeq(Message),
             write(' from '), writeq(Sender), nl.

See also
........

See before/3, abolish_events/5, current_event/5,
see define_events/5.

2.5.9 Message forwarding
------------------------

'built-in method'

2.5.9.1 ‘forward/1’
...................

2.5.9.2 Description
...................

     forward(Message)

User-defined method for forwarding unknown messages sent to an object
(using the (;;)/2 control construct), automatically called by
the runtime when defined.  This method is declared in the forwarding(1)
built-in protocol as a public predicate.  Note that you can
make its scope protected or private by using, respectively, see
protected or private implementation of the ‘forwarding’ protocol.

   ---------- Footnotes ----------

   (1) ../../../docs/forwarding_0.html#forwarding-0

2.5.9.3 Modes and number of proofs
..................................

     forward(+callable) - zero_or_more

2.5.9.4 Errors
..............

(none)

2.5.9.5 Examples
................

     :- object(proxy,
         implements(forwarding),
         ...).

         forward(Message) :-
             % delegate unknown messages to the "real" object
             [real::Message].

See also
........

See []/1.

2.5.10 Definite clause grammar rules
------------------------------------

'built-in method'

2.5.10.1 ‘call//1-N’
....................

2.5.10.2 Description
....................

     call(Closure)
     call(Closure, Arg1, ...)
     call(Object::Closure, Arg1, ...)
     call(::Closure, Arg1, ...)
     call(^^Closure, Arg1, ...)
     ...

This non-terminal takes a closure and is processed by
appending the two implicit grammar rule arguments to the arguments of
the closure.  This built-in non-terminal is interpreted as a private
non-terminal and thus cannot be used as a message to an object.

Using this non-terminal is recommended when calling a predicate whose
last two arguments are the two implicit grammar rule arguments to avoid
hard-coding assumptions about how grammar rules are compiled into
clauses.  Note that the compiler ensures zero overhead when using this
non-terminal with a bound argument at compile time.  To call a predicate
with a different argument order, use a lambda expression.
For example:

     square -->
         call([Number, Double]>>(Double is Number*Number)).

When using a backend Prolog compiler supporting a module
system, calls in the format ‘call(Module:Closure)’ may also be used.

2.5.10.3 Meta-non-terminal template
...................................

     call(0)
     call(1, *)
     call(2, *, *)
     ...

2.5.10.4 Modes and number of proofs
...................................

     call(+callable) - zero_or_more
     call(+callable, ?term) - zero_or_more
     call(+callable, ?term, ?term) - zero_or_more
     ...

2.5.10.5 Errors
...............

     ‘Closure’ is a variable:
          ‘instantiation_error’
     ‘Closure’ is neither a variable nor a callable term:
          ‘type_error(callable, Closure)’

2.5.10.6 Examples
.................

     Calls a goal, constructed by appending the two implicit grammar rule arguments to the closure, in in the context of the object or category containing the call:
          ‘call(Closure)’
     To make a 'super' call, constructed by appending the two implicit grammar rule arguments to the closure:
          ‘call(^^Closure)’
     To send a goal, constructed by appending the two implicit grammar rule arguments to the closure, as a message to self.:
          ‘call(::Closure)’
     To send a goal, constructed by appending the two implicit grammar rule arguments to the closure, as a message to an explicit object:
          ‘call(Object::Closure)’

See also
........

See eos//0, phrase//1, phrase/2,
phrase/3.

'built-in method'

2.5.10.7 ‘eos//0’
.................

2.5.10.8 Description
....................

     eos

This non-terminal matches the end-of-input.  It is implemented by
checking that the implicit difference list unifies with ‘[]-[]’.

2.5.10.9 Modes and number of proofs
...................................

     eos - zero_or_one

2.5.10.10 Errors
................

(none)

2.5.10.11 Examples
..................

     abc --> a, b, c, eos.

See also
........

See call//1-N, phrase//1, phrase/2,
phrase/3.

'built-in method'

2.5.10.12 ‘phrase//1’
.....................

2.5.10.13 Description
.....................

     phrase(GrammarRuleBody)

This non-terminal takes a grammar rule body and parses it using the two
implicit grammar rule arguments.  A common use is to wrap what otherwise
would be a naked meta-variable in a grammar rule body when
defining a meta non-terminal.

2.5.10.14 Meta-non-terminal template
....................................

     phrase(0)

2.5.10.15 Modes and number of proofs
....................................

     phrase(+callable) - zero_or_more

2.5.10.16 Errors
................

     ‘GrammarRuleBody’ is a variable:
          ‘instantiation_error’
     ‘GrammarRuleBody’ is neither a variable nor a callable term:
          ‘type_error(callable, GrammarRuleBody)’

2.5.10.17 Examples
..................

     (none)

See also
........

See call//1-N, phrase/2, phrase/3.

'built-in method'

2.5.10.18 ‘phrase/2’
....................

2.5.10.19 Description
.....................

     phrase(GrammarRuleBody, Input)
     phrase(::GrammarRuleBody, Input)
     phrase(Object::GrammarRuleBody, Input)

True when the ‘GrammarRuleBody’ grammar rule body can be applied to the
‘Input’ list of tokens.  In the most common case, ‘GrammarRuleBody’ is a
non-terminal defined by a grammar rule.  This built-in method is
declared private and thus cannot be used as a message to an object.
When using a backend Prolog compiler supporting a module
system, calls in the format ‘phrase(Module:GrammarRuleBody, Input)’ may
also be used.

This method is opaque to cuts in the first argument.  When the first
argument is sufficiently instantiated at compile time, the method call
is compiled in order to eliminate the implicit overheads of converting
the grammar rule body into a goal and meta-calling it.  For performance
reasons, the second argument is only type-checked at compile time.

2.5.10.20 Meta-predicate template
.................................

     phrase(2, *)

2.5.10.21 Modes and number of proofs
....................................

     phrase(+callable, ?list) - zero_or_more

2.5.10.22 Errors
................

     ‘GrammarRuleBody’ is a variable:
          ‘instantiation_error’
     ‘GrammarRuleBody’ is neither a variable nor a callable term:
          ‘type_error(callable, GrammarRuleBody)’

2.5.10.23 Examples
..................

     To parse a list of tokens using a local non-terminal:
          ‘phrase(NonTerminal, Input)’
     To parse a list of tokens using a non-terminal within the scope of self.:
          ‘phrase(::NonTerminal, Input)’
     To parse a list of tokens using a public non-terminal of an explicit object:
          ‘phrase(Object::NonTerminal, Input)’

See also
........

See call//1-N, phrase//1, phrase/3.

'built-in method'

2.5.10.24 ‘phrase/3’
....................

2.5.10.25 Description
.....................

     phrase(GrammarRuleBody, Input, Rest)
     phrase(::GrammarRuleBody, Input, Rest)
     phrase(Object::GrammarRuleBody, Input, Rest)

True when the ‘GrammarRuleBody’ grammar rule body can be applied to the
‘Input-Rest’ difference list of tokens.  In the most common case,
‘GrammarRuleBody’ is a non-terminal defined by a grammar rule.  This
built-in method is declared private and thus cannot be used as a message
to an object.  When using a backend Prolog compiler.
supporting a module system, calls in the format
‘phrase(Module:GrammarRuleBody, Input, Rest)’ may also be used.

This method is opaque to cuts in the first argument.  When the first
argument is sufficiently instantiated at compile time, the method call
is compiled in order to eliminate the implicit overheads of converting
the grammar rule body into a goal and meta-calling it.  For performance
reasons, the second and third arguments are only type-checked at compile
time.

2.5.10.26 Meta-predicate template
.................................

     phrase(2, *, *)

2.5.10.27 Modes and number of proofs
....................................

     phrase(+callable, ?list, ?list) - zero_or_more

2.5.10.28 Errors
................

     ‘GrammarRuleBody’ is a variable:
          ‘instantiation_error’
     ‘GrammarRuleBody’ is neither a variable nor a callable term:
          ‘type_error(callable, GrammarRuleBody)’

2.5.10.29 Examples
..................

     To parse a list of tokens using a local non-terminal:
          ‘phrase(NonTerminal, Input, Rest)’
     To parse a list of tokens using a non-terminal within the scope of self.:
          ‘phrase(::NonTerminal, Input, Rest)’
     To parse a list of tokens using a public non-terminal of an explicit object:
          ‘phrase(Object::NonTerminal, Input, Rest)’

See also
........

See call//1-N, phrase/2, phrase/3.

2.5.11 Term and goal expansion
------------------------------

'built-in method'

2.5.11.1 ‘expand_term/2’
........................

2.5.11.2 Description
....................

     expand_term(Term, Expansion)

Expands a term.  The most common use is to expand a grammar rule into a
clause.  Users may override the default Logtalk grammar rule translator
by defining clauses for the term_expansion/2 hook predicate.

The expansion works as follows: if the first argument is a variable,
then it is unified with the second argument; if the first argument is
not a variable and there are local or inherited clauses for the
‘term_expansion/2’ hook predicate within scope, then this predicate is
called to provide an expansion that is then unified with the second
argument; if the ‘term_expansion/2’ predicate is not used and the first
argument is a compound term with functor ‘-->/2’ then the default
Logtalk grammar rule translator is used, with the resulting clause being
unified with the second argument; when the translator is not used, the
two arguments are unified.  The ‘expand_term/2’ predicate may return a
single term or a list of terms.

This built-in method may be used to expand a grammar rule into a clause
for use with the built-in database methods.

Automatic term expansion is only performed at compile time (to expand
terms read from a source file) when using a hook object.
This predicate can be used by the user to manually perform term
expansion at runtime (for example, to convert a grammar rule into a
clause).

2.5.11.3 Modes and number of proofs
...................................

     expand_term(?term, ?term) - one

2.5.11.4 Errors
...............

(none)

2.5.11.5 Examples
.................

(none)

See also
........

See expand_goal/2, goal_expansion/2,
term_expansion/2.

'built-in method'

2.5.11.6 ‘term_expansion/2’
...........................

2.5.11.7 Description
....................

     term_expansion(Term, Expansion)

Defines an expansion for a term.  This predicate, when defined and
within scope, is automatically called by the expand_term/2.
method.  When that is not the case, the ‘expand_term/2’ method only uses
the default expansions.  Use of this predicate by the ‘expand_term/2’
method may be restricted by changing its default public scope.

The ‘term_expansion/2’ predicate may return a list of terms.  Returning
an empty list effectively suppresses the term.

Term expansion may be also be applied when compiling source files by
defining the object providing access to the ‘term_expansion/2’ clauses
as a hook object.  Clauses for the ‘term_expansion/2’
predicate defined within an object or a category are 'never' used in the
compilation of the object or the category itself.  Moreover, in this
context, terms wrapped using the {}/1 compiler bypass
control construct are not expanded and any expanded term wrapped in this
control construct will not be further expanded.

Objects and categories implementing this predicate should declare that
they implement the expanding(1) protocol if no ancestor already declares
it.  This protocol implementation relation can be declared as either
protected or private to restrict the scope of this predicate.

   ---------- Footnotes ----------

   (1) ../../../docs/expanding_0.html#expanding-0

2.5.11.8 Modes and number of proofs
...................................

     term_expansion(+nonvar, -nonvar) - zero_or_one
     term_expansion(+nonvar, -list(nonvar)) - zero_or_one

2.5.11.9 Errors
...............

(none)

2.5.11.10 Examples
..................

     term_expansion((:- license(default)), (:- license(gplv3))).
     term_expansion(data(Millimeters), data(Meters)) :- Meters is Millimeters / 1000.

See also
........

See expand_goal/2, expand_term/2,
goal_expansion/2, logtalk_load_context/2.

'built-in method'

2.5.11.11 ‘expand_goal/2’
.........................

2.5.11.12 Description
.....................

     expand_goal(Goal, ExpandedGoal)

Expands a goal.  The expansion works as follows: if the first argument
is a variable, then it is unified with the second argument; if the first
argument is not a variable and there are local or inherited clauses for
the ‘goal_expansion/2’ hook predicate within scope, then this predicate
is recursively called until a fixed-point is reached to provide an
expansion that is then unified with the second argument; if the
‘goal_expansion/2’ predicate is not within scope, the two arguments are
unified.

Automatic goal expansion is only performed at compile time (to expand
the body of clauses and meta-directives read from a source file) when
using hook objects.  This predicate can be used by the user
to manually perform goal expansion at runtime (for example, before
asserting a clause).

2.5.11.13 Modes and number of proofs
....................................

     expand_goal(?term, ?term) - one

2.5.11.14 Errors
................

(none)

2.5.11.15 Examples
..................

(none)

See also
........

See expand_term/2, goal_expansion/2,
term_expansion/2.

'built-in method'

2.5.11.16 ‘goal_expansion/2’
............................

2.5.11.17 Description
.....................

     goal_expansion(Goal, ExpandedGoal)

Defines an expansion for a goal.  The first argument is the goal to be
expanded.  The expanded goal is returned in the second argument.  This
predicate is called recursively on the expanded goal until a fixed point
is reached.  Thus, care must be taken to avoid compilation loops.  This
predicate, when defined and within scope, is automatically called by the
expand_goal/2 method.  Use of this predicate by the
‘expand_goal/2’ method may be restricted by changing its default public
scope.

Goal expansion may be also be applied when compiling source files by
defining the object providing access to the ‘goal_expansion/2’ clauses
as a hook object.  Clauses for the ‘goal_expansion/2’
predicate defined within an object or a category are 'never' used in the
compilation of the object or the category itself.  Moreover, in this
context, goals wrapped using the {}/1 compiler bypass
control construct are not expanded and any expanded goal wrapped in this
control construct will not be further expanded.

Objects and categories implementing this predicate should declare that
they implement the expanding(1) built-in protocol if no ancestor already
declares it.  This protocol implementation relation can be declared as
either protected or private to restrict the scope of this
predicate.

   ---------- Footnotes ----------

   (1) ../../../docs/expanding_0.html#expanding-0

2.5.11.18 Modes and number of proofs
....................................

     goal_expansion(+callable, -callable) - zero_or_one

2.5.11.19 Errors
................

(none)

2.5.11.20 Examples
..................

     goal_expansion(write(Term), (write_term(Term, []), nl)).
     goal_expansion(read(Term), (write('Input: '), {read(Term)})).

See also
........

See expand_goal/2, expand_term/2,
term_expansion/2, logtalk_load_context/2.

2.5.12 Coinduction hooks
------------------------

'built-in method'

2.5.12.1 ‘coinductive_success_hook/1-2’
.......................................

2.5.12.2 Description
....................

     coinductive_success_hook(Head, Hypothesis)
     coinductive_success_hook(Head)

User-defined hook predicates that are automatically called in case of
coinductive success when proving a query for a coinductive predicates.
The hook predicates are called with the head of the coinductive
predicate on coinductive success and, optionally, with the hypothesis
used that to reach coinductive success.

When both hook predicates are defined, the ‘coinductive_success_hook/1’
clauses are only used if no ‘coinductive_success_hook/2’ clause applies.
The compiler ensures zero performance penalties when defining
coinductive predicates without a corresponding definition for the
coinductive success hook predicates.

The compiler assumes that these hook predicates are defined as static
predicates in order to optimize their use.

2.5.12.3 Modes and number of proofs
...................................

     coinductive_success_hook(+callable, +callable) - zero_or_one
     coinductive_success_hook(+callable) - zero_or_one

2.5.12.4 Errors
...............

(none)

2.5.12.5 Examples
.................

     % Are there "occurrences" of arg1 in arg2?
     :- public(member/2).
     :- coinductive(member/2).

     member(X, [X| _]).
     member(X, [_| T]) :-
         member(X, T).

     % Are there infinitely many "occurrences" of arg1 in arg2?
     :- public(comember/2).
     :- coinductive(comember/2).
     comember(X, [_| T]) :-
         comember(X, T).

     coinductive_success_hook(member(_, _)) :-
         fail.
     coinductive_success_hook(comember(X, L)) :-
         member(X, L).

See also
........

See coinductive/1.

2.5.13 Message printing
-----------------------

'built-in method'

2.5.13.1 ‘print_message/3’
..........................

2.5.13.2 Description
....................

     print_message(Kind, Component, Term)

Built-in method for printing a message represented by a term, which is
converted to the message text using the
logtalk;;message_tokens(Term, Component) hook non-terminal.  This
method is declared in the logtalk(1) built-in object as a public
predicate.  The line prefix and the output stream used for each
‘Kind-Component’ pair can be found using the
logtalk;;message_prefix_stream(Kind, Component, Prefix, Stream).
hook predicate.

This predicate starts by converting the message term to a list of tokens
and by calling the logtalk;;message_hook(Message, Kind, Component, Tokens)
hook predicate.  If this predicate succeeds, the
‘print_message/3’ predicate assumes that the message have been
successfully printed.

By default: messages of kind ‘debug’ or ‘debug(_)’ are only printed when
the ‘debug’ flag is turned on; messages of kind ‘banner’, ‘comment’, or
‘comment(_)’ are only printed when the ‘report’ flag is set to ‘on’;
messages of kind ‘warning’ and ‘warning(_)’ are not printed when the
‘report’ flag is set to ‘off’; messages of kind ‘silent’ and ‘silent()’
are not printed (but can be intercepted).

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.13.3 Modes and number of proofs
...................................

     print_message(+nonvar, +nonvar, +nonvar) - one

2.5.13.4 Errors
...............

(none)

2.5.13.5 Examples
.................

     ..., logtalk::print_message(information, core, redefining_entity(object, foo)), ...

See also
........

See message_hook/4, message_prefix_stream/4,
message_tokens//2, print_message_tokens/3,
print_message_token/4, ask_question/5,
question_hook/6, question_prompt_stream/4.

'built-in method'

2.5.13.6 ‘message_tokens//2’
............................

2.5.13.7 Description
....................

     message_tokens(Message, Component)

User-defined non-terminal hook used to rewrite a message term into a
list of tokens and declared in the logtalk(1) built-in object as a
public, multifile, and dynamic non-terminal.  The list of tokens can be
printed by calling the print_message_tokens/3 method.  This
non-terminal hook is automatically called by the print_message/3
method.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.13.8 Modes and number of proofs
...................................

     message_tokens(+nonvar, +nonvar) - zero_or_more

2.5.13.9 Errors
...............

(none)

2.5.13.10 Examples
..................

     :- multifile(logtalk::message_tokens//2).
     :- dynamic(logtalk::message_tokens//2).

     logtalk::message_tokens(redefining_entity(Type, Entity), core) -->
         ['Redefining ~w ~q'-[Type, Entity], nl].

See also
........

See message_hook/4, message_prefix_stream/4,
print_message/3, print_message_tokens/3,
print_message_token/4, ask_question/5,
question_hook/6, question_prompt_stream/4.

'built-in method'

2.5.13.11 ‘message_hook/4’
..........................

2.5.13.12 Description
.....................

     message_hook(Message, Kind, Component, Tokens)

User-defined hook method for intercepting printing of a message,
declared in the logtalk(1) built-in object as a public, multifile, and
dynamic predicate.  This hook method is automatically called by the
print_message/3 method.  When the call succeeds, the
‘print_message/3’ method assumes that the message have been successfully
printed.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.13.13 Modes and number of proofs
....................................

     message_hook(@nonvar, @nonvar, @nonvar, @list(nonvar)) - zero_or_one

2.5.13.14 Errors
................

(none)

2.5.13.15 Examples
..................

     :- multifile(logtalk::message_hook/4).
     :- dynamic(logtalk::message_hook/4).

     % print silent messages instead of discarding them as default
     logtalk::message_hook(_, silent, core, Tokens) :-
         logtalk::message_prefix_stream(silent, core, Prefix, Stream),
         logtalk::print_message_tokens(Stream, Prefix, Tokens).

See also
........

See message_prefix_stream/4, message_tokens//2,
print_message/3, print_message_tokens/3,
print_message_token/4, ask_question/5,
question_hook/6, question_prompt_stream/4.

'built-in method'

2.5.13.16 ‘message_prefix_stream/4’
...................................

2.5.13.17 Description
.....................

     message_prefix_stream(Kind, Component, Prefix, Stream)

User-defined hook method for specifying the default prefix and stream
for printing a message for a given kind and component.  This
method is declared in the logtalk(1) built-in object as a public,
multifile, and dynamic predicate.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.13.18 Modes and number of proofs
....................................

     message_prefix_stream(?nonvar, ?nonvar, ?atom, ?stream_or_alias) - zero_or_more

2.5.13.19 Errors
................

(none)

2.5.13.20 Examples
..................

     :- multifile(logtalk::message_prefix_stream/4).
     :- dynamic(logtalk::message_prefix_stream/4).

     logtalk::message_prefix_stream(information, core, '% ', user_output).

See also
........

See message_hook/4, message_tokens//2,
print_message/3, print_message_tokens/3,
print_message_token/4, ask_question/5,
question_hook/6, question_prompt_stream/4.

'built-in method'

2.5.13.21 ‘print_message_tokens/3’
..................................

2.5.13.22 Description
.....................

     print_message_tokens(Stream, Prefix, Tokens)

Built-in method for printing a list of message tokens, declared in the
logtalk(1) built-in object as a public predicate.  This method is
automatically called by the print_message/3 method (assuming
that the message was not intercepted by a message_hook/4.
definition) and calls the user-defined hook predicate
print_message_token/4 for each token.  When a call to this hook
predicate succeeds, the ‘print_message_tokens/3’ predicate assumes that
the token have been printed.  When the call fails, the
‘print_message_tokens/3’ predicate uses a default printing procedure for
the token.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.13.23 Modes and number of proofs
....................................

     print_message_tokens(@stream_or_alias, +atom, @list(nonvar)) - zero_or_one

2.5.13.24 Errors
................

(none)

2.5.13.25 Examples
..................

     ...,
     logtalk::print_message_tokens(user_error, '% ', ['Redefining ~w ~q'-[object,foo], nl]),
     ...

See also
........

See message_hook/4, message_prefix_stream/4,
message_tokens//2, print_message/3,
print_message_token/4, ask_question/5,
question_hook/6, question_prompt_stream/4.

'built-in method'

2.5.13.26 ‘print_message_token/4’
.................................

2.5.13.27 Description
.....................

     print_message_token(Stream, Prefix, Token, Tokens)

User-defined hook method for printing a message token, declared in the
logtalk(1) built-in object as a public, multifile, and dynamic
predicate.  It allows the user to intercept the printing of a message
token.  This hook method is automatically called by the
print_message_tokens/3 built-in method for each token.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.13.28 Modes and number of proofs
....................................

     print_message_token(@stream_or_alias, @atom, @nonvar, @list(nonvar)) - zero_or_one

2.5.13.29 Errors
................

(none)

2.5.13.30 Examples
..................

     :- multifile(logtalk::print_message_token/4).
     :- dynamic(logtalk::print_message_token/4).

     % ignore all flush tokens
     logtalk::print_message_token(_Stream, _Prefix, flush, _Tokens).

See also
........

See message_hook/4, message_prefix_stream/4,
message_tokens//2, print_message/3,
print_message_tokens/3, ask_question/5,
question_hook/6, question_prompt_stream/4.

2.5.14 Question asking
----------------------

'built-in method'

2.5.14.1 ‘ask_question/5’
.........................

2.5.14.2 Description
....................

     ask_question(Question, Kind, Component, Check, Answer)

Built-in method for asking a question represented by a term, ‘Question’,
which is converted to the question text using the
logtalk;;message_tokens(Question, Component) hook predicate.  This
method is declared in the ‘logtalk’ built-in object as a public
predicate.  The default question prompt and the input stream used for
each ‘Kind-Component’ pair can be found using the
logtalk;;question_prompt_stream(Kind, Component, Prompt, Stream).
hook predicate.  The ‘Check’ argument is a closure that is
converted into a checking goal by extending it with the user supplied
answer.  This predicate implements a read-loop that terminates when the
checking predicate succeeds.

This predicate starts by calling the
logtalk;;question_hook(Question, Kind, Component, Check, Answer).
hook predicate.  If this predicate succeeds, the ‘ask_question/5’
predicate assumes that the question have been successfully asked and
replied.

2.5.14.3 Modes and number of proofs
...................................

     ask_question(+nonvar, +nonvar, +nonvar, +callable, -term) - one

2.5.14.4 Meta-predicate template
................................

     ask_question(*, *, *, 1, *)

2.5.14.5 Errors
...............

(none)

2.5.14.6 Examples
.................

     ...,
     logtalk::ask_question(enter_age, question, my_app, integer, Age),
     ...

See also
........

See question_hook/6, question_prompt_stream/4,
message_hook/4, message_prefix_stream/4,
message_tokens//2, print_message/3,
print_message_tokens/3, print_message_token/4.

'built-in method'

2.5.14.7 ‘question_hook/6’
..........................

2.5.14.8 Description
....................

     question_hook(Question, Kind, Component, Tokens, Check, Answer)

User-defined hook method for intercepting asking a question, declared in
the logtalk(1) built-in object as a public, multifile, and dynamic
predicate.  This hook method is automatically called by the
ask_question/5 method.  When the call succeeds, the
‘ask_question/5’ method assumes that the question have been successfully
asked and replied.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.14.9 Modes and number of proofs
...................................

     question_hook(+nonvar, +nonvar, +nonvar, +list(nonvar), +callable, -term) - zero_or_one

2.5.14.10 Meta-predicate template
.................................

     question_hook(*, *, *, *, 1, *)

2.5.14.11 Errors
................

(none)

2.5.14.12 Examples
..................

     :- multifile(logtalk::question_hook/6).
     :- dynamic(logtalk::question_hook/6).

     % use a pre-defined answer instead of asking the user
     logtalk::question_hook(upper_limit, question, my_app, _, _, 3.7).

See also
........

See ask_question/5, question_prompt_stream/4. See
message_hook/4, message_prefix_stream/4,
message_tokens//2, print_message/3,
print_message_tokens/3, print_message_token/4,

'built-in method'

2.5.14.13 ‘question_prompt_stream/4’
....................................

2.5.14.14 Description
.....................

     question_prompt_stream(Kind, Component, Prompt, Stream)

User-defined hook method for specifying the default prompt and input
stream for asking a question for a given kind and component.
This method is declared in the logtalk(1) built-in object as a public,
multifile, and dynamic predicate.

   ---------- Footnotes ----------

   (1) ../../../docs/logtalk_0.html#logtalk-0

2.5.14.15 Modes and number of proofs
....................................

     question_prompt_stream(?nonvar, ?nonvar, ?atom, ?stream_or_alias) - zero_or_more

2.5.14.16 Errors
................

(none)

2.5.14.17 Examples
..................

     :- multifile(logtalk::question_prompt_stream/4).
     :- dynamic(logtalk::question_prompt_stream/4).

     logtalk::question_prompt_stream(question, debugger, '    > ', user_input).

See also
........

See ask_question/5, question_hook/6,
message_hook/4, message_prefix_stream/4,
message_tokens//2, print_message/3,
print_message_tokens/3, print_message_token/4.

3 Tutorial
**********

3.1 List predicates
===================

In this example, we will illustrate the use of:

   - objects

   - protocols

by using common list utility predicates.

3.1.1 Defining a list object
----------------------------

We will start by defining an object, ‘list’, containing predicate
definitions for some common list predicates like ‘append/3’, ‘length/2’,
and ‘member/2’:

     :- object(list).

         :- public([
             append/3, length/2, member/2
         ]).

         append([], List, List).
         append([Head| Tail], List, [Head| Tail2]) :-
             append(Tail, List, Tail2).

         length(List, Length) :-
             length(List, 0, Length).

         length([], Length, Length).
         length([_| Tail], Acc, Length) :-
             Acc2 is Acc + 1,
             length(Tail, Acc2, Length).

         member(Element, [Element| _]).
         member(Element, [_| List]) :-
             member(Element, List).

     :- end_object.

What is different here from a regular Prolog program?  The definitions
of the list predicates are the usual ones.  We have two new directives,
see object/1-5 and end_object/0, that encapsulate the
object’s code.  In Logtalk, by default, all object predicates are
private; therefore, we have to explicitly declare all predicates that we
want to be public, that is, that we want to call from outside the
object.  This is done using the public/1 scope directive.

After we copy the object code to a text file and saved it under the name
‘list.lgt’, we need to change the Prolog working directory to the one
used to save our file (consult your Prolog compiler reference manual).
Then, after starting Logtalk (see the Installing and running Logtalk
section on the User Manual), we can compile and load the
object using the logtalk_load/1. Logtalk built-in predicate:

     | ?- logtalk_load(list).

     object list loaded
     yes

We can now try goals like:

     | ?- list::member(X, [1, 2, 3]).

     X = 1;
     X = 2;
     X = 3;
     no

or:

     | ?- list::length([1, 2, 3], L).

     L = 3
     yes

The infix operator (;;)/2 is used in Logtalk to send a
message to an object.  The message must match a public object predicate.
If we try to call a non-public predicate such as the ‘length/3’
auxiliary predicate an exception will be generated:

     | ?- list::length([1, 2, 3], 0, L).

     uncaught exception:
         error(
             existence_error(predicate_declaration, length/3),
             logtalk(list::length([1,2,3],0,_), ...)
         )

The exception term describes the type of error and the context where the
error occurred.

3.1.2 Defining a list protocol
------------------------------

As we saw in the above example, a Logtalk object may contain predicate
directives and predicate definitions (clauses).  The set of predicate
directives defines what we call the object’s 'protocol' or interface.
An interface may have several implementations.  For instance, we may
want to define a new object that implements the list predicates using
difference lists.  However, we do not want to repeat the predicate
directives in the new object.  Therefore, what we need is to split the
object’s protocol from the object’s implementation by defining a new
Logtalk entity known as a protocol.  Logtalk protocols are compilations
units, at the same level as objects and categories.  That said, let us
define a ‘listp’ protocol:

     :- protocol(listp).

         :- public([
             append/3, length/2, member/2
         ]).

     :- end_protocol.

Similar to what we have done for objects, we use the protocol/1-2
and end_protocol/0 directives to encapsulate the
predicate directives.  We can improve this protocol by documenting the
call/return modes and the number of proofs of each predicate using the
mode/2 directive:

     :- protocol(listp).

         :- public(append/3).
         :- mode(append(?list, ?list, ?list), zero_or_more).

         :- public(length/2).
         :- mode(length(?list, ?integer), zero_or_more).

         :- public(member/2).
         :- mode(member(?term, ?list), zero_or_more).

     :- end_protocol.

We now need to change our definition of the ‘list’ object by removing
the predicate directives and by declaring that the object implements the
‘listp’ protocol:

     :- object(list,
         implements(listp)).

         append([], List, List).
         append([Head| Tail], List, [Head| Tail2]) :-
             append(Tail, List, Tail2).
         ...

     :- end_object.

The protocol declared in ‘listp’ may now be alternatively implemented
using difference lists by defining a new object, ‘difflist’:

     :- object(difflist,
         implements(listp).

         append(L1-X, X-L2, L1-L2).
         ...

     :- end_object.

3.1.3 Summary
-------------

   * It is easy to define a simple object: just put your Prolog code
     inside starting and ending object directives and add the necessary
     scope directives.  The object will be self-defining and ready to
     use.

   * Define a protocol when you may want to provide or enable several
     alternative definitions to a given set of predicates.  This way we
     avoid needless repetition of predicate directives.

3.2 Dynamic object attributes
=============================

In this example, we will illustrate the use of:

   - categories

   - category predicates

   - dynamic predicates

by defining a category that implements a set of predicates for handling
dynamic object attributes.

3.2.1 Defining a category
-------------------------

We want to define a set of predicates to handle dynamic object
attributes.  We need public predicates to set, get, and delete
attributes, and a private dynamic predicate to store the attributes
values.  Let us name these predicates ‘set_attribute/2’ and
‘get_attribute/2’, for getting and setting an attribute value,
‘del_attribute/2’ and ‘del_attributes/2’, for deleting attributes, and
‘attribute_/2’, for storing the attributes values.

But we do not want to encapsulate these predicates in an object.  Why?
Because they are a set of useful, closely related, predicates that may
be used by several, unrelated, objects.  If defined at an object level,
we would be constrained to use inheritance in order to have the
predicates available to other objects.  Furthermore, this could force us
to use multi-inheritance or to have some kind of generic root object
containing all kinds of possible useful predicates.

For this kind of situation, Logtalk enables the programmer to
encapsulate the predicates in a 'category', so that they can be used in
any object.  A category is a Logtalk entity, at the same level as
objects and protocols.  It can contain predicates directives and/or
definitions.  Category predicates can be imported by any object, without
code duplication and without resorting to inheritance.

When defining category predicates, we need to remember that a category
can be imported by more than one object.  Thus, the calls to the
built-in methods that handle the private dynamic predicate (such as
assertz/1 or retract/1) must be made either in
the context of 'self', using the 'message to self' control structure,
see (;;)/1, or in the context of 'this' (i.e.  in the context of
the object importing the category).  This way, we ensure that when we
call one of the attribute predicates on an object, the intended object
own definition of ‘attribute_/2’ will be used.  The predicates
definitions are straightforward.  For example, if opting for storing the
attributes in 'self':

     :- category(attributes).

         :- public(set_attribute/2).
         :- mode(set_attribute(+nonvar, +nonvar), one).

         :- public(get_attribute/2).
         :- mode(get_attribute(?nonvar, ?nonvar), zero_or_more).

         :- public(del_attribute/2).
         :- mode(del_attribute(?nonvar, ?nonvar), zero_or_more).

         :- public(del_attributes/2).
         :- mode(del_attributes(@term, @term), one).

         :- private(attribute_/2).
         :- mode(attribute_(?nonvar, ?nonvar), zero_or_more).
         :- dynamic(attribute_/2).

         set_attribute(Attribute, Value):-
             ::retractall(attribute_(Attribute, _)),
             ::assertz(attribute_(Attribute, Value)).

         get_attribute(Attribute, Value):-
             ::attribute_(Attribute, Value).

         del_attribute(Attribute, Value):-
             ::retract(attribute_(Attribute, Value)).

         del_attributes(Attribute, Value):-
             ::retractall(attribute_(Attribute, Value)).

     :- end_category.

The alternative, opting for storing the attributes on 'this', is
similar: just delete the uses of the ‘(::)/1’ control structure from the
code above.

We have two new directives, see category/1-4 and
end_category/0, that encapsulate the category code.  If needed, we
can put the predicates directives inside a protocol that will be
implemented by the category:

     :- category(attributes,
         implements(attributes_protocol)).

         ...

     :- end_category.

Any protocol can be implemented by either an object, a category, or
both.

3.2.2 Importing the category
----------------------------

We reuse a category’s predicates by importing them into an object:

     :- object(person,
         imports(attributes)).

         ...

     :- end_object.

After compiling and loading this object and our category, we can now try
queries like:

     | ?- person::set_attribute(name, paulo).

     yes

     | ?- person::set_attribute(gender, male).

     yes

     | ?- person::get_attribute(Attribute, Value).

     Attribute = name, Value = paulo ;
     Attribute = gender, Value = male ;
     no

3.2.3 Summary
-------------

   - Categories are similar to objects: we just write our predicate
     directives and definitions bracketed by opening and ending category
     directives.

   - An object reuses a category by importing it.  The imported
     predicates behave as if they have been defined in the object
     itself.

   - When do we use a category instead of an object?  Whenever we have a
     set of closely related predicates that we want to reuse in several,
     unrelated, objects without being constrained by inheritance
     relations.  Thus, categories can be interpreted as object building
     components.

3.3 A reflective class-based system
===================================

When compiling an object, Logtalk distinguishes prototypes from instance
or classes by examining the object relations.  If an object instantiates
and/or specializes another object, then it is compiled as an instance or
class, otherwise it is compiled as a prototype.  A consequence of this
is that, in order to work with instance or classes, we always have to
define root objects for the instantiation and specialization hierarchies
(however, we are not restricted to a single hierarchy).  The best
solution is often to define a reflective class-based system
[Maes87], where every class is also an object and, as such, an
instance of some class.

In this example, we are going to define the basis for a reflective
class-based system, based on an extension of the ideas presented in
[Cointe87].  This extension provides, along with root objects
for the instantiation and specialization hierarchies, explicit support
for abstract classes [Moura94].

3.3.1 Defining the base classes
-------------------------------

We will start by defining three classes: ‘object’, ‘abstract_class’, and
‘class’.  The class ‘object’ will contain all predicates common to all
objects.  It will be the root of the inheritance graph:

     :- object(object,
         instantiates(class)).

         % predicates common to all objects

     :- end_object.

The class ‘abstract_class’ specializes ‘object’ by adding predicates
common to all classes.  It will be the default meta-class for abstract
classes:

     :- object(abstract_class,
         instantiates(class),
         specializes(object)).

         % predicates common to all classes

     :- end_object.

The class ‘class’ specializes ‘abstract_class’ by adding predicates
common to all instantiable classes.  It will be the root of the
instantiation graph and the default meta-class for instantiable classes:

     :- object(class,
         instantiates(class),
         specializes(abstract_class)).

         % predicates common to all instantiable classes

     :- end_object.

Note that all three objects are instances of class ‘class’.  The
instantiation and specialization relationships are chosen so that each
object may use the predicates defined in itself and in the other two
objects, with no danger of message lookup endless loops.

3.3.2 Summary
-------------

   - An object that does not instantiate or specialize other objects is
     always compiled as a prototype.

   - An instance must instantiate at least one object (its class).
     Similarly, a class must at least specialize or instantiate other
     object.

   - The distinction between abstract classes and instantiable classes
     is an operational one, depending on the class inherited methods.  A
     class is instantiable if inherits methods for creating instances.
     Conversely, a class is abstract if does not inherit any instance
     creation method.

3.4 Profiling programs
======================

In this example, we will illustrate the use of:

   - events

   - monitors

by defining a simple profiler that prints the starting and ending time
for processing a message sent to an object.

3.4.1 Messages as events
------------------------

In a pure object-oriented system, all computations start by sending
messages to objects.  We can thus define an 'event' as the sending of a
message to an object.  An event can then be specified by the tuple
‘(Object, Message, Sender)’.  This definition can be refined by
interpreting the sending of a message and the return of the control to
the object that has sent the message as two distinct events.  We call
these events respectively ‘before’ and ‘after’.  Therefore, we end up by
representing an event by the tuple ‘(Event, Object, Message, Sender)’.
For instance, if we send the message:

     | ?- foo::bar(X).

     X = 1
     yes

the two corresponding events will be:

     (before, foo, bar(X), user)
     (after, foo, bar(1), user)

Note that the second event is only generated if the message succeeds.
If the message as a goal have multiple solutions, then one ‘after’ event
will be generated for each solution.

Events are automatically generated by the message sending mechanisms for
each public message sent using the (;;)/2 operator.

3.4.2 Profilers as monitors
---------------------------

A monitor is an object that reacts whenever a spied event occurs.  The
monitor actions are defined by two event handlers: before/3.
for ‘before’ events and after/3 for ‘after’ events.  These
predicates are automatically called by the message sending mechanisms
when an event registered for the monitor occurs.  These event handlers
are declared as public predicates in the ‘monitoring’ built-in protocol.

In our example, we need a way to get the current time before and after
we process a message.  We will assume that we have a ‘time’ object
implementing a ‘cpu_time/1’ predicate that returns the current CPU time
for the Prolog session:

     :- object(time).

         :- public(cpu_time/1).
         :- mode(cpu_time(-number), one).
         ...

     :- end_object.

Our profiler will be named ‘stop_watch’.  It must define event handlers
for the ‘before’ and ‘after’ events that will print the event
description (object, message, and sender) and the current time:

     :- object(stop_watch,
         % event handler predicates protocol
         implements(monitoring)).

         :- uses(time, [cpu_time/1]).

         before(Object, Message, Sender) :-
             write(Object), write(' <-- '), writeq(Message),
             write(' from '), write(Sender), nl, write('STARTING at '),
             cpu_time(Seconds), write(Seconds), write(' seconds'), nl.

         after(Object, Message, Sender) :-
             write(Object), write(' <-- '), writeq(Message),
             write(' from '), write(Sender), nl, write('ENDING at '),
             cpu_time(Seconds), write(Seconds), write(' seconds'), nl.

     :- end_object.

After compiling and loading the ‘stop_watch’ object (and the objects
that we want to profile), we can use the define_events/5.
built-in predicate to set up our profiler.  For example, to profile all
messages that are sent to the object ‘foo’, we need to call the goal:

     | ?- define_events(_, foo, _, _, stop_watch).

     yes

This call will register ‘stop_watch’ as a monitor to all messages sent
to object ‘foo’, for both ‘before’ and ‘after’ events.  Note that we say
“as a monitor”, not “the monitor”: we can have any number of monitors
over the same events.

From now on, every time we sent a message to ‘foo’, the ‘stop_watch’
monitor will print the starting and ending times for the message
execution.  For instance:

     | ?- foo::bar(X).

     foo <-- bar(X) from user
     STARTING at 12.87415 seconds
     foo <-- bar(1) from user
     ENDING at 12.87419 seconds

     X = 1
     yes

To stop profiling the messages sent to ‘foo’ we use the
abolish_events/5 built-in predicate:

     | ?- abolish_events(_, foo, _, _, stop_watch).

     yes

This call will abolish all events defined over the object ‘foo’ assigned
to the ‘stop_watch’ monitor.

3.4.3 Summary
-------------

   - An event is defined as the sending of a (public) message to an
     object.

   - There are two kinds of events: ‘before’ events, generated before a
     message is processed, and ‘after’ events, generated after the
     message processing completed successfully.

   - Any object can be declared as a monitor to any event.  A monitor
     shall reference the ‘monitoring’ built-in protocol in the object
     opening directive.

   - A monitor defines event handlers, the predicates before/3
     and after/3, that are automatically called by the
     runtime engine when a spied event occurs.

   - Three built-in predicates, see define_events/5,
     current_event/5, and abolish_events/5, enables us
     define, query, and abolish both events and monitors.

4 FAQ
*****

4.1 General
===========

   * Why are all versions of Logtalk numbered 2.x or 3.x?.

   * Why do I need a Prolog compiler to use Logtalk?.

   * Is the Logtalk implementation based on Prolog modules?.

   * Does the Logtalk implementation use term-expansion?.

4.1.1 Why are all versions of Logtalk numbered 2.x or 3.x?
----------------------------------------------------------

The numbers “2” and “3” in the Logtalk version string refers to,
respectively, the second and the third generations of the Logtalk
language.  Development of Logtalk 2 started on January 1998, with the
first alpha release for registered users on July and the first public
beta on October.  The first stable version of Logtalk 2 was released on
February 9, 1999.  Development of Logtalk 3 started on April 2012, with
the first public alpha released on August 21, 2012.  The first stable
version of Logtalk 3 was released on January 7, 2015.

4.1.2 Why do I need a Prolog compiler to use Logtalk?
-----------------------------------------------------

Currently, the Logtalk language is implemented as a Prolog extension
instead of as a standalone compiler.  Compilation of Logtalk source
files is performed in two steps.  First, the Logtalk compiler converts a
source file to a Prolog file.  Second, the chosen Prolog compiler is
called by Logtalk to compile the intermediate Prolog file generated on
the first step.  The implementation of Logtalk as a Prolog extension
allows users to use Logtalk together with features only available on
specific Prolog compilers.

4.1.3 Is the Logtalk implementation based on Prolog modules?
------------------------------------------------------------

No.  Logtalk is (currently) implemented is plain Prolog code.  Only a
few Prolog compilers include a module system, with several compatibility
problems between them.  Moreover, the current ISO Prolog standard for
modules is next to worthless and is ignored by most of the Prolog
community.  Nevertheless, the Logtalk compiler is able to compile simple
modules (using a common subset of module directives) as objects for
backward-compatibility with existing code (see the Prolog integration and migration
for details).

4.1.4 Does the Logtalk implementation use term-expansion?
---------------------------------------------------------

No.  Term-expansion mechanisms are not standard and are not available in
all supported Prolog compilers.

4.2 Compatibility
=================

   * What are the backend Prolog compiler requirements to run Logtalk?.

   * Can I use constraint-based packages with Logtalk?.

   * Can I use Logtalk objects and Prolog modules at the same time?.

4.2.1 What are the backend Prolog compiler requirements to run Logtalk?
-----------------------------------------------------------------------

See the backend Prolog compiler requirements guide(1).

   ---------- Footnotes ----------

   (1) https://logtalk.org/backend_requirements.html

4.2.2 Can I use constraint-based packages with Logtalk?
-------------------------------------------------------

Usually, yes.  Some constraint-based packages may define operators which
clash with the ones defined by Logtalk.  In these cases, compatibility
with Logtalk depends on the constraint-based packages providing an
alternative for accessing the functionality provided by those operators.
When the constraint solver is encapsulated using a Prolog module, a
possible workaround is to use either explicit module qualification or
encapsulate the call using the {}/1 control construct (thus
bypassing the Logtalk compiler).

4.2.3 Can I use Logtalk objects and Prolog modules at the same time?
--------------------------------------------------------------------

Yes.  In order to call a module predicate from within an object (or
category) you may use an use_module/2 directive or use
explicit module qualification (possibly wrapping the call using the
Logtalk control construct {}/1 that allows bypassing of the
Logtalk compiler when compiling a predicate call).  Logtalk also allows
modules to be compiled as objects (see the Prolog integration and migration
for details).

4.3 Installation
================

   * The integration scripts/shortcuts are not working!.

   * I get errors when starting up Logtalk after upgrading to the latest version!.

4.3.1 The integration scripts/shortcuts are not working!
--------------------------------------------------------

Check that the ‘LOGTALKHOME’ and ‘LOGTALKUSER’ environment variables are
defined, that the Logtalk user folder is available on the location
pointed by ‘LOGTALKUSER’ (you can create this folder by running the
‘logtalk_user_setup’ shell script), and that the Prolog compilers that
you want to use are supported and available from the system path.  If
the problem persists, run the shell script that creates the integration
script or shortcut manually and check for any error message or
additional instructions.  For some Prolog compilers such as XSB and
Ciao, the first call of the integration script or shortcut must be made
by an administrator user.  If you are using Windows, make sure that any
anti-virus or other security software that you might have installed is
not silently blocking some of the installer tasks.

4.3.2 I get errors when starting up Logtalk after upgrading to the latest version!
----------------------------------------------------------------------------------

Changes in the Logtalk compiler between releases may render Prolog
adapter files from older versions incompatible with new ones.  You may
need to update your local Logtalk user files by running e.g.  the
‘logtalk_user_setup’ shell script.  Check the ‘UPGRADING.md’ file on the
root of the Logtalk installation directory and the release notes for any
incompatible changes to the adapter files.

4.4 Portability
===============

   * Are my Logtalk applications portable across Prolog compilers?.

   * Are my Logtalk applications portable across operating systems?.

4.4.1 Are my Logtalk applications portable across Prolog compilers?
-------------------------------------------------------------------

Yes, as long you don’t use built-in predicates or special features only
available on some Prolog compilers.  There is a portability.
compiler flag that you can set to instruct Logtalk to print a warning
for each occurrence of non-ISO Prolog standard features such as
proprietary built-in predicates.  In addition, it is advisable that you
constrain, if possible, the use of platform or compiler dependent code
to a small number of objects with clearly defined protocols.  You may
also use Logtalk support for conditional compilation to compile
different entity or predicate definitions depending on the backend
Prolog compiler being used.

4.4.2 Are my Logtalk applications portable across operating systems?
--------------------------------------------------------------------

Yes, as long you don’t use built-in predicates or special features that
your chosen backend Prolog compiler only supports in some
operating-systems.  You may need to change the end-of-lines characters
of your source files to match the ones on the target operating system
and the expectations of your Prolog compiler.  Some Prolog compilers
silently fail to compile source files with the wrong end-of-lines
characters.

4.5 Programming
===============

   * Should I use prototypes or classes in my application?.

   * Can I use both classes and prototypes in the same application?.

   * Can I mix classes and prototypes in the same hierarchy?.

   * Can I use a protocol or a category with both prototypes and classes?.

   * What support is provided in Logtalk for defining and using components?.

   * What support is provided in Logtalk for reflective programming?.

4.5.1 Should I use prototypes or classes in my application?
-----------------------------------------------------------

Prototypes and classes provide different patterns of code reuse.  A
prototype encapsulates code that can be used by itself and by its
descendent prototypes.  A class encapsulates code to be used by its
descendent instances.  Prototypes provide the best replacement to the
use of modules as encapsulation units, avoiding the need to instantiate
a class in order to access its code.

4.5.2 Can I use both classes and prototypes in the same application?
--------------------------------------------------------------------

Yes.  In addition, you may freely exchange messages between prototypes,
classes, and instances.

4.5.3 Can I mix classes and prototypes in the same hierarchy?
-------------------------------------------------------------

No.  However, you may define as many prototype hierarchies and class
hierarchies and classes as needed by your application.

4.5.4 Can I use a protocol or a category with both prototypes and classes?
--------------------------------------------------------------------------

Yes.  A protocol may be implemented by both prototypes and classes in
the same application.  Likewise, a category may be imported by both
prototypes and classes in the same application.

4.5.5 What support is provided in Logtalk for defining and using components?
----------------------------------------------------------------------------

Logtalk supports component-based programming (since its inception on
January 1998), by using 'categories' (which are first-class entities
like objects and protocols).  Logtalk categories can be used with both
classes and prototypes and are inspired on the Smalltalk-80
(documentation-only) concept of method categories and on Objective-C
categories, hence the name.  For more information, please consult the
Categories section and the examples provided with the
distribution.

4.5.6 What support is provided in Logtalk for reflective programming?
---------------------------------------------------------------------

Logtalk supports meta-classes, behavioral reflection through the use of
event-driven programming, and structural reflection through the use of a
set of built-in predicates and built-in methods which allow us to query
the system about existing entities, entity relations, and entity
predicates.

4.6 Troubleshooting
===================

   * Using compiler options on calls to the Logtalk compiling and loading predicates do not work!.

   * Gecko-based browsers (e.g. Firefox) show non-rendered HTML entities when browsing XML documenting files!.

   * *note Compiling a source file results in errors or warnings but the
     Logtalk compiler reports a successful compilation with zero errors
     and zero warnings!: 85f.

4.6.1 Using compiler options on calls to the Logtalk compiling and loading predicates do not work!
--------------------------------------------------------------------------------------------------

Using compiler options on calls to the Logtalk logtalk_compile/2
and logtalk_load/2 built-in predicates only apply the
file being compiled.  If the first argument is a loader file,
the compiler options will only be used in the compilation of the loader
file itself, not in the compilation of the files loaded by the loader
file.  The solution is to edit the loader file and add the compiler
options to the calls that compile/load the individual files.

4.6.2 Gecko-based browsers (e.g. Firefox) show non-rendered HTML entities when browsing XML documenting files!
--------------------------------------------------------------------------------------------------------------

Using Gecko-based browsers (e.g.  Firefox) show non-rendered HTML
entities (e.g.  ‘&ndash;’) when browsing XML documenting files after
running the ‘lgt2xml’ shell script in the directory containing the XML
documenting files.  This is a consequence of the lack of support for the
‘disable-output-escaping’ attribute in the browser XSLT processor.  The
workaround is to use other browser (e.g.  Safari or Opera) or to use
instead the ‘lgt2html’ shell script in the directory containing the XML
documenting files to convert them to (X)HTML files for browsing.

4.6.3 Compiling a source file results in errors or warnings but the Logtalk compiler reports a successful compilation with zero errors and zero warnings!
---------------------------------------------------------------------------------------------------------------------------------------------------------

This may happen when your Prolog compiler implementation of the ISO
Prolog standard ‘write_canonical/2’ built-in predicate is buggy and
writes terms that cannot be read back when consulting the intermediate
Prolog files generated by the Logtalk compiler.  Often, syntax errors
found when consulting result in error messages but not in exceptions as
the Prolog compiler tries to continue the compilation despite the
problems found.  As the Logtalk compiler relies on the exception
mechanisms to catch compilation problems, it may report zero errors and
zero warnings despite the error messages.  Send a bug report to the
Prolog compiler developers asking them to fix the ‘write_canonical/2’
buggy implementation.

4.7 Usability
=============

   * Is there a shortcut for compiling and loading source files?.

   * Is there an equivalent directive to the ensure_loaded/1 Prolog directive?.

   * Are there shortcuts for the make functionality?.

4.7.1 Is there a shortcut for compiling and loading source files?
-----------------------------------------------------------------

Yes.  With most backend Prolog compilers, you can use ‘{File}’ as a
shortcut for ‘logtalk_load(File)’.  For compiling and loading multiple
files simply use ‘{File1, File2, ...}’.  See the documentation of the
logtalk_load/1 predicate for details.

4.7.2 Is there an equivalent directive to the ‘ensure_loaded/1’ Prolog directive?
---------------------------------------------------------------------------------

You can use the goal ‘logtalk_load(File, [reload(skip)])’ to ensure that
‘File’ is only loaded once.  See the documentation of the
logtalk_load/2 predicate for details.

4.7.3 Are there shortcuts for the make functionality?
-----------------------------------------------------

Yes.  With most backend Prolog compilers, you can use ‘{*}’ as a
shortcut for ‘logtalk_make(all)’ to reload all files modified since last
compiled and loaded, ‘{!}’ as a shortcut for ‘logtalk_make(clean)’ to
delete all intermediate Prolog files generated by the compilation of
Logtalk source files, ‘{?}’ as a shortcut for ‘logtalk_make(missing)’ to
list missing entities and predicates, and ‘{@}’ as a shortcut for
‘logtalk_make(circular)’ to list circular references.  See the
documentation of the logtalk_make/1 predicate for details.

4.8 Deployment
==============

   * Can I create standalone applications with Logtalk?.

4.8.1 Can I create standalone applications with Logtalk?
--------------------------------------------------------

It depends on the Prolog compiler that you use to run Logtalk.  Assuming
that your Prolog compiler supports the creation of standalone
executables, your application must include the adapter file for your
compiler and the Logtalk compiler and runtime.  The distribution
includes embedding scripts for selected backend Prolog compilers and
embedding examples.

For instructions on how to embed Logtalk and Logtalk applications see
the embedding guide(1).

   ---------- Footnotes ----------

   (1) https://logtalk.org/embedding.html

4.9 Performance
===============

   * Is Logtalk implemented as a meta-interpreter?.

   * What kind of code Logtalk generates when compiling objects? Dynamic code? Static code?.

   * How about message-sending performance? Does Logtalk use static binding or dynamic binding?.

   * How does Logtalk performance compare with plain Prolog and with Prolog modules?.

4.9.1 Is Logtalk implemented as a meta-interpreter?
---------------------------------------------------

No.  Objects and their encapsulated predicates are compiled, not
meta-interpreted.  In particular, inheritance relations are pre-compiled
for improved performance.  Moreover, no meta-interpreter is used even
for objects compiled in debug mode.

4.9.2 What kind of code Logtalk generates when compiling objects? Dynamic code? Static code?
--------------------------------------------------------------------------------------------

Static objects are compiled to static code.  Static objects containing
dynamic predicates are also compiled to static code, except, of course,
for the dynamic predicates themselves.  Dynamic objects are necessarily
compiled to dynamic code.  As in Prolog programming, for best
performance, dynamic object predicates and dynamic objects should only
be used when truly needed.

4.9.3 How about message-sending performance? Does Logtalk use static binding or dynamic binding?
------------------------------------------------------------------------------------------------

Logtalk supports both static binding and dynamic binding.  When static
binding is not possible, Logtalk uses dynamic binding coupled with a
caching mechanism that avoids repeated lookups of predicate declarations
and predicate definitions.  This is a solution common to other
programming languages supporting dynamic binding.  Message lookups are
automatically cached the first time a message is sent.  Cache entries
are automatically removed when loading entities or using Logtalk dynamic
features that invalidate the cached lookups.  Whenever static binding is
used, message sending performance is essentially the same as a predicate
call in plain Prolog.  Performance of dynamic binding when lookups are
cached is close to the performance that would be achieved with static
binding.  See the User Manual section on performance for more
details.

4.9.4 Which Prolog-dependent factors are most crucial for good Logtalk performance?
-----------------------------------------------------------------------------------

Logtalk compiles objects assuming first-argument indexing for static
code.  First-argument indexing of dynamic code, when available, helps to
improve performance due to the automatic caching of method lookups and
the necessary use of book-keeping tables by the runtime engine (this is
specially important when using event-driven programming).  Dynamic
objects and static objects containing dynamic predicates also benefit
from first-argument indexing of dynamic predicates.  The availability of
multi-argument indexing, notably for dynamic predicates, also benefits
dynamic binding performance.

4.9.5 How does Logtalk performance compare with plain Prolog and with Prolog modules?
-------------------------------------------------------------------------------------

Plain Prolog, Prolog modules, and Logtalk objects provide different
trade-offs between performance and features.  In general, for a given
predicate definition, the best performance will be attained using plain
Prolog, second will be Prolog modules (assuming no explicitly qualified
calls are used), and finally Logtalk objects.  Whenever static binding
is used, the performance of Logtalk is equal or close to that of plain
Prolog (depending on the Prolog virtual machine implementation and
compiler optimizations).  See the simple benchmark test results(1) using
some popular Prolog compilers.

   ---------- Footnotes ----------

   (1) https://logtalk.org/performance.html

4.10 Licensing
==============

   * What’s the Logtalk distribution license?.

   * Can Logtalk be used in commercial applications?.

   * What’s the final license for a combination of Logtalk with a Prolog compiler?.

4.10.1 What’s the Logtalk distribution license?
-----------------------------------------------

Logtalk follows the Apache License 2.0(1).

   ---------- Footnotes ----------

   (1) https://github.com/LogtalkDotOrg/logtalk3/blob/master/LICENSE.txt

4.10.2 Can Logtalk be used in commercial applications?
------------------------------------------------------

Yes, the Apache License 2.0 allows commercial use.  See e.g.  the Apache
License and Distribution FAQ(1).

   ---------- Footnotes ----------

   (1) https://www.apache.org/foundation/license-faq.html

4.10.3 What’s the final license for a combination of Logtalk with a Prolog compiler?
------------------------------------------------------------------------------------

See the licensing guide(1) for details and relevant resources.

   ---------- Footnotes ----------

   (1) https://logtalk.org/licensing.html

4.11 Support
============

   * Are there professional consulting, training and supporting services?.

4.11.1 Are there professional consulting, training and supporting services?
---------------------------------------------------------------------------

Yes.  Please visit logtalk.pt(1) for professional consulting,
developing, training, and other supporting services.

   ---------- Footnotes ----------

   (1) https://logtalk.pt

5 Developer Tools
*****************

The documentation of each developer tool can also be found in the tool
directory in the ‘NOTES.md’ file.

5.1 Overview
============

The following developer tools are available, each one with its own
‘loader.lgt’ loader file (except for the built-in ‘linter’ and ‘make’
tools, which are integrated with the compiler/runtime) and ‘NOTES.md’
documentation files:

   - ‘asdf’

   - ‘assertions’

   - ‘code_metrics’

   - ‘dead_code_scanner’

   - ‘debug_messages’

   - ‘debugger’

   - ‘diagrams’

   - ‘doclet’

   - ‘help’

   - ‘issue_creator’

   - ‘lgtdoc’

   - ‘lgtunit’

   - ‘linter’

   - ‘make’

   - ‘packs’

   - ‘ports_profiler’

   - ‘profiler’

   - ‘tutor’

   - ‘wrapper’

5.1.1 Loading the developer tools
---------------------------------

To load the main developer tools, use the following goal:

     | ?- logtalk_load(tools(loader)).

The ‘ports_profiler’ tool is not loaded by default, however, as it
conflicts with the ‘debugger’ tool as both provide a debug handler that
must be unique in a running session.

The ‘profiler’ tool is also not loaded by default as it provides
integration with selected backend Prolog compiler profilers that are not
portable.

The ‘tutor’ tool is also not loaded by default given its useful mainly
for new users that need help understanding compiler warning and error
messages.

The ‘wrapper’ tool is also not loaded by default given its specialized
purpose and beta status.

To load a specific tool, either change your Prolog working directory to
the tool folder and then compile and load the corresponding loader
utility file or simply use library notation as argument for the
compiling and loading predicates.  For example:

     | ?- logtalk_load(lgtunit(loader)).

5.1.2 Tools documentation
-------------------------

Specific notes about each tool can be found in the corresponding
‘NOTES.md’ files.  HTML documentation for each tool API can be found on
the ‘docs’ directory (open the ‘../docs/index.html’ file with your web
browser).  The documentation for these tools can be regenerated using
the shell scripts ‘../scripts/update_html_docs.sh’ and
‘../scripts/update_svg_diagrams.sh’.

5.1.3 Tools common flags
------------------------

The ‘lgtdoc’ and ‘lgtunit’ tools share a ‘suppress_path_prefix’ flag
that can be used to suppress a prefix when printing file paths.  For
example (after loading the tools):

     | ?- set_logtalk_flag(suppress_path_prefix, '/home/jdoe/').

5.1.4 Tools requirements
------------------------

Some of the developer tools have third-party dependencies.  For example,
the ‘lgtdoc’ tool depends on XSLT processors to generate documentation
final formats and uses Sphinx for the preferred HTML final format.  Be
sure to consult the tools documentation details on those requirements
and possible alternatives.  For convenience, follows a global list of
the main tool requirements and suggestions for installing them per
operating-system.  If your operating-system or a dependency for it is
not listed, see the dependency websites for installation instructions.

5.1.4.1 Tool dependencies for full functionality
................................................

   - ‘diagrams’: Graphviz

   - ‘help’: ‘info’

   - ‘issue_creator’: ‘gh’, ‘glab’

   - ‘lgtdoc’: Sphinx, ‘libxslt’, ‘fop’

   - ‘lgtunit’: Allure

   - ‘packs’: ‘coreutils’, ‘libarchive’, ‘gnupg2’, ‘git’, ‘curl’,
     ‘direnv’

5.1.4.2 Python dependencies (all operating-systems)
...................................................

     $ pip install --upgrade pygments
     $ pip install --upgrade sphinx
     $ pip install --upgrade sphinx_rtd_theme

5.1.4.3 macOS - MacPorts
........................

     $ sudo port install graphviz
     $ sudo port install texinfo
     $ sudo port install libxslt fop
     $ sudo port install coreutils libarchive gnupg2 git curl direnv
     $ sudo port install gh glab

5.1.4.4 macOS - Homebrew
........................

     $ brew install graphviz
     $ brew install texinfo
     $ brew install libxslt fop
     $ brew install allure
     $ brew install coreutils libarchive gnupg2 git curl direnv
     $ brew install gh glab

5.1.4.5 Ubuntu
..............

     $ sudo apt install graphviz
     $ sudo apt install info
     $ sudo apt install xsltproc fop
     $ sudo apt-add-repository ppa:qameta/allure && sudo apt install allure
     $ sudo apt install libarchive-tools gnupg2 git curl direnv

5.1.4.6 RedHat
..............

     $ sudo dnf install graphviz
     $ sudo dnf install libxslt fop
     $ sudo dnf install bsdtar gnupg2 git curl direnv

5.1.4.7 Windows - Chocolatey
............................

     > choco install graphviz
     > choco install xsltproc apache-fop
     > choco install gnupg git
     > choco install gh glab

5.1.4.8 Windows - installers
............................

     ‘https://www.graphviz.org/download/’
     ‘https://docs.qameta.io/allure-report/’
     ‘https://www.gnupg.org/’
     ‘https://gitforwindows.org’
     ‘https://cli.github.com’
     ‘https://glab.readthedocs.io’

5.1.4.9 Windows - PowerShell add-ons
....................................

     PS> Install-Module -Name Set-PsEnv

5.2 ‘asdf’
==========

A Logtalk plugin for the asdf(1) extendable version manager is available
at:

‘https://github.com/LogtalkDotOrg/asdf-logtalk’

This plugin provides an alternative to the ‘logtalk_version_select’
script that can be useful to manage Logtalk versions when developing
solutions that use other languages and tools that can also be handled by
‘asdf’.

   ---------- Footnotes ----------

   (1) https://asdf-vm.com

5.3 ‘assertions’
================

The ‘assertions.lgt’ file contains definitions for two meta-predicates,
‘assertion/1-2’, which allows using of assertions on your source code to
print warning and error messages (using the message printing mechanism).
The ‘assertions_messages.lgt’ file defines the default message
translations generated on assertions succeed, fail, or throw an
exception.

5.3.1 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#assertions’

5.3.2 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(assertions(loader)).

5.3.3 Testing
-------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(assertions(tester)).

5.3.4 Adding assertions to your source code
-------------------------------------------

The ‘assertion/1’ predicate takes a goal as argument.  For example:

     foo(L) :-
         assertions::assertion(non_empty_list(L)),
         ...

The ‘assertion/2’ predicate takes as arguments a term for passing
context information and a goal.  Using again a unit test as an example:

     foo(L) :-
         assertions::assertion(foo_list_alerts, non_empty_list(L)),
         ...

When using a large number of assertions, you can use a lighter syntax by
adding a ‘uses/2’ directive.  For example:

     :- uses(assertions, [assertion/1, assertion/2]).

5.3.5 Automatically adding file and line context information to assertions
--------------------------------------------------------------------------

The ‘assertions/1’ parametric object can be used as a hook object to
automatically add file and line context information, represented by the
term ‘file_lines(File, BeginLine-EndLine)’, to calls to the
‘assertion/1’ predicate by goal-expanding it to calls to the
‘assertion/2’ predicate (the expansion assumes that a ‘uses/2’ directive
is being used in the code that will be expanded to direct ‘assertion/1’
calls to the ‘assertions’ object).  For example, assuming the file using
assertions is named ‘source’, it would be compiled and loaded using the
call:

     logtalk_load(source, [hook(assertions(debug))])

5.3.6 Suppressing assertion calls from source code
--------------------------------------------------

The ‘assertions/1’ parametric object can be used as a hook object to
suppress calls to the ‘assertion/1-2’ predicates using goal-expansion
(the expansion assumes ‘assertions::assertion/1-2’ messages).  For
example, assuming the file using assertions is named ‘source’, it would
be compiled and loaded using the call:

     logtalk_load(source, [hook(assertions(production))])

5.3.7 Redirecting assertion failure messages
--------------------------------------------

By default, assertion failures and errors are printed to the standard
output stream.  These messages, however, can be intercepted by defining
the ‘logtalk::message_hook/4’ multifile predicate.  For example:

     :- category(redirect_assertions_messages).

         :- multifile(logtalk::message_hook/4).
         :- dynamic(logtalk::message_hook/4).

         logtalk::message_hook(Message, error, assertions, _) :-
             writeq(my_log_file, Message), write(my_log_file, '.\n').

     :- end_category.

5.3.8 Converting assertion failures into errors
-----------------------------------------------

If you want an assertion failure to result in a failure or a runtime
error, you can intercept the assertion failure messages, optionally
still printing them, and throw an error.  For example:

     :- category(assertions_failures_to_errors).

         :- multifile(logtalk::message_hook/4).
         :- dynamic(logtalk::message_hook/4).

         logtalk::message_hook(Message, error, assertions, Tokens) :-
             % uncomment the next two lines to also print the default message
             % logtalk::message_prefix_stream(error, assertions, Prefix, Stream),
             % logtalk::print_message_tokens(Stream, Prefix, Tokens),
             throw(error(Message, _)).

     :- end_category.

In alternative, if you want assertions to always trigger an exception,
use instead the ‘lgtunit’ tool ‘assertions/1-2’ public predicates.

5.4 ‘code_metrics’
==================

The purpose of this tool is to assess qualities of source code that may
predict negative aspects such as entity coupling, cohesion, complexity,
error-proneness, and overall maintainability.  It is meant to be
extensible via the addition of objects implementing new metrics.

This tool provides predicates for computing metrics for source files,
entities, libraries, files, and directories.  The actual availability of
a particular predicate depends on the specific metric.  One set of
predicates prints, by default, the computed metric values to the
standard output.  A second set of predicates computes and returns a
score (usually a compound term with the computed metric values as
arguments).

5.4.1 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#code-metrics’

5.4.2 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(code_metrics(loader)).

5.4.3 Testing
-------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(code_metrics(tester)).

5.4.4 Available metrics
-----------------------

Currently, the following metrics are provided:

   - Number of Clauses (‘noc_metric’)

   - Number of Rules (‘nor_metric’)

   - Unique Predicate Nodes (‘upn_metric’)

   - Cyclomatic Complexity (‘cc_metric’)

   - Depth of Inheritance (‘dit_metric’)

   - Efferent coupling, afferent coupling, instability, and abstractness
     (‘coupling_metric’)

   - Documentation (‘doc_metric’)

   - Source code size (‘size_metric’)

   - Halstead complexity (‘halstead_metric’ and
     ‘halstead_metric(Stroud)’)

A helper object, ‘code_metrics’, is also provided allowing running all
loaded individual metrics.  For code coverage metrics, see the ‘lgtunit’
tool documentation.

5.4.5 Coupling metrics
----------------------

   - Efferent coupling (‘Ce’): Number of entities that an entity depends
     on.  These include objects receiving messages from the entity plus
     the implemented protocols, imported categories, and
     extended/instantiated/specialized objects.

   - Afferent coupling (‘Ca’): Number of entities that depend on an
     entity.  For a protocol, the number of protocols that extend it
     plus the number of objects and categories that implement it.  For a
     category, the number of objects that import it.  For an object, the
     number of categories and objects that send messages to it plus the
     number of objects that extend/instantiate/specialize it.

   - Instability: Computed as ‘Ce / (Ce + Ca)’.  Measures the entity
     resilience to change.  Ranging from 0.0 to 1.0, with 0.0 indicating
     a maximally stable entity and 1.0 indicating a maximally unstable
     entity.  Ideally, an entity is either maximally stable or maximally
     unstable.

   - Abstractness: Computed as the ratio between the number of static
     predicates with scope directives without a local definition and the
     number of static predicates with scope directives.  Measures the
     rigidity of an entity.  Ranging from 0.0 to 1.0, with 0.0
     indicating a fully concrete entity and 1.0 indicating a fully
     abstract entity.

The dependencies count include direct entity relations plus predicate
calls or dynamic updates to predicates in external objects or
categories.

For more information on the interpretation of the coupling metric
scores, see e.g.  the original paper by Robert Martin:

     @inproceedings{citeulike:1579528,
         author = "Martin, Robert",
         booktitle = "Workshop Pragmatic and Theoretical Directions in Object-Oriented Software Metrics",
         citeulike-article-id = 1579528,
         citeulike-linkout-0 = "http://www.objectmentor.com/resources/articles/oodmetrc.pdf",
         keywords = "diplomarbeit",
         organization = "OOPSLA'94",
         posted-at = "2007-08-21 11:08:44",
         priority = 0,
         title = "OO Design Quality Metrics - An Analysis of Dependencies",
         url = "http://www.objectmentor.com/resources/articles/oodmetrc.pdf",
         year = 1994
     }

The coupling metric was also influenced by the metrics rating system in
Microsoft Visual Studio and aims to eventually emulate the functionality
of a maintainability index score.

5.4.6 Halstead metric
---------------------

Predicates declared, user-defined, and called are interpreted as
'operators'.  Built-in predicates and built-in control constructs are
ignored.  Predicate arguments are abstracted, assumed distinct, and
interpreted as 'operands'.  Note that this definition of operands is a
significant deviation from the original definition, which used syntactic
literals.  A computation closer to the original definition of the metric
would require switching to use the parser to collect information on
syntactic literals, which would imply a much large computation cost.

The computation of this metric is parameterized by the 'Stroud'
coefficient for computing the time required to program (default is 18).
The following individual measures are computed:

   - Number of distinct predicates (declared, defined, called, or
     updated; ‘Pn’).

   - Number of predicate arguments (assumed distinct; ‘PAn’).

   - Number of predicate calls/updates + number of clauses (‘Cn’).

   - Number of predicate call/update arguments + number of clause head
     arguments (‘CAn’).

   - Entity vocabulary (‘EV’).  Computed as ‘EV = Pn + PAn’.

   - Entity length (‘EL’).  Computed as ‘EL = Cn + CAn’.

   - Volume (‘V’).  Computed as ‘V = EL * log2(EV)’.

   - Difficulty (‘D’).  Computed as ‘D = (Pn/2) * (CAn/An)’.

   - Effort (‘E’).  Computed as ‘E = D * V’.

   - Time required to program (‘T’).  Computed as ‘T = E/k’ seconds
     (where ‘k’ is the Stroud number; defaults to 18).

   - Number of delivered bugs (‘B’).  Computed as ‘B = V/3000’.

5.4.7 UPN metric
----------------

The Unique Predicate Nodes (UPN) metric is described in the following
paper:

     @article{MOORES199845,
         title = "Applying Complexity Measures to Rule-Based Prolog Programs",
         journal = "Journal of Systems and Software",
         volume = "44",
         number = "1",
         pages = "45 - 52",
         year = "1998",
         issn = "0164-1212",
         doi = "https://doi.org/10.1016/S0164-1212(98)10042-0",
         url = "http://www.sciencedirect.com/science/article/pii/S0164121298100420",
         author = "Trevor T Moores"
     }

The nodes include called and updated predicates independently of where
they are defined.  It also includes multifile predicates contributed to
other entities.

5.4.8 Cyclomatic complexity metric
----------------------------------

The cyclomatic complexity metric evaluates an entity code complexity by
measuring the number of linearly independent paths through the code.  In
its current implementation, all defined predicates that are not called
or updated are counted as graph connected components (the reasoning
being that these predicates can be considered entry points).  The
implementation uses the same predicate abstraction as the UPN metric.
The defined predicates include multifile predicate definitions
contributed by the entity to other entities.

For more details on this metric, see the original paper by Thomas J.
McCabe:

     @inproceedings{McCabe:1976:CM:800253.807712,
         author = "McCabe, Thomas J.",
         title = "A Complexity Measure",
         booktitle = "Proceedings of the 2Nd International Conference on Software Engineering",
         series = "ICSE '76",
         year = 1976,
         location = "San Francisco, California, USA",
         pages = "407--",
         url = "http://dl.acm.org/citation.cfm?id=800253.807712",
         acmid = 807712,
         publisher = "IEEE Computer Society Press",
         address = "Los Alamitos, CA, USA",
         keywords = "Basis, Complexity measure, Control flow, Decomposition, Graph theory, Independence, Linear, Modularization, Programming, Reduction, Software, Testing",
     }

5.4.9 Usage
-----------

All metrics require the source code to be analyzed to be loaded with the
‘source_data’ flag turned on.  For usage examples, see the ‘SCRIPT.txt’
file in the tool directory.

Be sure to fully understand the metrics individual meanings and any
implementation limitations before using them to support any evaluation
or decision process.

5.4.10 Excluding code from analysis
-----------------------------------

A set of options are available to specify code that should be excluded
when applying code metrics:

   -
          ‘exclude_directories(Directories)’
          list of directories to exclude (default is ‘[]’); all sub-directories of the excluded directories are also excluded; directories may be listed by full or relative path

   -
          ‘exclude_files(Files)’
          list of source files to exclude (default is ‘[]’); files may be listed by full path or basename, with or without extension

   -
          ‘exclude_libraries(Libraries)’
          list of libraries to exclude (default is ‘[startup, scratch_directory]’)

   -
          ‘exclude_entities(Entities)’
          list of entities to exclude (default is ‘[]’)

5.4.11 Defining new metrics
---------------------------

New metrics can be implemented by defining an object that imports the
‘code_metric’ category and implements its score predicates.  There is
also a ‘code_metrics_utilities’ category that defines useful predicates
for the definition of metrics.

5.4.12 Third-party tools
------------------------

‘cloc’ is an open-source command-line program that counts blank lines,
comment lines, and lines of source code in many programming languages
including Logtalk.  Available at ‘https://github.com/AlDanial/cloc’

‘ohcount’ is an open-source command-line program that counts blank
lines, comment lines, and lines of source code in many programming
languages including Logtalk.  Available at
‘https://github.com/blackducksoftware/ohcount’

‘tokei’ is an open-source command-line program that counts blank lines,
comment lines, and lines of source code in many programming languages
including Logtalk.  Available at ‘https://github.com/XAMPPRocky/tokei’

5.4.13 Applying metrics to Prolog modules
-----------------------------------------

Some of the metrics can also be applied to Prolog modules that Logtalk
is able to compile as objects.  For example, if the Prolog module file
is named ‘module.pl’, try:

     | ?- logtalk_load(module, [source_data(on)]).

Due to the lack of standardization of module systems and the abundance
of proprietary extensions, this solution is not expected to work for all
cases.

5.4.14 Applying metrics to plain Prolog code
--------------------------------------------

Some of the metrics can also be applied to plain Prolog code.  For
example, if the Prolog file is named ‘code.pl’, simply define an object
including its code:

     :- object(code).
         :- include('code.pl').
     :- end_object.

Save the object to an e.g.  ‘code.lgt’ file in the same directory as the
Prolog file and then load it in debug mode:

     | ?- logtalk_load(code, [source_data(on)]).

In alternative, use the ‘object_wrapper_hook’ provided by the
‘hook_objects’ library:

     | ?- logtalk_load(hook_objects(loader)).
     ...

     | ?- logtalk_load(code, [hook(object_wrapper_hook), source_data(on)]).

With either wrapping solution, pay special attention to any compilation
warnings that may signal issues that could prevent the plain Prolog code
of working when wrapped by an object.

5.5 ‘dead_code_scanner’
=======================

This tool detects 'likely' dead code in Logtalk entities and in Prolog
modules compiled as objects.  Predicates (and non-terminals) are
classified as dead code when:

   - There is no scope directive for them and they are not called,
     directly or indirectly, by any predicate with a (local or
     inherited) scope directive.

   - They are listed in ‘uses/2’ and ‘use_module/2’ directives but not
     called.

Besides dead code, this tool can also help detect other problems in the
code that often result in reporting false positives.  For example, typos
in ‘alias/2’ directives, missing scope directives, and missing
‘meta_non_terminal/1’ and ‘meta_predicate/1’ directives.

Given the possibility of false positives, care must be taken before
deleting reported dead code to ensure that it’s, in fact, code that is
not used.  A common cause of false positives is the use of conditional
compilation directives to provide implementations for predicates missing
in some systems or different predicate implementations per
operating-system.

The ‘dead_code_scanner.lgt’ source file implements the scanning
predicates for finding dead code in entities, libraries, and
directories.  The source file ‘dead_code_scanner_messages.lgt’ defines
the default translations for the messages printed when scanning for dead
code.  These messages can be intercepted to customize output, e.g.  to
make it less verbose, or for integration with e.g.  GUI IDEs and
continuous integration servers.

5.5.1 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#dead-code-scanner’

For sample queries, please see the ‘SCRIPT.txt’ file in the tool
directory.

5.5.2 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(dead_code_scanner(loader)).

5.5.3 Testing
-------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(dead_code_scanner(tester)).

5.5.4 Usage
-----------

This tool provides a set of predicates that allows scanning entities,
libraries, files, and directories.  See the tool API documentation for
details.  The source code to be analyzed should be loaded with the
‘source_data’ and ‘optimize’ flags turned on (possibly set from a loader
file).

As an example, assume that we want to scan an application with a library
alias ‘my_app’.  The following goals could be used:

     | ?- set_logtalk_flag(source_data, on),
          set_logtalk_flag(optimize, on).
     yes

     | ?- logtalk_load(my_app(loader)).
     ...
     yes

     | ?- dead_code_scanner::library(my_app).
     ...

For complex applications that make use of sub-libraries, there are also
‘rlibrary/1-2’ predicates that performs a recursive scan of a library
and all its sub-libraries.  Conversely, we may be interested in scanning
a single entity:

     | ?- dead_code_scanner::entity(some_object).
     ...

For other usage examples, see the ‘SCRIPT.txt’ file in the tool
directory.

5.5.5 Excluding code from analysis
----------------------------------

A set of options are available to specify code that should be excluded
when looking for unused predicates (and non-terminals):

   -
          ‘exclude_directories(Directories)’
          list of directories to exclude (default is ‘[]’); all sub-directories of the excluded directories are also excluded; directories may be listed by full or relative path

   -
          ‘exclude_files(Files)’
          list of source files to exclude (default is ‘[]’); files may be listed by full path or basename, with or without extension

   -
          ‘exclude_libraries(Libraries)’
          list of libraries to exclude (default is ‘[startup, scratch_directory]’)

   -
          ‘exclude_entities(Entities)’
          list of entities to exclude (default is ‘[]’)

5.5.6 Integration with the ‘make’ tool
--------------------------------------

The ‘loader.lgt’ file sets a make target action that will call the
‘dead_code_scanner::all’ goal whenever the ‘logtalk_make(check)’ goal
(or its top-level abbreviation, ‘{?}’) is called.

5.5.7 Caveats
-------------

Use of local meta-calls with goal arguments only known at runtime can
result in false positives.  When using library or user-defined
meta-predicates, compilation of the source files with the ‘optimize’
flag turned on may allow meta-calls to be resolved at compile time and
thus allow calling information for the meta-arguments to be recorded,
avoiding false positives for predicates that are only meta-called.

5.5.8 Scanning Prolog modules
-----------------------------

This tool can also be applied to Prolog modules that Logtalk is able to
compile as objects.  For example, if the Prolog module file is named
‘module.pl’, try:

     | ?- logtalk_load(module, [source_data(on)]).

Due to the lack of standardization of module systems and the abundance
of proprietary extensions, this solution is not expected to work for all
cases.

5.5.9 Scanning plain Prolog files
---------------------------------

This tool can also be applied to plain Prolog code.  For example, if the
Prolog file is named ‘code.pl’, simply define an object including its
code:

     :- object(code).
         :- include('code.pl').
     :- end_object.

Save the object to an e.g.  ‘code.lgt’ file in the same directory as the
Prolog file and then load it in debug mode:

     | ?- logtalk_load(code, [source_data(on), optimize(on)]).

In alternative, use the ‘object_wrapper_hook’ provided by the
‘hook_objects’ library:

     | ?- logtalk_load(hook_objects(loader)).
     ...

     | ?- logtalk_load(code, [hook(object_wrapper_hook), source_data(on), optimize(on)]).

With either wrapping solution, pay special attention to any compilation
warnings that may signal issues that could prevent the plain Prolog from
being fully analyzed when wrapped by an object.

5.6 ‘debug_messages’
====================

By default, ‘debug’ and ‘debug(Group)’ messages are only printed when
the ‘debug’ flag is turned on.  These messages are also suppressed when
compiling code with the ‘optimize’ flag turned on.  This tool supports
selective enabling of ‘debug’ and ‘debug(Group)’ messages in normal and
debug modes.

5.6.1 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#debug-messages’

For general information on debugging, open in a web browser the
following link and consult the debugging section of the User Manual:

‘../../manuals/userman/debugging.html’

5.6.2 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(debug_messages(loader)).

5.6.3 Testing
-------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(debug_messages(tester)).

5.6.4 Usage
-----------

The tool provides two sets of predicates.  The first set allows enabling
and disabling of all ‘debug’ and ‘debug(Group)’ messages for a given
component.  The second set allows enabling and disabling of
‘debug(Group)’ messages for a given group and component for fine-grained
control.

Upon loading the tool, all debug messages are skipped.  The user is then
expected to use the tool API to selectively enable the messages that
will be printed.  As an example, consider the following object, part of
a ‘xyz’ component:

     :- object(foo).

         :- public([bar/0, baz/0]).
         :- uses(logtalk, [print_message/3]).

         bar :-
             print_message(debug(bar), xyz, @'bar/0 called').

         baz :-
             print_message(debug(baz), xyz, @'baz/0 called').

     :- end_object.

Assuming the object ‘foo’ is compiled and loaded in normal or debug
mode, after also loading this tool, ‘bar/0’ and ‘baz/0’ messages will
not print any debug messages:

     | ?- {debug_messages(loader), foo}.
     ...
     yes

     | ?- foo::(bar, baz).
     yes

We can then enable all debug messages for the ‘xyz’ component:

     | ?- debug_messages::enable(xyx).
     yes

     | ?- foo::(bar, baz).
     bar/0 called
     baz/0 called
     yes

Or we can selectively enable only debug messages for a specific group:

     | ?- debug_messages::disable(xyx).
     yes

     | ?- debug_messages::enable(xyx, bar).
     yes

     | ?- foo::(bar, baz).
     bar/0 called
     yes

5.7 ‘debugger’
==============

This tool provides the default Logtalk command-line debugger.  Unlike
Prolog systems, the Logtalk debugger is a regular application, using a
public API. As a consequence, it must be explicitly loaded by the
programmer, either manually at the top-level interpreter or
automatically from a settings file.

5.7.1 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#debugger’

5.7.2 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(debugger(loader)).

When the code to be debugged runs computationally expensive
initializations, loading this tool after the code may have a noticeable
impact in loading times.

5.7.3 Testing
-------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(debugger(tester)).

5.7.4 Usage
-----------

Debugging Logtalk source code (with this debugger) requires compiling
source files using the ‘debug(on)’ compiler flag.  For example:

     | ?- logtalk_load(my_buggy_code, [debug(on)]).

In alternative, you may also turn on the ‘debug’ flag globally by
typing:

     | ?- set_logtalk_flag(debug, on).

But note that loader files may override this flag setting (e.g.  by
using ‘debug(off)’ or ‘optimize(on)’ options for loaded files).  If
that’s the case, you will need to either edit the loader files or write
customized loader files enabling debugging.  For detailed information on
using the debugger, consult the debugging section of the User Manual:

‘../../manuals/userman/debugging.html’

The ‘debugger_messages.lgt’ source file defines the default debugger
message translations.

The ‘dump_trace.lgt’ provides a simple solution for dumping a goal trace
to a file.  For example:

     | ?- dump_trace::start_redirect_to_file('trace.txt', some_goal),
          dump_trace::stop_redirect_to_file.

A full trace can also be obtained at the top-level by using the ‘S’
(Skip) command at the call port for the top-levle goal when tracing it.

5.7.5 Alternative debugger tools
--------------------------------

Logtalk provides basic support for the SWI-Prolog graphical tracer.  The
'required' settings are described in the ‘settings-sample.lgt’ file.
Logtalk queries can be traced using this tool by using the ‘gtrace/0-1’
predicates.  For example:

     | ?- gtrace(foo::bar).

or:

     | ?- gtrace, foo::bar.

You can also use the ‘gspy/1’ predicate to spy a Logtalk predicate
specified as ‘Entity::Functor/Arity’ when using the graphical tracer.
When using this tool, internal Logtalk compiler/runtime predicates and
compiled predicates that resulted from the term-expansion mechanism may
be exposed in some cases.  This issue is shared with Prolog code and
results from the non-availability of source code for the predicates
being traced.

5.7.6 Known issues
------------------

Line number spy points (aka breakpoints) require a Prolog backend
compiler that supports accessing read term starting line but only some
backends (B-Prolog, GNU Prolog, JIProlog, LVM, SICStus Prolog,
SWI-Prolog, Trealla Prolog, and YAP) provide accurate line numbers.

As a workaround, you can check the start line number for an entity
predicate definition using a query such as:

     | ?- object_property(Entity, defines(Functor/Arity, Properties)).

and checking the returned ‘line_count/1’ property to find if there’s any
offset to the source file number of the predicate clause that you want
to trace.  This issue, if present, usually only affects the first
predicate clause.

Line number spy points are currently not available when using XSB as the
Prolog backend compiler.

5.8 ‘diagrams’
==============

This tool generates 'library', 'directory', 'file', 'entity', and
'predicate' diagrams for source files and for libraries of source files
using the Logtalk reflection API to collect the relevant information and
a graph language for representing the diagrams.  Limited support is also
available for generating diagrams for Prolog module applications.  It’s
also possible in general to generate predicate cross-referencing
diagrams for plain Prolog files.

Linking 'library diagrams' to 'entity diagrams' to 'predicate
cross-referencing diagrams' and linking 'directory diagrams' to 'file
diagrams' is also supported when using SVG output.  This feature allows
using diagrams for understanding the architecture of applications by
navigating complex code and zooming into details.  SVG output can also
easily link to both source code repositories and API documentation.
This allows diagrams to be used for source code navigation.

Diagrams can also be used to uncover code issues.  For example,
comparing 'loading diagrams' with 'dependency diagrams' can reveal
implicit dependencies.  Loading diagrams can reveal circular
dependencies that may warrant code refactoring.  Entity diagrams can
provide a good overview of code coupling.  Predicate cross-referencing
diagrams can be used to visually access entity code complexity,
complementing the ‘code_metrics’ tool.

All diagrams support a comprehensive set of options, discussed below, to
customize the final contents and appearance.

Diagram generation can be easily automated using the ‘doclet’ tool and
the ‘logtalk_doclet’ scripts.  See the ‘doclet’ tool examples and
documentation for details.  See also the ‘diagrams’ tool own ‘lgt2svg’
Bash and PowerShell scripts.

5.8.1 Requirements
------------------

A recent version of Graphviz is required for generating diagrams in the
final formats.  It can be installed using a Graphviz installer or e.g.
the following per operating-system commands:

5.8.1.1 macOS - MacPorts
........................

     $ sudo port install graphviz

5.8.1.2 macOS - Homebrew
........................

     $ brew install graphviz

5.8.1.3 Ubuntu
..............

     $ sudo apt install graphviz

5.8.1.4 Windows - Chocolatey
............................

     > choco install graphviz

5.8.1.5 Installers
..................

‘https://www.graphviz.org/download/’

On Linux systems, use the distribution own package manager to install
any missing command.

5.8.2 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#diagrams’

For sample queries, please see the ‘SCRIPT.txt’ file in the tool
directory.

5.8.3 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(diagrams(loader)).

5.8.4 Testing
-------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(diagrams(tester)).

5.8.5 Supported diagrams
------------------------

The following entity diagrams are supported:

   - 'entity diagrams' showing entity public interfaces, entity
     inheritance relations, and entity predicate cross-reference
     relations

   - 'predicate cross-reference diagrams' (between entities or within an
     entity)

   - 'inheritance diagrams' showing entity inheritance relations

   - 'uses diagrams' showing which entities use resources from other
     entities

The following library diagrams are supported:

   - 'library loading diagrams' showing which libraries load other
     libraries

   - 'library dependency diagrams' showing which libraries contain
     entities with references to entities defined in other libraries

The following file diagrams are supported:

   - 'file loading diagrams' showing which files load or include other
     files

   - 'file dependency diagrams' showing which files contain entities
     with references to entities defined in other files

File dependency diagrams are specially useful in revealing dependencies
that are not represented in file loading diagrams due to files being
loaded indirectly by files external to the libraries being documented.

The following directory diagrams are supported:

   - 'directory loading diagrams' showing which directories contain
     files that load files in other directories

   - 'directory dependency diagrams' showing which directories contain
     entities with references to entities defined in other directories

Comparing directory (or file) loading diagrams with directory (or file)
dependency diagrams allows comparing what is explicitly loaded with the
actual directory (or file) dependencies, which are inferred from the
source code.

Library and directory dependency diagrams are specially useful for large
applications where file diagrams would be too large and complex to be
useful, specially when combined with the 'zoom' option to link to,
respectively, entity and file diagrams.

A utility object, ‘diagrams’, is provided for generating all supported
diagrams in one step.  This object provides an interface common to all
diagrams but note that some predicates that generate diagrams only make
sense for some types of diagrams.  For best results and fine-grained
customization of each diagram, the individual diagram objects should be
used with the intended set of options.

5.8.6 Graph elements
--------------------

Limitations in both the graph language and UML forces the invention of a
modeling language that can represent all kinds of Logtalk entities and
entity relations.  Currently we use the following Graphviz DOT shapes
(libraries, entities, predicates, and files) and arrows (entity,
predicate, and file relations):

   -
          libraries
          ‘tab’ (lightsalmon)

   -
          library loading and dependency relations
          ‘normal’ (arrow ending with a black triangle)

   -
          objects (classes, instances, and prototypes)
          ‘box’ (rectangle, yellow for instances/classes and beige for prototypes)

   -
          protocols
          ‘note’ (aqua marine rectangle with folded right-upper corners)

   -
          categories
          ‘component’ (light cyan rectangle with two small rectangles intercepting the left side)

   -
          modules
          ‘tab’ (plum rectangle with small tab at top)

   -
          public predicates
          ‘box’ (springgreen)

   -
          public, multifile, predicates
          ‘box’ (skyblue)

   -
          protected predicates
          ‘box’ (yellow)

   -
          private predicates
          ‘box’ (indianred)

   -
          external predicates
          ‘box’ (beige)

   -
          exported module predicates
          ‘box’ (springgreen)

   -
          directories
          ‘tab’ (lightsalmon)

   -
          directory loading and dependency relations
          ‘normal’ (arrow ending with a black triangle)

   -
          files
          ‘box’ (pale turquoise rectangle)

   -
          file loading and dependency relations
          ‘normal’ (arrow ending with a black triangle)

   -
          specialization relation
          ‘onormal’ (arrow ending with a white triangle)

   -
          instantiation relation
          ‘normal’ (arrow ending with a black triangle)

   -
          extends relation
          ‘vee’ (arrow ending with a “v”)

   -
          implements relation
          ‘dot’ (arrow ending with a black circle)

   -
          imports relation
          ‘box’ (arrow ending with a black square)

   -
          complements relation
          ‘obox’ (arrow ending with a white square)

   -
          uses and use module relations
          ‘rdiamond’ (arrow ending with a black half diamond)

   -
          predicate calls
          ‘normal’ (arrow ending with a black triangle)

   -
          dynamic predicate updates
          ‘diamond’ (arrow ending with a black diamond)

The library, directory, file, entity, and predicate nodes that are not
part of the predicates, entities, files, or libraries for which we are
generating a diagram use a dashed border, a darker color, and are
described as external.

Note that all the elements above can have captions.  See below the
diagrams ‘node_type_captions/1’ and ‘relation_labels/1’ output options.

5.8.7 Supported graph languages
-------------------------------

Currently only the DOT graph language is supported (tested with Graphviz
version 10.0 on macOS; visit the ‘http://www.graphviz.org/’ website for
more information).  There’s also preliminary support for Mermaid (which
is not loaded by default as its current version lacks required features
for parity with Graphviz).

The diagrams ‘.dot’ files are created on the current directory by
default.  These files can be easily converted into a printable format
such as SVG, PDF, or Postscript.  For example, using the ‘dot’
command-line executable we can simply type:

     dot -Tpdf diagram.dot > diagram.pdf

This usually works fine for entity and predicate call cross-referencing
diagrams.  For directory and file diagrams, the ‘fdp’ and ‘circo’
command-line executables may produce better results.  For example:

     fdp -Tsvg diagram.dot > diagram.svg
     circo -Tsvg diagram.dot > diagram.svg

It’s also worth to experiment with different layouts to find the one
that produces the best results (see the ‘layout/1’ option described
below).

Some output formats such as SVG support tooltips and URL links, which
can be used for showing e.g.  entity types, relation types, file paths,
and for navigating to files and directories of files (libraries) or to
API documentation.  See the relevant diagram options below in order to
take advantage of these features (see the discussion below on “linking
diagrams”).

Sample helper scripts are provided for batch converting a directory of
‘.dot’ files to ‘.svg’ files:

   - ‘lgt2svg.sh’ for POSIX systems

   - ‘lgt2svg.ps1’ for Windows systems

   - ‘lgt2svg.js’ and ‘lgt2svg.bat’ for Windows systems (deprecated)

The scripts assume that the Graphviz command-line executables are
available from the system path (the default is the ‘dot’ executable but
the scripts accept a command-line option to select in alternative the
‘circo’, ‘fdp’, or ‘neato’ executables).

When generating diagrams for multiple libraries or directories, it’s
possible to split a diagram with several disconnected library or
directory graphs using the ‘ccomps’ command-line executable.  For
example:

     ccomps -x -o subdiagram.dot diagram.dot

For more information on the DOT language and related tools see:

     http://www.graphviz.org/

When using Windows, there are known issues with some Prolog compilers
due to the internal representation of paths.  If you encounter problems
with a specific backend Prolog compiler, try if possible to use another
supported backend Prolog compiler when generating diagrams.

For printing large diagrams, you will need to either use a tool to slice
the diagram in page-sized pieces or, preferably, use software capable of
tiled printing (e.g.  Adobe Reader).  You can also hand-edit the
generated ‘.dot’ files and play with settings such as aspect ratio for
fine-tuning the diagrams layout.

5.8.8 Customization
-------------------

A set of options are available to specify the details to include in the
generated diagrams.  For entity diagrams the options are:

   -
          ‘layout(Layout)’
          diagram layout (one of the atoms ‘{top_to_bottom,bottom_to_top,left_to_right,right_to_left}’; default is ‘bottom_to_top’)

   -
          ‘title(Title)’
          diagram title (an atom; default is ‘''’)

   -
          ‘date(Boolean)’
          print current date and time (‘true’ or ‘false’; default is ‘true’)

   -
          ‘versions(Boolean)’
          print Logtalk and backend version data (‘true’ or ‘false’; default is ‘false’)

   -
          ‘interface(Boolean)’
          print public predicates (‘true’ or ‘false’; default is ‘true’)

   -
          ‘file_labels(Boolean)’
          print file labels (‘true’ or ‘false’; default is ‘true’)

   -
          ‘file_extensions(Boolean)’
          print file name extensions (‘true’ or ‘false’; default is ‘true’)

   -
          ‘relation_labels(Boolean)’
          print entity relation labels (‘true’ or ‘false’; default is ‘true’)

   -
          ‘externals(Boolean)’
          print external nodes (‘true’ or ‘false’; default is ‘true’)

   -
          ‘node_type_captions(Boolean)’
          print node type captions (‘true’ or ‘false’; default is ‘true’)

   -
          ‘inheritance_relations(Boolean)’
          print inheritance relations (‘true’ or ‘false’; default is ‘true’ for entity inheritance diagrams and ‘false’ for other entity diagrams)

   -
          ‘provide_relations(Boolean)’
          print provide relations (‘true’ or ‘false’; default is ‘false’)

   -
          ‘xref_relations(Boolean)’
          print predicate call cross-reference relations (‘true’ or ‘false’; default depends on the specific diagram)

   -
          ‘xref_calls(Boolean)’
          print predicate cross-reference calls (‘true’ or ‘false’; default depends on the specific diagram)

   -
          ‘output_directory(Directory)’
          directory for the .dot files (an atom; default is ‘'./dot_dias'’)

   -
          ‘exclude_directories(Directories)’
          list of directories to exclude (default is ‘[]’); all sub-directories of the excluded directories are also excluded; directories may be listed by full or relative path

   -
          ‘exclude_files(Files)’
          list of source files to exclude (default is ‘[]’); files may be listed by full path or basename, with or without extension

   -
          ‘exclude_libraries(Libraries)’
          list of libraries to exclude (default is ‘[startup, scratch_directory]’)

   -
          ‘exclude_entities(Entities)’
          list of entities to exclude (default is ‘[]’)

   -
          ‘path_url_prefixes(PathPrefix, CodeURLPrefix, DocURLPrefix)’
          code and documenting URL prefixes for a path prefix used when generating cluster, library, directory, file, and entity links (atoms; no default; can be specified multiple times)

   -
          ‘url_prefixes(CodeURLPrefix, DocURLPrefix)’
          default URL code and documenting URL prefixes used when generating cluster, library, file, and entity links (atoms; no default)

   -
          ‘entity_url_suffix_target(Suffix, Target)’
          extension for entity documenting URLs (an atom; default is ‘'.html'’) and target separating symbols (an atom; default is ‘'#'’)

   -
          ‘omit_path_prefixes(Prefixes)’
          omit common path prefixes when printing directory paths and when constructing URLs (a list of atoms; default is a list with the user home directory)

   -
          ‘zoom(Boolean)’
          generate sub-diagrams and add links and zoom icons to library and entity nodes (‘true’ or ‘false’; default is ‘false’)

   -
          ‘zoom_url_suffix(Suffix)’
          extension for linked diagrams (an atom; default is ‘'.svg'’)

In the particular case of cross-referencing diagrams, there are also the
options:

   -
          ‘recursive_relations(Boolean)’
          print recursive predicate relations (‘true’ or ‘false’; default is ‘false’)

   -
          ‘url_line_references(Host)’
          syntax for the URL source file line part (an atom; possible values are ‘{github,gitlab,bitbucket}’; default is ‘github’); when using this option, the ‘CodeURLPrefix’ should be a permanent link (i.e. it should include the commit SHA1)

   -
          ‘predicate_url_target_format(Generator)’
          documentation final format generator (an atom; default is ‘sphinx’)

For directory and file diagrams the options are:

   -
          ‘layout(Layout)’
          diagram layout (one of the atoms ‘{top_to_bottom,bottom_to_top,left_to_right,right_to_left}’; default is ‘top_to_bottom’)

   -
          ‘title(Title)’
          diagram title (an atom; default is ‘''’)

   -
          ‘date(Boolean)’
          print current date and time (‘true’ or ‘false’; default is ‘true’)

   -
          ‘versions(Boolean)’
          print Logtalk and backend version data (‘true’ or ‘false’; default is ‘false’)

   -
          ‘directory_paths(Boolean)’
          print file directory paths (‘true’ or ‘false’; default is ‘false’)

   -
          ‘file_extensions(Boolean)’
          print file name extensions (‘true’ or ‘false’; default is ‘true’)

   -
          ‘path_url_prefixes(PathPrefix, CodeURLPrefix, DocURLPrefix)’
          code and documenting URL prefixes for a path prefix used when generating cluster, directory, file, and entity links (atoms; no default; can be specified multiple times)

   -
          ‘url_prefixes(CodeURLPrefix, DocURLPrefix)’
          default URL code and documenting URL prefixes used when generating cluster, library, file, and entity links (atoms; no default)

   -
          ‘omit_path_prefixes(Prefixes)’
          omit common path prefixes when printing directory paths and when constructing URLs (a list of atoms; default is a list with the user home directory)

   -
          ‘relation_labels(Boolean)’
          print entity relation labels (‘true’ or ‘false’; default is ‘false’)

   -
          ‘externals(Boolean)’
          print external nodes (‘true’ or ‘false’; default is ‘true’)

   -
          ‘node_type_captions(Boolean)’
          print node type captions (‘true’ or ‘false’; default is ‘false’)

   -
          ‘output_directory(Directory)’
          directory for the .dot files (an atom; default is ‘'./dot_dias'’)

   -
          ‘exclude_directories(Directories)’
          list of directories to exclude (default is ‘[]’)

   -
          ‘exclude_files(Files)’
          list of source files to exclude (default is ‘[]’)

   -
          ‘zoom(Boolean)’
          generate sub-diagrams and add links and zoom icons to library and entity nodes (‘true’ or ‘false’; default is ‘false’)

   -
          ‘zoom_url_suffix(Suffix)’
          extension for linked diagrams (an atom; default is ‘'.svg'’)

For library diagrams the options are:

   -
          ‘layout(Layout)’
          diagram layout (one of the atoms ‘{top_to_bottom,bottom_to_top,left_to_right,right_to_left}’; default is ‘top_to_bottom’)

   -
          ‘title(Title)’
          diagram title (an atom; default is ‘''’)

   -
          ‘date(Boolean)’
          print current date and time (‘true’ or ‘false’; default is ‘true’)

   -
          ‘versions(Boolean)’
          print Logtalk and backend version data (‘true’ or ‘false’; default is ‘false’)

   -
          ‘directory_paths(Boolean)’
          print file directory paths (‘true’ or ‘false’; default is ‘false’)

   -
          ‘path_url_prefixes(PathPrefix, CodeURLPrefix, DocURLPrefix)’
          code and documenting URL prefixes for a path prefix used when generating cluster, library, file, and entity links (atoms; no default; can be specified multiple times)

   -
          ‘url_prefixes(CodeURLPrefix, DocURLPrefix)’
          default URL code and documenting URL prefixes used when generating cluster, library, file, and entity links (atoms; no default)

   -
          ‘omit_path_prefixes(Prefixes)’
          omit common path prefixes when printing directory paths and when constructing URLs (a list of atoms; default is a list with the user home directory)

   -
          ‘relation_labels(Boolean)’
          print entity relation labels (‘true’ or ‘false’; default is ‘false’)

   -
          ‘externals(Boolean)’
          print external nodes (‘true’ or ‘false’; default is ‘true’)

   -
          ‘node_type_captions(Boolean)’
          print node type captions (‘true’ or ‘false’; default is ‘false’)

   -
          ‘output_directory(Directory)’
          directory for the .dot files (an atom; default is ‘'./dot_dias'’)

   -
          ‘exclude_directories(Directories)’
          list of directories to exclude (default is ‘[]’)

   -
          ‘exclude_files(Files)’
          list of source files to exclude (default is ‘[]’)

   -
          ‘exclude_libraries(Libraries)’
          list of libraries to exclude (default is ‘[startup, scratch_directory]’)

   -
          ‘zoom(Boolean)’
          generate sub-diagrams and add links and zoom icons to library and entity nodes (‘true’ or ‘false’; default is ‘false’)

   -
          ‘zoom_url_suffix(Suffix)’
          extension for linked diagrams (an atom; default is ‘'.svg'’)

When using the ‘zoom(true)’ option, the ‘layout(Layout)’ option applies
only to the top diagram; sub-diagrams will use their own layout default.

The option ‘omit_path_prefixes(Prefixes)’ with a non-empty list of
prefixes should preferably be used together with the option
‘directory_paths(true)’ when generating library or file diagrams that
reference external libraries or files.  To confirm the exact default
options used by each type of diagram, send the ‘default_options/1’
message to the diagram object.

Be sure to set the ‘source_data’ flag ‘on’ before compiling the
libraries or files for which you want to generated diagrams.

Support for displaying Prolog modules and Prolog module files in
diagrams of Logtalk applications:

   -
          ECLiPSe
          file diagrams don’t display module files

   -
          SICStus Prolog
          file diagrams don’t display module files

   -
          SWI-Prolog
          full support (uses the SWI-Prolog ‘prolog_xref’ library)

   -
          YAP
          full support (uses the YAP ‘prolog_xref’ library)

5.8.9 Linking diagrams
----------------------

When using SVG output, it’s possible to generate diagrams that link to
other diagrams, to API documentation, and to source code repositories
(to both files and directories).

For generating links between diagrams, use the ‘zoom(true)’ option.
This option allows (1) linking library diagrams to entity diagrams to
predicate cross-referencing diagrams and (2) linking directory diagrams
to file diagrams.  The sub-diagrams are automatically generated.  For
example, using the predicates that generate library diagrams will
automatically also generate the entity and predicate cross-referencing
diagrams.

To generate links to API documentation and source code repositories, use
the options ‘path_url_prefixes/3’ (or ‘url_prefixes/2’ for simpler
cases) and ‘omit_path_prefixes/1’.  The idea is that the
‘omit_path_prefixes/1’ option specifies local file prefixes that will be
cut and replaced by the URL prefixes (which can be path prefix specific
when addressing multiple code repositories).  To generate local file
system URLs, define the empty atom, ‘''’, as a prefix.  As an example,
consider the Logtalk library.  Its source code is available from a
GitHub repository and its documentation is published in the Logtalk
website.  The relevant URLs in this case are:

   - ‘https://github.com/LogtalkDotOrg/logtalk3/’ (source code)

   - ‘https://logtalk.org/library/’ (API documentation)

Git source code URLs should include the commit SHA1 to ensure that
entity and predicate file line information in the URLs remain valid if
the code changes in later commits.  Assuming a ‘GitHub’ variable bound
to the SHA1 commit URL we want to reference, an inheritance diagram can
be generated using the goal:

     | ?- GitHub  = 'https://github.com/LogtalkDotOrg/logtalk3/commit/eb156d46e135ac47ef23adcc5d20d49dd8b66abb',
          APIDocs = 'https://logtalk.org/library/',
          logtalk_load(diagrams(loader)),
          set_logtalk_flag(source_data, on),
          logtalk_load(library(all_loader)),
          inheritance_diagram::rlibrary(library, [
              title('Logtalk library'),
              node_type_captions(true),
              zoom(true),
              path_url_prefixes('$LOGTALKUSER/', GitHub, APIDocs),
              path_url_prefixes('$LOGTALKHOME/', GitHub, APIDocs),
              omit_path_prefixes(['$LOGTALKUSER/', '$LOGTALKHOME/', '$HOME/'])
          ]).

The two ‘path_url_prefixes/3’ options take care of source code and API
documentation for entities loaded either from the Logtalk installation
directory (whose location is given by the ‘LOGTALKHOME’ environment
variable) or from the Logtalk user directory (whose location is given by
the ‘LOGTALKUSER’ environment variable).  As we also don’t want any
local operating-system paths to be exposed in the diagram, we use the
‘omit_path_prefixes/1’ option to suppress those path prefixes, Note that
all the paths and URLs must end with a slash for proper handling.  The
‘git’ library may be useful to retrieve the commit SHA1 from a local
repo directory.

For both ‘path_url_prefixes/3’ and ‘omit_path_prefixes/1’ options, when
a path prefix is itself a prefix of another path, the shorter path must
come last to ensure correct links.

See the ‘SCRIPT.txt’ file in the tool directory for additional examples.
To avoid retyping such complex goals when updating diagrams, use the
‘doclet’ tool to save and reapply them easily (e.g.  by using the ‘make’
tool with the ‘documentation’ target).

5.8.10 Creating diagrams for Prolog module applications
-------------------------------------------------------

Currently limited to SWI-Prolog and YAP Prolog module applications due
to the lack of a comprehensive reflection API in other Prolog systems.

Simply load your Prolog module application and its dependencies and then
use diagram entity, directory, or file predicates.  Library diagram
predicates are not supported.  See the ‘SCRIPT.txt’ file in the tool
directory for some usage examples.  Note that support for diagrams with
links to API documentation is quite limited, however, due to the lack of
Prolog standards.

5.8.11 Creating diagrams for plain Prolog files
-----------------------------------------------

This tool can also be used to create predicate cross-referencing
diagrams for plain Prolog files.  For example, if the Prolog file is
named ‘code.pl’, simply define an object including its code:

     :- object(code).
         :- include('code.pl').
     :- end_object.

Save the object to an e.g.  ‘code.lgt’ file in the same directory as the
Prolog file and then load it and create the diagram:

     | ?- logtalk_load(code),
          xref_diagram::entity(code).

An alternative is to use the ‘object_wrapper_hook’ provided by the
‘hook_objects’ library:

     | ?- logtalk_load(hook_objects(loader)).
     ...

     | ?- logtalk_load(code, [hook(object_wrapper_hook)]),
          xref_diagram::entity(code).

5.8.12 Other notes
------------------

Generating complete diagrams requires that all referenced entities are
loaded.  When that is not the case, notably when generating
cross-referencing diagrams, missing entities can result in incomplete
diagrams.

When generating entity predicate call cross-reference diagrams, caller
nodes are not created for auxiliary predicates.  For example, if the
‘meta_compiler’ library is used to optimize meta-predicates calls, the
diagrams may show predicates that are not apparently called by any other
predicate when the callers are from the optimized meta-predicate goals
(which are called via library generated auxiliary predicates).  A
workaround in this case would be creating a dedicated loader file that
doesn’t load (and apply) the ‘meta_compiler’ library when generating the
diagrams.

The zoom icons, ‘zoom.png’ and ‘zoom.svg’ have been designed by Xinh
Studio:

‘https://www.iconfinder.com/xinhstudio’

Currently, only the ‘zoom.png’ file is used.  A copy of this file must
exist in any directory used for publishing diagrams using it.  The
‘lgt2svg’ scripts take care of copying this file.

When generating diagrams in SVG format, a copy of the ‘diagrams.css’
file must exist in any directory used for publishing diagrams using it.
The ‘lgt2svg’ scripts also take care of copying this file.

The Graphviz command-line utilities, e.g.  ‘dot’, are notorious for
random crashes (segmentation faults usually), often requiring re-doing
conversions from ‘.dot’ files to other formats.  A possible workaround
is to repeat the command until it completes without error.  See for
example the ‘lgt2svg.sh’ script.

5.9 ‘doclet’
============

This folder provides a simple tool for (re)generating documentation for
a project.  The tool defines a ‘doclet’ object that is expected to be
extended by the user to specify a sequence of goals and a sequence of
shell commands that load the application and (re)generate its
documentation.

Doclet source files are preferably named ‘doclet.lgt’ (or
‘doclet.logtalk’) and the doclet object are usually named after the
application or library to be documented with a ‘_doclet’ suffix.  By
using an ‘initialization/1’ directive to automatically send the
‘update/0’ message that generates the documentation upon doclet loading,
we can abstract the name of the doclet object.  The usual query to load
and run a doclet is therefore:

     | ?- logtalk_load([doclet(loader), doclet]).

For usage examples see the ‘sample_doclet.lgt’, ‘doclet1.lgt’,
‘zoom_doclet.lgt’, and ‘tools_doclet.lgt’ source files.

5.9.1 API documentation
-----------------------

This tool API documentation is available at:

‘../../docs/library_index.html#doclet’

For sample queries, please see the ‘SCRIPT.txt’ file in the tool
directory.

5.9.2 Loading
-------------

This tool can be loaded using the query:

     | ?- logtalk_load(doclet(loader)).

5.9.3 Automating running doclets
--------------------------------

You can use the ‘scripts/logtalk_doclet.sh’ Bash shell script for
automating running doclets.  The script expects the doclet source files
to be named either ‘doclet.lgt’ or ‘doclet.logtalk’.  See the
‘scripts/NOTES.md’ file or the script man page for details.

5.9.4 Integration with the make tool
------------------------------------

Loading this tool adds a definition for the
‘logtalk_make_target_action/1’ hook predicate for the target
‘documentation’.  The hook definition sends an ‘update/0’ message to
each loaded doclet.

5.10 ‘help’
===========

This tool provides basic on-line help for Logtalk features and libraries
when running in most operating-systems.  For help on the Logtalk
compiler error and warning messages, see the ‘tutor’ tool.

5.10.1 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#help’

For sample queries, please see the ‘SCRIPT.txt’ file in the tool
directory.

5.10.2 Loading
--------------

     | ?- logtalk_load(help(loader)).

5.10.3 Testing
--------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(help(tester)).

5.10.4 Supported operating-systems
----------------------------------

Currently, support is limited to Linux, macOS, and Windows.

On Windows, the ‘start’ command must be available.  On Linux, the
‘xdg-open’ command must be available.  On macOS, the command ‘open’ is
used.

This tool relies on the library portable operating-system access
abstraction.

5.10.5 Usage
------------

After loading the tool, use the query ‘help::help’ to get started.

5.10.6 Experimental features
----------------------------

On POSIX systems, when using Ciao Prolog, ECLiPSe, GNU Prolog (1.5.1 or
later version), LVM, SICStus Prolog, SWI-Prolog, Trealla Prolog, or XSB
as the backend, ‘apis/1’ and ‘handbook/0-1’ predicates are made
available.  These predicates open inline at the top-level interpreter
the Texinfo versions of the Handbook and the APIs documentation.  The
optional argument is a starting node, which can be an atom, a predicate
indicator, or a non-terminal indicator.  When there are several nodes
for the same argument (e.g.  multiple implementations of the ‘member/2’
predicate), one of them will be displayed.  Some examples:

     | ?- help::handbook.

     | ?- help::handbook(base64).

     | ?- help::handbook(logtalk_load/2).

     | ?- help::apis.

     | ?- help::apis(check/2).

     | ?- help::apis(message_tokens//2).

Although less useful, you can also browse the ‘man’ pages of Logtalk
scripts.  For example:

     | ?- help::man(logtalk_tester).

When you finish consult the documentation and quit the ‘info’ process,
you will be back to the top-level prompt (if you find that the top-level
have scrolled from its last position, try to set your terminal terminfo
to ‘xterm-256colour’).

If you’re running Logtalk from a git clone of its repo, you will need to
run the ‘scripts/update_html_docs.sh’ or ‘scripts/update_html_docs.ps1’
scripts to generate the APIs documentation ‘.info’ file and also run the
‘manuals/sources/build_manuals.sh’ or
‘manuals/sources/build_manuals.ps1’ scripts to generated the Handbook
‘.info’ file.  In alternative, you can download the ‘.info’ files for
the latest stable release from the Logtalk website and save them to the
‘docs’ and ‘manuals’ directories.

The required ‘info’ command is provided by the third-party ‘texinfo’
package (tested with version 6.8).  On macOS, this package can be
installed with either MacPorts:

     $ sudo port install texinfo

Or using Homebrew:

     $ brew install texinfo

On Linux systems, use the distribution own package manager to install
the ‘texinfo’ package.  For example, in Ubuntu systems:

     $ sudo apt install info

5.10.7 Known issues
-------------------

The open commands used to open documentation URLs drop the fragment
part, thus preventing navigating to the specified position on the
documentation page.

When browsing the Texinfo versions of the Handbook and the APIs
documentation generated with a recent version of Sphinx and using a
recent version of Texinfo, the Texinfo search feature often displays the
previous nodes of the searched nodes.

ECLiPSe defines a ‘help’ prefix operator that forces wrapping this atom
between parenthesis when sending messages to the tool.  E.g.  use
‘(help)::help’ instead of ‘help::help’.

5.11 ‘issue_creator’
====================

This is a complementary tool for the ‘lgtunit’ tool for automatically
creating bug report issues for failed tests in GitHub or GitLab servers.

5.11.1 Requirements
-------------------

This tool requires that the GitHub and GitLab CLIs be installed.  For
the installation instructions see:

   - GitHub: ‘https://cli.github.com’

   - GitLab: ‘https://glab.readthedocs.io’

5.11.2 Loading
--------------

This tool can be loaded using the query:

     | ?- logtalk_load(issue_creator(loader)).

But in the most common usage scenario, this tool is automatically loaded
by the ‘logtalk_tester’ automation script.

5.11.3 Usage
------------

The ‘logtalk_tester’ automation script accepts a ‘-b’ option for
automatically using this tool (see the script man page for details).  In
the most simple case, this option possible values are ‘github’ and
‘gitlab’.  For example:

     $ logtalk_tester \
         -p gnu \
         -b github \
         -s "/home/jdoe/foo/" \
         -u https://github.com/jdoe/foo/tree/55aa900775befa135e0d5b48ea63098df8b97f5c/

The ‘logtalk_tester’ script 'must' be called from a git repo directory
or one of its sub-directories, which is a common setup in CI/CD
pipelines.  Moreover, prior to running the tests, the CLI must be used,
if required, to authenticate and login to the server where the bug
report issues will be created:

   - GitHub: ‘gh auth login --hostname <string> --with-token <
     token.txt’

   - GitLab: ‘glab auth login --hostname <string> --token <string>’

The access token must ensure the necessary scopes that allow bug reports
to be created.  See the CLIs documentation for details.  Typically, the
‘auth’ command is called from the CI/CD pipeline definition scripts.
However, depending on the CI/CD workflow, the authentication may be done
implicitly.

The bug reports are created using by default the label ‘bug’ and
assigned to the author of the latest commit of the git repo.  The ‘-b’
option can also be used to override the label with a comma separated set
of labels.  For example, to use both ‘bug’ and ‘auto’ labels:

     $ logtalk_tester \
         -p gnu \
         -b github:bug,auto \
         -s "/home/jdoe/foo/" \
         -u https://github.com/jdoe/foo/tree/55aa900775befa135e0d5b48ea63098df8b97f5c/

Note that the labels 'must' be predefined in the issue tracker server
for the bug report to be successfully created.

The bug reports use Markdown formatting, which is the default in GitHub
and GitLab issue trackers.

But reports are only created for non-flaky tests.  The bug report title
and labels are used to prevent creating duplicated bug reports.
Therefore, the same labels should be used for multiple runs of the same
tests and preserved when editing the bug reports.

There are cases where we may want to postpone or temporarily disable the
automatic creation of bug reports.  E.g.  a WIP branch that’s known to
break multiple tests.  A solution is to define a pull/merge request
label, e.g.  ‘NO_AUTO_BUG_REPORTS’, that can then be checked by the
CI/CD workflow.  For example, we can test the presence of that label to
set a ‘AUTO_BUG_REPORTS’ environment variable to either an empty string
or a ‘-b’ option and use:

     logtalk_tester.sh $AUTO_BUG_REPORTS -p ...

5.11.4 Known issues
-------------------

GitLab creates CI/CD pipelines in a detached HEAD state.  As a
consequence, the bug reports always show “Git branch: HEAD”.

This tool is in a beta stage of development.  Your feedback is most
appreciated.

5.12 ‘lgtdoc’
=============

This is the default Logtalk documenting tool for generating API
documentation for libraries and applications.  It uses the structural
reflection API to extract and output in XML format relevant
documentation about a source file, a library or directory of source
files, or all loaded source files.  The tool predicates accept several
options for generating the XML files, including the output directory.

The ‘lgtdoc/xml’ directory contains several ready to use Bash and
PowerShell scripts for converting the XML documenting files into final
formats including (X)HTML, PDF, Markdown, and reStructuredText (for use
with Sphinx), or plain text files.  The scripts are described in their
‘man’ pages and made available in the system path by default.  See also
the ‘lgtdoc/xml/NOTES.md’ for details, including the 'required
third-party software'.

5.12.1 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#lgtdoc’

5.12.2 Loading
--------------

This tool can be loaded using the query:

     | ?- logtalk_load(lgtdoc(loader)).

5.12.3 Testing
--------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(lgtdoc(tester)).

5.12.4 Documenting source code
------------------------------

For information on documenting your source code, notably on documenting
directives, consult the documenting section of the User Manual:

‘../../manuals/userman/documenting.html’

Extracting documenting information from your source code using with this
tool requires compiling the source files using the ‘source_data(on)’
compiler flag.  For example:

     | ?- logtalk_load(source_file, [source_data(on)]).

Usually, this flag is set for all application source files in the
corresponding loader file.  In alternative, you may also turn on the
‘source_data’ flag globally by typing:

     | ?- set_logtalk_flag(source_data, on).

The tool API allows generating documentation for libraries, directories,
and files, complemented with library, directory, entity, and predicate
indexes.  Note that the source files to be documented 'must' be loaded
prior to using this tool predicates to generate the documentation.

5.12.5 Generating documentation
-------------------------------

For a simple application, assuming a library alias is defined for it
(e.g.  ‘my_app’), and at the top-level interpreter, we can generate the
application documentation by typing:

     | ?- {my_app(loader)}.
     ...

     | ?- {lgtdoc(loader)}.
     ...

     | ?- lgtdoc::library(my_app).
     ...

By default, the documenting XML files are created in a ‘xml_docs’
directory in the current working directory.  But usually all documenting
files are collected for both the application and the libraries it uses
in a common directory so that all documentation links resolved properly.
The ‘lgtdoc’ predicates can take a list of options to customize the
generated XML documenting files.  See the remarks section in the
lgtdocp(1) protocol documentation for details on the available options.

After generating the XML documenting files, these can be easily
converted into final formats using the provided scripts.  For example,
assuming that we want to generate HTML documentation:

     $ cd xml_docs
     $ lgt2html -t "My app"

To generate the documentation in Sphinx format instead (as used by
Logtalk itself for its APIs):

     $ cd xml_docs
     $ lgt2rst -s -- -q -p "Application name" -a "Author name" -v "Version X.YZ.P"
     $ make html

In this case, the generated documentation will be in the
‘xml_docs/_build/html/’ directory.  See the scripts man pages or call
them using the ‘-h’ option to learn more about their supported options.

For more complex applications, you can use the ‘doclet’ tool to define a
'doclet' to automate all the steps required to generate documentation.
The 'doclet' message that triggers the process can also be sent
automatically when the ‘make’ tool is used with the ‘documentation’
target.

   ---------- Footnotes ----------

   (1) https://logtalk.org/docs/library_index.html#lgtdoc

5.12.6 Documentation linter checks
----------------------------------

When the ‘lgtdoc_missing_directives’ flag is set to ‘warning’ (its usual
default value), the ‘lgtdoc’ tool prints warnings on missing entity
‘info/1’ directives and missing predicate ‘info/2’ and ‘mode/2’
directives.

When the ‘lgtdoc_missing_info_key’ flag is set to ‘warning’ (its usual
default value), the ‘lgtdoc’ tool prints warnings on entity ‘info/1’
directive and predicate ‘info/2’ directive missing de facto required
keys (e.g ‘comment’, ‘parameters’ or ‘parnames’ for parametric entities,
‘arguments’ or ‘argnames’ for predicates/non-terminals with arguments).

When the ‘lgtdoc_invalid_dates’ flag is set to ‘warning’ (its usual
default value), the ‘lgtdoc’ tool prints warnings on invalid dates
(including dates in the future) in ‘info/1’ directives.

When the ‘lgtdoc_non_standard_exceptions’ flag is set to ‘warning’ (its
usual default value), the ‘lgtdoc’ tool prints warnings on non-standard
exceptions.  This linter check is particularly effective in detecting
typos when specifying standard exceptions.

When the ‘lgtdoc_missing_punctuation’ flag is set to ‘warning’ (its
usual default value), the ‘lgtdoc’ tool prints warnings on missing
ending periods (full stops), exclamation marks, or question marks in
‘info/1-2’ directives (in comments, remarks, parameter descriptions, and
argument descriptions).

Set a flag value to ‘silent’ to turn off the corresponding linter
warnings.

5.13 ‘lgtunit’
==============

The ‘lgtunit’ tool provides testing support for Logtalk.  It can also be
used for testing plain Prolog code and Prolog module code.

This tool is inspired by the xUnit frameworks architecture and by the
works of Joachim Schimpf (ECLiPSe library ‘test_util’) and Jan
Wielemaker (SWI-Prolog ‘plunit’ package).

Tests are defined in objects, which represent a 'test set' or 'test
suite'.  In simple cases, we usually define a single object containing
the tests.  But it is also possible to use parametric test objects or
multiple objects defining parametrizable tests or test subsets for
testing more complex units and facilitate tests maintenance.  Parametric
test objects are specially useful to test multiple implementations of
the same protocol using a single set of tests by passing the
implementation object as a parameter value.

5.13.1 Main files
-----------------

The ‘lgtunit.lgt’ source file implements a framework for defining and
running unit tests in Logtalk.  The ‘lgtunit_messages.lgt’ source file
defines the default translations for the messages printed when running
unit tests.  These messages can be intercepted to customize output, e.g.
to make it less verbose, or for integration with e.g.  GUI IDEs and
continuous integration servers.

Other files part of this tool provide support for alternative output
formats of test results and are discussed below.

5.13.2 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#lgtunit’

5.13.3 Loading
--------------

This tool can be loaded using the query:

     | ?- logtalk_load(lgtunit(loader)).

5.13.4 Testing
--------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(lgtunit(tester)).

5.13.5 Writing and running tests
--------------------------------

In order to write your own unit tests, define objects extending the
‘lgtunit’ object.  You may start by copying the ‘tests-sample.lgt’ file
(at the root of the Logtalk distribution) to a ‘tests.lgt’ file in your
project directory and edit it to add your tests:

     :- object(tests,
         extends(lgtunit)).

         % test definitions
         ...

     :- end_object.

The section on test dialects below describes in detail how
to write tests.  See the ‘tests’ top directory for examples of actual
unit tests.  Other sources of examples are the ‘library’ and ‘examples’
directories.

The tests must be term-expanded by the ‘lgtunit’ object by compiling the
source files defining the test objects using the option ‘hook(lgtunit)’.
For example:

     | ?- logtalk_load(lgtunit(loader)),
          logtalk_load(tests, [hook(lgtunit)]).

As the term-expansion mechanism applies to all the contents of a source
file, the source files defining the test objects should preferably not
contain entities other than the test objects.  Additional code necessary
for the tests should go to separate files.  In general, the tests
themselves can be compiled in 'optimized' mode.  Assuming that’s the
case, also use the ‘optimize(on)’ compiler option for faster tests
execution.

The term-expansion performed by the ‘lgtunit’ object sets the test
object ‘source_data’ flag to ‘on’ and the ‘context_switching_calls’ flag
to ‘allow’ for code coverage and debugging support.  But these settings
can always be overriden in the test objects.

The ‘tester-sample.lgt’ file (at the root of the Logtalk distribution)
exemplifies how to compile and load ‘lgtunit’ tool, the source code
under testing, the unit tests, and for automatically run all the tests
after loading:

     :- initialization((
         % minimize compilation reports to the essential ones (errors and warnings)
         set_logtalk_flag(report, warnings),
         % load any necessary library files for your application; for example
         logtalk_load(basic_types(loader)),
         % load the unit test tool
         logtalk_load(lgtunit(loader)),
         % load your application files (e.g. "source.lgt") enabling support for
         % code coverage, which requires compilation in debug mode and collecting
         % source data information; if code coverage is not required, remove the
         % "debug(on)" option for faster execution
         logtalk_load(source, [source_data(on), debug(on)]),
         % compile the unit tests file expanding it using "lgtunit" as the hook
         % object to preprocess the tests; if you have failing tests, add the
         % option debug(on) to debug them (see "tools/lgtunit/NOTES.md" for
         % debugging advice); tests should be loaded after the code being tested
         % is loaded to avoid warnings such as references to unknown entities
         logtalk_load(tests, [hook(lgtunit)]),
         % run all the unit tests; assuming your tests object is named "tests"
         tests::run
     )).

You may copy this sample file to a ‘tester.lgt’ file in your project
directory and edit it to load your project and tests files.  The
‘logtalk_tester’ testing automation script defaults to look for test
driver files named ‘tester.lgt’ or ‘tester.logtalk’ (if you have
work-in-progress test sets that you don’t want to run by default, simply
use a different file name such as ‘tester_wip.lgt’; you can still run
them automated by using ‘logtalk_tester -n tester_wip’).

Debugged test sets should preferably be compiled in optimal mode,
specially when containing deterministic tests and when using the utility
benchmarking predicates.

Assuming a ‘tester.lgt’ driver file as exemplified above, the tests can
be run by simply loading this file:

     | ?- logtalk_load(tester).

Assuming your test object is named ‘tests’, you can re-run the tests by
typing:

     | ?- tests::run.

You can also re-run a single test (or a list of tests) using the ‘run/1’
predicate:

     | ?- tests::run(test_identifier).

When testing complex 'units', it is often desirable to split the tests
between several test objects or using parametric test objects to be able
to run the same tests using different parameters (e.g.  different data
sets or alternative implementations of the same protocol).  In this
case, you can run all test subsets using the goal:

     | ?- lgtunit::run_test_sets([test_set_1, test_set_2, ...]).

where the ‘run_test_sets/1’ predicate argument is a list of two or more
test object identifiers.  This predicate makes possible to get a single
code coverage report that takes into account all the tests.

It’s also possible to automatically run loaded tests when using the
‘make’ tool by calling the goal that runs the tests from a definition of
the hook predicate ‘logtalk_make_target_action/1’.  For example, by
adding to the tests ‘tester.lgt’ driver file the following code:

     % integrate the tests with logtalk_make/1
     :- multifile(logtalk_make_target_action/1).
     :- dynamic(logtalk_make_target_action/1).

     logtalk_make_target_action(check) :-
         tests::run.

Alternatively, you can define the predicate ‘make/1’ inside the test set
object.  For example:

     :- object(tests, extends(lgtunit)).

         make(check).
         ...

     :- end_object.

This clause will cause all tests to be run when calling the
‘logtalk_make/1’ predicate with the target ‘check’ (or its top-level
shortcut, ‘{?}’).  The other possible target is ‘all’ (with top-level
shortcut ‘{*}’).

Note that you can have multiple test driver files.  For example, one
driver file that runs the tests collecting code coverage data and a
quicker driver file that skips code coverage and compiles the code to be
tested in optimized mode.

5.13.6 Automating running tests
-------------------------------

You can use the ‘scripts/logtalk_tester.sh’ Bash shell script or the
‘scripts/logtalk_tester.ps1’ PowerShell script for automating running
unit tests (e.g.  from a CI/CD pipeline).  For example, assuming your
current directory (or sub-directories) contain one or more ‘tester.lgt’
files:

     $ logtalk_tester -p gnu

The only required argument is the identifier of the backend Prolog
system.  For other options, see the ‘scripts/NOTES.md’ file or type:

     $ logtalk_tester -h

On POSIX systems, you can also access extended documentation by
consulting the script man page:

     $ man logtalk_tester

The scripts support the same set of options but the option for passing
additional arguments to the tests use different syntax.  For example:

     $ logtalk_tester -p gnu -- foo bar baz

     PS> logtalk_tester -p gnu -a foo,bar,baz

On POSIX systems, assuming Logtalk was installed using one of the
provided installers or installation scripts, there is also a ‘man’ page
for the script:

     $ man logtalk_tester

Alternatively, an HTML version of this man page can be found at:

‘https://logtalk.org/man/logtalk_tester.html’

The ‘logtalk_tester.ps1’ PowerShell script timeout option requires that
Git for Windows is also installed as it requires the GNU timeout command
bundled with it.

In alternative to using the ‘logtalk_tester.ps1’ PowerShell script, the
Bash shell version of the automation script can also be used in Windows
operating-systems with selected backends by using the Bash shell
included in the Git for Windows installer.  That requires defining a
‘.profile’ file setting the paths to the Logtalk scripts and the Prolog
backend executables.  For example:

     $ cat ~/.profile
     # YAP
     export PATH="/C/Program Files/Yap64/bin":$PATH
     # GNU Prolog
     export PATH="/C/GNU-Prolog/bin":$PATH
     # SWI/Prolog
     export PATH="/C/Program Files/swipl/bin":$PATH
     # ECLiPSe
     export PATH="/C/Program Files/ECLiPSe 7.0/lib/x86_64_nt":$PATH
     # SICStus Prolog
     export PATH="/C/Program Files/SICStus Prolog VC16 4.6.0/bin":$PATH
     # Logtalk
     export PATH="$LOGTALKHOME/scripts":"$LOGTALKHOME/integration":$PATH

The Git for Windows installer also includes GNU ‘coreutils’ and its
‘timeout’ command, which is used by the ‘logtalk_tester’ script ‘-t’
option.

Note that some tests may give different results when run from within the
Bash shell compared with running the tests manually using a Windows GUI
version of the Prolog backend.  Some backends may also not be usable for
automated testing due to the way their are made available as Windows
applications.

Additional advice on testing and on automating testing using continuous
integration servers can be found at:

‘https://logtalk.org/testing.html’

5.13.7 Parametric test objects
------------------------------

Parameterized unit tests can be easily defined by using parametric test
objects.  A typical example is testing multiple implementations of the
same protocol.  In this case, we can use a parameter to pass the
specific implementation being tested.  For example, assume that we want
to run the same set of tests for the library ‘random_protocol’ protocol.
We can write:

     :- object(tests(_RandomObject_),
         extends(lgtunit)).

         :- uses(_RandomObject_, [
             random/1, between/3, member/2,
             ...
         ]).

         test(between_3_in_interval) :-
             between(1, 10, Random),
             1 =< Random, Random =< 10.

         ...

     :- end_object.

We can then test a specific implementation by instantiating the
parameter.  For example:

     | ?- tests(fast_random)::run.

Or use the ‘lgtunit::run_test_sets/1’ predicate to test all the
implementations:

     | ?- lgtunit::run_test_sets([
             tests(backend_random),
             tests(fast_random),
             tests(random)
          ]).

5.13.8 Test dialects
--------------------

Multiple test 'dialects' are supported by default.  See the next section
on how to define your own test dialects.  In all dialects, a 'ground
callable term', usually an atom, is used to uniquely identify a test.
This simplifies reporting failed tests and running tests selectively.
An error message is printed if invalid or duplicated test identifiers
are found.  These errors must be corrected otherwise the reported test
results can be misleading.  Ideally, tests should have descriptive names
that clearly state the purpose of the test and what is being tested.

Unit tests can be written using any of the following predefined
dialects:

     test(Test) :- Goal.

This is the most simple dialect, allowing the specification of tests
that are expected to succeed.  The argument of the ‘test/1’ predicate is
the test identifier, which must be unique.  A more versatile dialect is:

     succeeds(Test) :- Goal.
     deterministic(Test) :- Goal.
     fails(Test) :- Goal.
     throws(Test, Ball) :- Goal.
     throws(Test, Balls) :- Goal.

This is a straightforward dialect.  For ‘succeeds/1’ tests, ‘Goal’ is
expected to succeed.  For ‘deterministic/1’ tests, ‘Goal’ is expected to
succeed once without leaving a choice-point.  For ‘fails/1’ tests,
‘Goal’ is expected to fail.  For ‘throws/2’ tests, ‘Goal’ is expected to
throw the exception term ‘Ball’ or one of the exception terms in the
list ‘Balls’.  The specified exception must subsume the actual exception
for the test to succeed.

An alternative test dialect that can be used with more expressive power
is:

     test(Test, Outcome) :- Goal.

The possible values of the outcome argument are:

   -
          ‘true’
          The test is expected to succeed.

   -
          ‘true(Assertion)’
          The test is expected to succeed and satisfy the ‘Assertion’ goal.

   -
          ‘deterministic’
          The test is expected to succeed once without leaving a choice-point.

   -
          ‘deterministic(Assertion)’
          The test is expected to succeed once without leaving a choice-point and satisfy the ‘Assertion’ goal.

   -
          ‘subsumes(Expected, Result)’
          The test is expected to succeed binding ‘Result’ to a term that is subsumed by the ‘Expected’ term.

   -
          ‘variant(Term1, Term2)’
          The test is expected to succeed binding ‘Term1’ to a term that is a variant of the ‘Term2’ term.

   -
          ‘exists(Assertion)’
          A solution exists for the test goal that satisfies the ‘Assertion’ goal.

   -
          ‘all(Assertion)’
          All test goal solutions satisfy the ‘Assertion’ goal.

   -
          ‘fail’
          The test is expected to fail.

   -
          ‘false’
          The test is expected to fail.

   -
          ‘error(Error)’
          The test is expected to throw the exception term ‘error(ActualError, _)’ where ‘ActualError’ is subsumed ‘Error’.

   -
          ‘errors(Errors)’
          The test is expected to throw an exception term ‘error(ActualError, _)’ where ‘ActualError’ is subsumed by an element of the list ‘Errors’.

   -
          ‘ball(Ball)’
          The test is expected to throw the exception term ‘ActualBall’ where ‘ActualBall’ is subsumed ‘Ball’.

   -
          ‘balls(Balls)’
          The test is expected to throw an exception term ‘ActualBall’ where ‘ActualBall’ is subsumed by an element of the list ‘Balls’.

In the case of the ‘true(Assertion)’, ‘deterministic(Assertion)’, and
‘all(Assertion)’ outcomes, a message that includes the assertion goal is
printed for assertion failures and errors to help to debug failed unit
tests.  Same for the ‘subsumes(Expected, Result)’ and ‘variant(Term1,
Term2)’ assertions.  Note that this message is only printed when the
test goal succeeds as its failure will prevent the assertion goal from
being called.  This allows distinguishing between test goal failure and
assertion failure.

Note that the ‘all(Assertion)’ outcome simplifies pinpointing which test
goal solution failed the assertion.  See also the section below on
testing non-deterministic predicates.

The ‘fail’ and ‘false’ outcomes are better reserved to cases where there
is a single test goal.  With multiple test goals, the test will succeed
when 'any' of those goals fail.

Some tests may require individual condition, setup, or cleanup goals.
In this case, the following alternative test dialect can be used:

     test(Test, Outcome, Options) :- Goal.

The currently supported options are (non-recognized options are
ignored):

   -
          ‘condition(Goal)’
          Condition for deciding if the test should be run or skipped (default goal is ‘true’).

   -
          ‘setup(Goal)’
          Setup goal for the test (default goal is ‘true’).

   -
          ‘cleanup(Goal)’
          Cleanup goal for the test (default goal is ‘true’).

   -
          ‘flaky’
          Declare the test as a flaky test.

   -
          ‘note(Term)’
          Annotation to print (between parenthesis by default) after the test result (default is ‘''’); the annotation term can share variables with the test goal, which can be used to pass additional information about the test result.

Also supported is QuickCheck testing where random tests are
automatically generated and run given a predicate mode template with
type information for each argument (see the section below for more
details):

     quick_check(Test, Template, Options).
     quick_check(Test, Template).

The valid options are the same as for the ‘test/3’ dialect plus all the
supported QuickCheck specific options (see the QuickCheck section below
for details).

For examples of how to write unit tests, check the ‘tests’ folder or the
‘testing’ example in the ‘examples’ folder in the Logtalk distribution.
Most of the provided examples also include unit tests, some of them with
code coverage.

5.13.9 User-defined test dialects
---------------------------------

Additional test dialects can be easily defined by extending the
‘lgtunit’ object and by term-expanding the new dialect into one of the
default dialects.  As an example, suppose that you want a dialect where
you can simply write a file with tests defined by clauses using the
format:

     test_identifier :-
         test_goal.

First, we define an expansion for this file into a test object:

     :- object(simple_dialect,
         implements(expanding)).

         term_expansion(begin_of_file, [(:- object(tests,extends(lgtunit)))]).
         term_expansion((Head :- Body), [test(Head) :- Body]).
         term_expansion(end_of_file, [(:- end_object)]).

     :- end_object.

Then we can use this hook object to expand and run tests written in this
dialect by using a ‘tester.lgt’ driver file with contents such as:

     :- initialization((
         set_logtalk_flag(report, warnings),
         logtalk_load(lgtunit(loader)),
         logtalk_load(library(hook_flows_loader)),
         logtalk_load(simple_dialect),
         logtalk_load(tests, [hook(hook_pipeline([simple_dialect,lgtunit]))]),
         tests::run
     )).

The hook pipeline first applies our ‘simple_dialect’ expansion followed
by the default ‘lgtunit’ expansion.  This solution allows other hook
objects (e.g.  required by the code being tested) to also be used by
updating the pipeline.

5.13.10 QuickCheck
------------------

QuickCheck was originally developed for Haskell.  Implementations for
several other programming languages soon followed.  QuickCheck provides
support for 'property-based testing'.  The idea is to express properties
that predicates must comply with and automatically generate tests for
those properties.  The ‘lgtunit’ tool supports both ‘quick_check/2-3’
test dialects, as described above, and ‘quick_check/1-3’ public
predicates for interactive use:

     quick_check(Template, Result, Options).
     quick_check(Template, Options).
     quick_check(Template).

The following options are supported:

   - ‘n/1’: number of random tests that will be generated and run
     (default is 100).

   - ‘s/1’: maximum number of shrink operations when a counter-example
     is found (default is 64).

   - ‘ec/1’: boolean option deciding if type edge cases are tested
     before generating random tests (default is ‘true’).

   - ‘rs/1’: starting seed to be used when generating the random tests
     (no default).

   - ‘pc/1’: pre-condition closure for generated tests (extended with
     the test arguments; no default).

   - ‘l/1’: label closure for classifying the generated tests (extended
     with the test arguments plus the label argument; no default).

   - ‘v/1’: boolean option for verbose reporting of generated random
     tests (default is ‘false’).

   - ‘pb/2’: progress bar option for executed random tests when the
     verbose option is false (first argument is a boolean, default is
     ‘false’; second argument is the tick number, a positive integer).

The ‘quick_check/1’ predicate uses the default option values.  The
‘quick_check/1-2’ predicates print the test results and are thus better
reserved for testing at the top-level interpreter.  The ‘quick_check/3’
predicate returns results in reified form:

   - ‘passed(SequenceSeed, Discarded, Labels)’

   - ‘failed(Goal, SequenceSeed, TestSeed)’

   - ‘error(Error, Goal, SequenceSeed, TestSeed)’

   - ‘broken(Why, Culprit)’

The ‘broken(Why, Culprit)’ result only occurs when the user-defined
testing setup is broken.  For example, a non-callable template (e.g.  a
non-existing predicate), an invalid option, a problem with the
pre-condition closure or with the label closure (e.g.  a pre-condition
that always fails or a label that fails to classify a generated test),
or errors/failures when generating tests (e.g.  due to an unknown type
being used in the template or a broken custom type arbitrary value
generator).

The ‘Goal’ argument is the random test that failed.

The ‘SequenceSeed’ argument is the starting seed used to generate the
sequence of random tests.  The ‘TestSeed’ is the seed used to generate
the test that failed.  Both seems should be regarded as opaque terms.
When the test seed equal to the sequence seed, this means means that the
failure or error occurred while using only type edge cases.  See below
how to use the seeds when testing bug fixes.

The ‘Discarded’ argument returns the number of generated tests that were
discarded for failing to comply a pre-condition specified using the
‘pc/1’ option.  This option is specially useful when constraining or
enforcing a relation between the generated arguments and is often used
as an alternative to define a custom type.  For example, if we define
the following predicate:

     condition(I) :-
         between(0, 127, I).

we can then use it to filter the generated tests:

     | ?- lgtunit::quick_check(integer(+byte), [pc(condition)]).
     % 100 random tests passed, 94 discarded
     % starting seed: seed(416,18610,17023)
     yes

The ‘Labels’ argument returns a list of pairs ‘Label-N’ where ‘N’ is the
number of generated tests that are classified as ‘Label’ by a closure
specified using the ‘l/1’ option.  For example, assuming the following
predicate definition:

     label(I, Label) :-
         (   I mod 2 =:= 0 ->
             Label = even
         ;   Label = odd
         ).

we can try:

     | ?- lgtunit::quick_check(integer(+byte), [l(label), n(10000)]).
     % 10000 random tests passed, 0 discarded
     % starting seed: seed(25513,20881,16407)
     % even: 5037/10000 (50.370000%)
     % odd: 4963/10000 (49.630000%)
     yes

The label statistics are key to verify that the generated tests provide
the necessary coverage.  The labelling predicates can return a single
test label or a list of test labels.  Labels should be ground and are
typically atoms.  To examine the generated tests themselves, you can use
the verbose option, ‘v/1’.  For example:

     | ?- lgtunit::quick_check(integer(+integer), [v(true), n(7), pc([I]>>(I>5))]).
     % Discarded: integer(0)
     % Passed:    integer(786)
     % Passed:    integer(590)
     % Passed:    integer(165)
     % Discarded: integer(-412)
     % Passed:    integer(440)
     % Discarded: integer(-199)
     % Passed:    integer(588)
     % Discarded: integer(-852)
     % Discarded: integer(-214)
     % Passed:    integer(196)
     % Passed:    integer(353)
     % 7 random tests passed, 5 discarded
     % starting seed: seed(23671,3853,29824)
     yes

When a counter-example is found, the verbose option also prints the
shrink steps.  For example:

     | ?- lgtunit::quick_check(atom(+atomic), [v(true), ec(false)]).
     % Passed:    atom('dyO=Xv_MX-3b/U4KH U')
     *     Failure:   atom(-198)
     *     Shrinked:  atom(-99)
     *     Shrinked:  atom(-49)
     *     Shrinked:  atom(-24)
     *     Shrinked:  atom(-12)
     *     Shrinked:  atom(-6)
     *     Shrinked:  atom(-3)
     *     Shrinked:  atom(-1)
     *     Shrinked:  atom(0)
     *     quick check test failure (at test 2 after 8 shrinks):
     *       atom(0)
     *     starting seed: seed(3172,9814,20125)
     *     test seed:     seed(7035,19506,18186)
     no

The template can be a ‘(::)/2’, ‘(<<)/2’, or ‘(:)/2’ qualified callable
term.  When the template is an unqualified callable term, it will be
used to construct a goal to be called in the context of the 'sender'
using the ‘(<<)/2’ debugging control construct.  Another simple example
by passing a template that will trigger a failed test (as the
‘random::random/1’ predicate always returns non-negative floats):

     | ?- lgtunit::quick_check(random::random(-negative_float)).
     *     quick check test failure (at test 1 after 0 shrinks):
     *       random::random(0.09230089279334841)
     *     starting seed: seed(3172,9814,20125)
     *     test seed:     seed(3172,9814,20125)
     no

When QuickCheck exposes a bug in the tested code, we can use the
reported counter-example to help diagnose it and fix it.  As tests are
randomly generated, we can use the starting seed reported with the
counter-example to confirm the bug fix by calling the ‘quick_check/2-3’
predicates with the ‘rs(Seed)’ option.  For example, assume the
following broken predicate definition:

     every_other([], []).
     every_other([_, X| L], [X | R]) :-
         every_other(L, R).

The predicate is supposed to construct a list by taking every other
element of an input list.  Cursory testing may fail to notice the bug:

     | ?- every_other([1,2,3,4,5,6], List).
     List = [2, 4, 6]
     yes

But QuickCheck will report a bug with lists with an odd number of
elements with a simple property that verifies that the predicate always
succeed and returns a list of integers:

     | ?- lgtunit::quick_check(every_other(+list(integer), -list(integer))).
     *     quick check test failure (at test 2 after 0 shrinks):
     *       every_other([0],A)
     *     starting seed: seed(3172,9814,20125)
     *     test seed:     seed(3172,9814,20125)
     no

We could fix this particular bug by rewriting the predicate:

     every_other([], []).
     every_other([H| T], L) :-
         every_other(T, H, L).

     every_other([], X, [X]).
     every_other([_| T], X, [X| L]) :-
         every_other(T, L).

By retesting with the same test seed that uncovered the bug, the same
random test that found the bug will be generated and run again:

     | ?- lgtunit::quick_check(
             every_other(+list(integer), -list(integer)),
             [rs(seed(3172,9814,20125))]
          ).
     % 100 random tests passed, 0 discarded
     % starting seed: seed(3172,9814,20125)
     yes

Still, after verifying the bug fix, is also a good idea to re-run the
tests using the sequence seed instead as bug fixes sometimes cause
regressions elsewhere.

When retesting using the ‘logtalk_tester’ automation script, the
starting seed can be set using the ‘-r’ option.  For example:

     $ logtalk_tester -r "seed(3172,9814,20125)"

We could now move to other properties that the predicate should comply
(e.g.  all elements in the output list being present in the input list).
Often, both traditional unit tests and QuickCheck tests are used,
complementing each other to ensure the required code coverage.

Another example using a Prolog module predicate:

     | ?- lgtunit::quick_check(
             pairs:pairs_keys_values(
                 +list(pair(atom,integer)),
                 -list(atom),
                 -list(integer)
             )
          ).
     % 100 random tests passed, 0 discarded
     % starting seed: seed(3172,9814,20125)
     yes

As illustrated by the examples above, properties are expressed using
predicates.  In the most simple cases, that can be the predicate that we
are testing itself.  But, in general, it will be an auxiliary predicate
calling the predicate or predicates being tested and checking properties
that the results must comply with.

The QuickCheck test dialects and predicates take as argument the mode
template for a property, generate random values for each input argument
based on the type information, and check each output argument.  For
common types, the implementation tries first (by default) common edge
cases (e.g.  empty atom, empty list, or zero) before generating
arbitrary values.  When the output arguments check fails, the QuickCheck
implementation tries (by default) up to 64 shrink operations of the
counter-example to report a simpler case to help debugging the failed
test.  Edge cases, generating of arbitrary terms, and shrinking terms
make use of the library ‘arbitrary’ category via the ‘type’ object (both
entities can be extended by the user by defining clauses for multifile
predicates).

The mode template syntax is the same used in the ‘info/2’ predicate
directives with an additional notation, ‘{}/1’, for passing argument
values as-is instead of generating random values for these arguments.
For example, assume that we want to verify the ‘type::valid/2’
predicate, which takes as first argument a type.  Randomly generating
random types would be cumbersome at best but the main problem is that we
need to generate random values for the second argument according to the
first argument.  Using the ‘{}/1’ notation we can solve this problem for
any specific type, e.g.  integer, by writing:

     | ?- lgtunit::quick_check(type::valid({integer}, +integer)).

We can also test all (ground, i.e.  non-parametric) types with arbitrary
value generators by writing:

     | ?- forall(
             (type::type(Type), ground(Type), type::arbitrary(Type)),
             lgtunit::quick_check(type::valid({Type}, +Type))
          ).

You can find the list of the basic supported types for using in the
template in the API documentation for the library entities ‘type’ and
‘arbitrary’.  Note that other library entities, including third-party or
your own, can contribute with additional type definitions as both ‘type’
and ‘arbitrary’ entities are user extensible by defining clauses for
their multifile predicates.

The user can define new types to use in the property mode templates to
use with its QuickCheck tests by defining clauses for the ‘type’ library
object and the ‘arbitrary’ library category multifile predicates.
QuickCheck will use the later to generate arbitrary input arguments and
the former to verify output arguments.  As a toy example, assume that
the property mode template have an argument of type ‘bit’ with possible
values ‘0’ and ‘1’.  We would then need to define:

     :- multifile(type::type/1).
     type::type(bit).

     :- multifile(type::check/2).
     type::check(bit, Term) :-
         once((Term == 0; Term == 1)).

     :- multifile(arbitrary::arbitrary/1).
     arbitrary::arbitrary(bit).

     :- multifile(arbitrary::arbitrary/2).
     arbitrary::arbitrary(bit, Arbitrary) :-
         random::member(Arbitrary, [0, 1]).

5.13.11 Skipping tests
----------------------

A test object can define the ‘condition/0’ predicate (which defaults to
‘true’) to test if some necessary condition for running the tests holds.
The tests are skipped if the call to this predicate fails or generates
an error.

Individual tests that for some reason should be unconditionally skipped
can have the test clause head prefixed with the ‘(-)/1’ operator.  For
example:

     - test(not_yet_ready) :-
         ...

In this case, it’s a good idea to use the ‘test/3’ dialect with a
‘note/1’ option that briefly explains why the test is being skipped.
For example:

     - test(xyz_reset, true, [note('Feature xyz reset not yet implemented')]) :-
         ...

The number of skipped tests is reported together with the numbers of
passed and failed tests.  To skip a test depending on some condition,
use the ‘test/3’ dialect and the ‘condition/1’ option.  For example:

     test(test_id, true, [condition(current_prolog_flag(bounded,true))) :-
         ...

The test is skipped if the condition goal fails or generates an error.
The conditional compilation directives can also be used in alternative
but note that in this case there will be no report on the number of
skipped tests.

5.13.12 Checking test goal results
----------------------------------

Checking test goal results can be performed using the ‘test/2-3’
supported outcomes such as ‘true(Assertion)’ and
‘deterministic(Assertion)’.  For example:

     test(compare_3_order_less, deterministic(Order == (<))) :-
         compare(Order, 1, 2).

For the other test dialects, checking test goal results can be performed
by calling the ‘assertion/1-2’ utility predicates or by writing the
checking goals directly in the test body.  For example:

     test(compare_3_order_less) :-
         compare(Order, 1, 2),
         ^^assertion(Order == (<)).

or:

     succeeds(compare_3_order_less) :-
         compare(Order, 1, 2),
         Order == (<).

Using assertions is, however, preferable to directly check test results
in the test body as it facilitates debugging by printing the unexpected
results when the assertions fail.

The ‘assertion/1-2’ utility predicates are also useful for the
‘test/2-3’ dialects when we want to check multiple assertions in the
same test.  For example:

     test(dictionary_clone_4_01, true) :-
         as_dictionary([], Dictionary),
         clone(Dictionary, DictionaryPairs, Clone, ClonePairs),
         empty(Clone),
         ^^assertion(original_pairs, DictionaryPairs == []),
         ^^assertion(clone_pairs, ClonePairs == []).

Ground results can be compared using the standard ‘==/2’ term equality
built-in predicate.  Non-ground results can be compared using the
‘variant/2’ predicate provided by ‘lgtunit’.  The standard
‘subsumes_term/2’ built-in predicate can be used when testing a compound
term structure while abstracting some of its arguments.  Floating-point
numbers can be compared using the ‘=~=/2’, ‘approximately_equal/3’,
‘essentially_equal/3’, and ‘tolerance_equal/4’ predicates provided by
‘lgtunit’.  Using the ‘=/2’ term unification built-in predicate is
almost always an error as it would mask test goals failing to bind
output arguments.  The ‘lgtunit’ tool implements a linter check for the
use of unification goals in test outcome assertions.  In the rare cases
that a unification goal is intended, wrapping the ‘(=)/2’ goal using the
‘{}/1’ control construct avoids the linter warning.

5.13.13 Testing local predicates
--------------------------------

The ‘(<<)/2’ debugging control construct can be used to access and test
object local predicates (i.e.  predicates without a scope directive).
In this case, make sure that the ‘context_switching_calls’ compiler flag
is set to ‘allow’ for those objects.  This is seldom required, however,
as local predicates are usually auxiliary predicates called by public
predicates and thus tested when testing those public predicates.  The
code coverage support can pinpoint any local predicate clause that is
not being exercised by the tests.

5.13.14 Testing non-deterministic predicates
--------------------------------------------

For testing non-deterministic predicates (with a finite and manageable
number of solutions), you can wrap the test goal using the standard
‘findall/3’ predicate to collect all solutions and check against the
list of expected solutions.  When the expected solutions are a set, use
in alternative the standard ‘setof/3’ predicate.

If you want to check that all solutions of a non-deterministic predicate
satisfy an assertion, use the ‘test/2’ or ‘test/3’ test dialect with the
‘all(Assertion)’ outcome.  For example:

     test(atom_list, all(atom(Item))) :-
         member(Item, [a, b, c]).

See also the next section on testing 'generators'.

If you want to check that a solution exists for a non-deterministic
predicate that satisfies an assertion, use the ‘test/2’ or ‘test/3’ test
dialect with the ‘exists(Assertion)’ outcome.  For example:

     test(at_least_one_atom, exists(atom(Item))) :-
         member(Item, [1, foo(2), 3.14, abc, 42]).

5.13.15 Testing generators
--------------------------

To test all solutions of a predicate that acts as a 'generator', we can
use either the ‘all/1’ outcome or the ‘forall/2’ predicate as the test
goal with the ‘assertion/2’ predicate called to report details on any
solution that fails the test.  For example:

     test(test_solution_generator, all(test(X,Y,Z))) :-
         generator(X, Y, Z).

or:

     :- uses(lgtunit, [assertion/2]).
     ...

     test(test_solution_generator_2) :-
         forall(
             generator(X, Y, Z),
             assertion(generator(X), test(X,Y,Z))
         ).

While using the ‘all/1’ outcome results in a more compact test
definition, using the ‘forall/2’ predicate allows customizing the
assertion description.  In the example above, we use the ‘generator(X)’
description instead of the ‘test(X,Y,Z)’ description implicit when we
use the ‘all/1’ outcome.

5.13.16 Testing input/output predicates
---------------------------------------

Extensive support for testing input/output predicates is provided, based
on similar support found on the Prolog conformance testing framework
written by Péter Szabó and Péter Szeredi.

Two sets of predicates are provided, one for testing text input/output
and one for testing binary input/output.  In both cases, temporary files
(possibly referenced by a user-defined alias) are used.  The predicates
allow setting, checking, and cleaning text/binary input/output.  These
predicate are declared as protected and thus called using the ‘(^^/1)’
control construct.

As an example of testing an input predicate, consider the standard
‘get_char/1’ predicate.  This predicate reads a single character (atom)
from the current input stream.  Some test for basic functionality could
be:

     test(get_char_1_01, true(Char == 'q')) :-
         ^^set_text_input('qwerty'),
         get_char(Char).

     test(get_char_1_02, true(Assertion)) :-
         ^^set_text_input('qwerty'),
         get_char(_Char),
         ^^text_input_assertion('werty', Assertion).

As you can see in the above example, the testing pattern consist on
setting the input for the predicate being tested, calling it, and then
checking the results.  It is also possible to work with streams other
than the current input/output streams by using the ‘lgtunit’ predicate
variants that take a stream alias as argument.  For example, when
testing the standard ‘get_char/2’ predicate, we could write:

     test(get_char_2_01, true(Char == 'q')) :-
         ^^set_text_input(in, 'qwerty'),
         get_char(in, Char).

     test(get_char_2_02, true(Assertion)) :-
         ^^set_text_input(in, 'qwerty'),
         get_char(in, _Char),
         ^^text_input_assertion(in, 'werty', Assertion).

Testing output predicates follows a similar pattern by using instead the
‘set_text_output/1-2’ and ‘text_output_assertion/2-3’ predicates.  For
example:

     test(put_char_2_02, true(Assertion)) :-
         ^^set_text_output(out, 'qwert'),
         put_char(out, y),
         ^^text_output_assertion(out, 'qwerty', Assertion).

The ‘set_text_output/1’ predicate diverts only the standard output
stream (to a temporary file) using the standard ‘set_output/1’
predicate.  Most backend Prolog systems also support writing to the de
facto standard error stream.  But there’s no standard solution to divert
this stream.  However, several systems provide a ‘set_stream/2’ or
similar predicate that can be used for stream redirection.  For example,
assume that you wanted to test a backend Prolog system warning when an
‘initialization/1’ directive fails that is written to ‘user_error’.  An
hypothetical test could be:

     test(singletons_warning, true(Assertion)) :-
         ^^set_text_output(''),
         current_output(Stream),
         set_stream(Stream, alias(user_error)),
         consult(broken_file),
         ^^text_output_assertion('WARNING: initialization/1 directive failed', Assertion).

For testing binary input/output predicates, equivalent testing
predicates are provided.  There is also a small set of helper predicates
for dealing with stream handles and stream positions.  For testing with
files instead of streams, testing predicates are provided that allow
creating text and binary files with given contents and check text and
binary files for expected contents.

For more practical examples, check the included tests for Prolog
standard conformance of built-in input/output predicates.

5.13.17 Suppressing tested predicates output
--------------------------------------------

Sometimes predicates being tested output text or binary data that at
best clutters testing logs and at worse can interfere with parsing of
test logs.  If that output itself is not under testing, you can suppress
it by using the goals ‘^^suppress_text_output’ or
‘^^suppress_binary_output’ at the beginning of the tests.  For example:

     test(proxies_04, true(Color == yellow)) :-
         ^^suppress_text_output,
         {circle('#2', Color)}::print.

The ‘suppress_text_output/0’ and ‘suppress_binary_output/0’ predicates
work by redirecting standard output to the operating-system null device.
But the application may also output to e.g.  ‘user_error’ and other
streams.  If this output must also be suppressed, several alternatives
are described next.

Output of expected warnings can be suppressed by turning off the
corresponding linter flags.  In this case, it is advisable to restrict
the scope of the flag value changes as much as possible.

Output of expected compiler errors can be suppressed by defining
suitable clauses for the ‘logtalk::message_hook/4’ hook predicate.  For
example:

     :- multifile(logtalk::message_hook/4).
     :- dynamic(logtalk::message_hook/4).

     % ignore expected domain error
     logtalk::message_hook(compiler_error(_,_,error(domain_error(foo,bar),_)), error, core, _).

In this case, it is advisable to restrict the scope of the clauses as
much as possible to exact exception terms.  For the exact message terms,
see the ‘core_messages’ category source file.  Defining this hook
predicate can also be used to suppress all messages from a given
component.  For example:

     :- multifile(logtalk::message_hook/4).
     :- dynamic(logtalk::message_hook/4).

     logtalk::message_hook(_Message, _Kind, code_metrics, _Tokens).

Note that there’s no portable solution to suppress 'all' output.
However, several systems provide a ‘set_stream/2’ or similar predicate
that can be used for stream redirection.  Check the documentation of the
backend Prolog systems you’re using for details.

5.13.18 Tests with timeout limits
---------------------------------

There’s no portable way to call a goal with a timeout limit.  However,
some backend Prolog compilers provide this functionality:

   - B-Prolog: ‘time_out/3’ built-in predicate

   - ECLiPSe: ‘timeout/3’ and ‘timeout/7’ library predicates

   - LVM: ‘call_with_timeout/2-3’ built-in predicates

   - SICStus Prolog: ‘time_out/3’ library predicate

   - SWI-Prolog: ‘call_with_time_limit/2’ library predicate

   - Trealla Prolog: ‘call_with_time_limit/2’ and ‘time_out/3’ library
     predicates

   - XSB: ‘timed_call/2’ built-in predicate

   - YAP: ‘time_out/3’ library predicate

Logtalk provides a ‘timeout’ portability library implementing a simple
abstraction for those backend Prolog compilers.

The ‘logtalk_tester’ automation script accepts a timeout option that can
be used to set a limit per test set.

5.13.19 Setup and cleanup goals
-------------------------------

A test object can define ‘setup/0’ and ‘cleanup/0’ goals.  The ‘setup/0’
predicate is called, when defined, before running the object unit tests.
The ‘cleanup/0’ predicate is called, when defined, after running all the
object unit tests.  The tests are skipped when the setup goal fails or
throws an error.  For example:

     cleanup :-
         this(This),
         object_property(This, file(_,Directory)),
         atom_concat(Directory, serialized_objects, File),
         catch(ignore(os::delete_file(File)), _, true).

Per test setup and cleanup goals can be defined using the ‘test/3’
dialect and the ‘setup/1’ and ‘cleanup/1’ options.  The test is skipped
when the setup goal fails or throws an error.  Note that a broken test
cleanup goal doesn’t affect the test but may adversely affect any
following tests.  Variables in the setup and cleanup goals are shared
with the test body.

5.13.20 Test annotations
------------------------

It’s possible to define per unit and per test annotations to be printed
after the test results or when tests are skipped.  This is particularly
useful when some units or some unit tests may be run while still being
developed.  Annotations can be used to pass additional information to a
user reviewing test results.  By intercepting the unit test framework
message printing calls (using the ‘message_hook/4’ hook predicate), test
automation scripts and integrating tools can also access these
annotations.

Units can define a global annotation using the predicate ‘note/1’.  To
define per test annotations, use the ‘test/3’ dialect and the ‘note/1’
option.  For example, you can inform why a test is being skipped by
writing:

     - test(foo_1, true, [note('Waiting for Deep Thought answer')]) :-
         ...

Another common use is to return the execution time of one of the test
sub-goals.  For example:

     test(foobar, true, [note(bar(seconds-Time))]) :-
         foo(...),
         benchmark(bar(...), Time).

Annotations are written, by default, between parenthesis after and in
the same line as the test results.

5.13.21 Test execution times and memory usage
---------------------------------------------

Individual test CPU and wall execution times (in seconds) are reported
by default when running the tests.  Total CPU and wall execution times
for passed and failed tests are reported after the tests complete.
Starting and ending date and time when running a set of tests is also
reported by default.  The ‘lgtunit’ object also provides several public
benchmarking predicates that can be useful for e.g.  reporting test
sub-goals execution times using either CPU or wall clocks.  When running
multi-threaded code, the CPU time may or may not include all threads CPU
time depending on the backend.

Test memory usage is not reported by default due to the lack of a
portable solution to access memory data.  However, several backend
Prolog systems provide a ‘statistics/2’ or similar predicate that can be
used for a custom solution.  Depending on the system, individual keys
may be provided for each memory area (heap, trail, atom table, …).
Aggregating keys may also be provided.  As an hypothetical example,
assume you’re running Logtalk with a backend providing a ‘statistics/2’
predicate with a ‘memory_used’ key:

     test(ack_3, true(Result == 125), [note(memory-Memory)]) :-
         statistics(memory_used, Memory0),
         ack::ack(3, 4, Result),
         statistics(memory_used, Memory1),
         Memory is Memory1 - Memory0.

Consult the documentation of the backend Prolog systems for actual
details.

5.13.22 Working with test data files
------------------------------------

Frequently tests make use of test data files that are usually stored in
the test set directory or in sub-directories.  These data files are
referenced using their relative paths.  But to allow the tests to run
independently of the Logtalk process current directory, the relative
paths often must be expanded into an absolute path before being passed
to the predicates being tested.  The ‘file_path/2’ protected predicate
can be used in the test definitions to expand the relative paths.  For
example:

     % check that the encoding/1 option is accepted
     test(lgt_unicode_open_4_01, true) :-
         ^^file_path(sample_utf_8, Path),
         open(Path, write, Stream, [encoding('UTF-8')]),
         close(Stream).

The absolute path is computed relative to the path of 'self', i.e.
relative to the path of the test object that received the message that
runs the tests.

It’s also common for tests to create temporary files and directories
that should be deleted after the tests completion.  The ‘clean_file/1’
and ‘clean_directory/1’ protected predicates can be used for this
purpose.  For example, assuming that the tests create a ‘foo.txt’ text
file and a ‘tmp’ directory in the same directory of the tests object:

     cleanup :-
         ^^clean_file('foo.txt'),
         ^^clean_directory('tmp').

Similar to the ‘file_path/2’ predicate, relative paths are interpreted
as relative to the path of the test object.  This predicate also closes
any open stream connected to the file before deleting it.

5.13.23 Flaky tests
-------------------

Flaky tests are tests that pass or fail non-deterministically, usually
due to external conditions (e.g.  computer or network load).  Thus,
flaky tests often don’t result from bugs in the code being tested itself
but from test execution conditions that are not predictable.  The
‘flaky/0’ test option declares a test to be flaky.  For example:

     test(foo, true, [flaky]) :-
         ...

For backawards compatibility, the ‘note/1’ annotation can also be used
to alert that a test failure is for a flaky test when its argument is an
atom containing the sub-atom ‘flaky’.

The testing automation support outputs the text ‘[flaky]’ when reporting
failed flaky tests.  Moreover, the ‘logtalk_tester’ automation script
will ignore failed flaky tests when setting its exit status.

5.13.24 Mocking
---------------

Sometimes the code being tested performs complex tasks that are not
feasible or desirable when running tests.  For example, the code may
perform a login operation requiring the user to provide a username and a
password using some GUI widget.  In this case, the tests may required
the login operation to still be performed but using canned data (also
simplifying testing automation).  I.e.  we want to 'mock' (as in
'imitate') the login procedure.  Ideally, this should be accomplished
without requiring any changes to the code being tested.  Logtalk
provides two solutions that can be used for mocking: 'term-expansion'
and 'hot patching'.  A third solution is possible if the code we want to
mock uses the 'message printing mechanism'.

Using the term-expansion mechanism, we would define a 'hook object' that
expands the login predicate into a fact:

     :- object(mock_login,
         implements(expanding)).

         term_expansion((login(_, _) :- _), login(jdoe, test123)).

     :- end_object.

The tests driver file would then load the application object responsible
for user management using this hook object:

     :- initialization((
         ...,
         logtalk_load(mock_login),
         logtalk_load(user_management, [hook(mock_login)]),
         ...
     )).

Using hot patching, we would define a 'complementing category' patching
the object that defines the login predicate:

     :- category(mock_login,
         complements(user_management)).

         login(jdoe, test123).

     :- end_category.

The tests driver file would then set the ‘complements’ flag to ‘allow’
and load the patch after loading application code:

     :- initialization((
         ...,
         set_logtalk_flag(complements, allow),
         logtalk_load(application),
         logtalk_load(mock_login),
         ...
     )).

There are pros and cons for each solution.  Term-expansion works by
defining hook objects that are used at compile time while hot patching
happens at runtime.  Complementing categories can also be dynamically
created, stacked, and abolished.  Hot patching disables static binding
optimizations but that’s usually not a problem as the code being tested
if often compiled in debug mode to collect code coverage data.  Two
advantages of the term-expansion solution is that it allows defining
conditions for expanding terms and goals and can replace both predicate
definitions and predicate calls.  Limitations in the current Prolog
standards prevent patching callers to local predicates being patched.
But often both solutions can be used with the choice depending on code
clarity and user preference.  See the Handbook sections on
term-expansion and hot patching for more details on these mechanisms.

In those cases where the code we want to mock uses the message printing
mechanism, the solution is to intercept and rewrite the messages being
printed and/or the questions being asked using the
‘logtalk::message_hook/4’ and ‘logtalk::question_hook/6’ hook
predicates.

5.13.25 Debugging messages in tests
-----------------------------------

Sometimes is useful to write debugging or logging messages from tests
when running them manually.  But those messages are better suppressed
when running the tests automated.  A common solution is to use debug
'meta-messages'.  For example:

     :- uses(logtalk, [
         print_message(debug, my_app, Message) as dbg(Message)
     ]).

     test(some_test_id, ...) :-
         ...,
         dbg('Some intermediate value'-Value),
         ...,
         dbg([Stream]>>custom_print_goal(Stream, ...)),
         ...

The messages are only printed (and the user-defined printing goals are
only called) when the ‘debug’ flag is turned on.  Note that this doesn’t
require compiling the tests in debug mode: you simply toggle the flag to
toggle the debug messages.  Also note that the ‘print_message/3’ goals
are suppressed by compiler when compiling with the ‘optimize’ flag
turned on.

5.13.26 Debugging failed tests
------------------------------

Debugging of failed unit tests is simplified by using test assertions as
the reason for the assertion failures is printed out.  Thus, use
preferably the ‘test/2-3’ dialects with ‘true(Assertion)’,
‘deterministic(Assertion)’, ‘subsumes(Expected, Result)’, or
‘variant(Term1, Term2)’ outcomes.  If a test checks multiple assertions,
you can use the predicate ‘assertion/2’ in the test body.  In the case
of QuickCheck tests, the ‘v(true)’ verbose option can be used to print
the generated test case that failed if necessary.

If the assertion failures don’t provide enough information, you can use
the ‘debugger’ tool to debug failed unit tests.  Start by compiling the
unit test objects and the code being tested in debug mode.  Load the
debugger and trace the test that you want to debug.  For example,
assuming your tests are defined in a ‘tests’ object and that the
identifier of test to be debugged is ‘test_foo’:

     | ?- logtalk_load(debugger(loader)).
     ...

     | ?- debugger::trace.
     ...

     | ?- tests::run(test_foo).
     ...

You can also compile the code and the tests in debug mode but without
using the ‘hook/1’ compiler option for the tests compilation.  Assuming
that the ‘context_switching_calls’ flag is set to ‘allow’, you can then
use the ‘(<<)/2’ debugging control construct to debug the tests.  For
example, assuming that the identifier of test to be debugged is
‘test_foo’ and that you used the ‘test/1’ dialect:

     | ?- logtalk_load(debugger(loader)).
     ...

     | ?- debugger::trace.
     ...

     | ?- tests<<test(test_foo).
     ...

In the more complicated cases, it may be worth to define
‘loader_debug.lgt’ and ‘tester_debug.lgt’ driver files that load code
and tests in debug mode and also load the debugger.

5.13.27 Code coverage
---------------------

If you want entity predicate clause coverage information to be collected
and printed, you will need to compile the entities that you’re testing
using the flags ‘debug(on)’ and ‘source_data(on)’.  Be aware, however,
that compiling in debug mode results in a performance penalty.

A single test object may include tests for one or more entities
(objects, protocols, and categories).  The entities being tested by a
unit test object for which code coverage information should be collected
must be declared using the ‘cover/1’ predicate.  For example, to collect
code coverage data for the objects ‘foo’ and ‘bar’ include in the tests
object the two clauses:

     cover(foo).
     cover(bar).

Code coverage is listed using the predicates clause indexes (counting
from one).  For example, using the ‘points’ example in the Logtalk
distribution:

     % point: default_init_option/1 - 2/2 - (all)
     % point: instance_base_name/1 - 1/1 - (all)
     % point: move/2 - 1/1 - (all)
     % point: position/2 - 1/1 - (all)
     % point: print/0 - 1/1 - (all)
     % point: process_init_option/1 - 1/2 - [1]
     % point: position_/2 - 0/0 - (all)
     % point: 7 out of 8 clauses covered, 87.500000% coverage

The numbers after the predicate indicators represents the clauses
covered and the total number of clauses.  E.g.  for the
‘process_init_option/1’ predicate, the tests cover 1 out of 2 clauses.
After these numbers, we either get ‘(all)’ telling us that all clauses
are covered or a list of indexes for the covered clauses.  E.g.  only
the first clause for the ‘process_init_option/1’ predicate, ‘[1]’.
Summary clause coverage numbers are also printed for entities and for
clauses across all entities.

In the printed predicate clause coverage information, you may get a
total number of clauses smaller than the covered clauses.  This results
from the use of dynamic predicates with clauses asserted at runtime.
You may easily identify dynamic predicates in the results as their
clauses often have an initial count equal to zero.

The list of indexes of the covered predicate clauses can be quite long.
Some backend Prolog compilers provide a flag or a predicate to control
the depth of printed terms that can be useful:

   - CxProlog: ‘write_depth/2’ predicate

   - ECLiPSe: ‘print_depth’ flag

   - LVM 3.2.0 or later: ‘answer_write_options’ flag

   - SICStus Prolog: ‘toplevel_print_options’ flag

   - SWI-Prolog 7.1.10 or earlier: ‘toplevel_print_options’ flag

   - SWI-Prolog 7.1.11 or later: ‘answer_write_options’ flag

   - Trealla Prolog: ‘answer_write_options’ flag

   - XSB: ‘set_file_write_depth/1’ predicate

   - YAP: ‘write_depth/2-3’ predicates

Code coverage is only available when testing Logtalk code.  But Prolog
modules can often be compiled as Logtalk objects and plain Prolog code
may be wrapped in a Logtalk object.  For example, assuming a ‘module.pl’
module file, we can compile and load the module as an object by simply
calling:

     | ?- logtalk_load(module).
     ...

The module exported predicates become object public predicates.  For a
plain Prolog file, say ‘plain.pl’, we can define a Logtalk object that
wraps the code using an ‘include/1’ directive:

     :- object(plain).

         :- include('plain.pl').

     :- end_object.

The object can also declare as public the top Prolog predicates to
simplify writing the tests.  In alternative, we can use the
‘object_wrapper_hook’ provided by the ‘hook_objects’ library:

     | ?- logtalk_load(hook_objects(loader)).
     ...

     | ?- logtalk_load(plain, [hook(object_wrapper_hook)]).
     ...

These workarounds may thus allow generating code coverage data also for
Prolog code by defining tests that use the ‘(<<)/2’ debugging control
construct to call the Prolog predicates.

See also the section below on exporting code coverage results to XML
files, which can be easily converted and published as e.g.  HTML
reports.

5.13.28 Utility predicates
--------------------------

The ‘lgtunit’ tool provides several public utility predicates to
simplify writing unit tests and for general use:

   -
          ‘variant(Term1, Term2)’
          To check when two terms are a variant of each other (e.g. to check expected test results against actual results when they contain variables).

   -
          ‘assertion(Goal)’
          To generate an exception in case the goal argument fails or throws an error.

   -
          ‘assertion(Description, Goal)’
          To generate an exception in case the goal argument fails or throws an error (the first argument allows assertion failures to be distinguished when using multiple assertions).

   -
          ‘approximately_equal(Number1, Number2)’
          For number approximate equality using the ‘epsilon’ arithmetic constant value.

   -
          ‘approximately_equal(Number1, Number2, Epsilon)’
          For number approximate equality. Weaker equality than essential equality.

   -
          ‘essentially_equal(Number1, Number2, Epsilon)’
          For number essential equality. Stronger equality than approximate equality.

   -
          ‘tolerance_equal(Number1, Number2, RelativeTolerance, AbsoluteTolerance)’
          For number equality within tolerances.

   -
          ‘Number1 =~= Number2’
          For number (or list of numbers) close equality (usually floating-point numbers).

   -
          ‘benchmark(Goal, Time)’
          For timing a goal.

   -
          ‘benchmark_reified(Goal, Time, Result)’
          Reified version of ‘benchmark/2’.

   -
          ‘benchmark(Goal, Repetitions, Time)’
          For finding the average time to prove a goal.

   -
          ‘benchmark(Goal, Repetitions, Clock, Time)’
          For finding the average time to prove a goal using a ‘cpu’ or a ‘wall’ clock.

   -
          ‘deterministic(Goal)’
          For checking that a predicate succeeds without leaving a choice-point.

   -
          ‘deterministic(Goal, Deterministic)’
          Reified version of the ‘deterministic/1’ predicate.

The ‘assertion/1-2’ predicates can be used in the body of tests where
using two or more assertions is convenient or in the body of tests
written using the ‘test/1’, ‘succeeds/1’, and ‘deterministic/1’ dialects
to help differentiate between the test goal and checking the test goal
results and to provide more informative test failure messages.

When the assertion, benchmarking, and deterministic meta-predicates call
a local predicate of the tests object, you must call them using an
implicit or explicit message instead of a using 'super' call.  For
example, to use an implicit message to call the ‘assertion/1-2’
meta-predicates, add the following directive to the tests object:

     :- uses(lgtunit, [assertion/1, assertion/2]).

The reason this is required is that meta-predicates goals arguments are
always called in the context of the 'sender', which would be the
‘lgtunit’ object in the case of a ‘(^^)/2’ call (as it preserves both
'self' and 'sender' and the tests are internally run by a message sent
from the ‘lgtunit’ object to the tests object).

As the ‘benchmark/2-4’ predicates are meta-predicates, turning on the
‘optimize’ compiler flag is advised to avoid runtime compilation of the
meta-argument, which would add an overhead to the timing results.  But
this advice conflicts with collecting code coverage data, which requires
compilation in debug mode.  The solution is to use separate test objects
for benchmarking and for code coverage.  Note that the CPU and wall
execution times (in seconds) for each individual test are reported by
default when running the tests.

The ‘(=~=)/2’ predicate is typically used by adding the following
directive to the object (or category) calling it:

     :- uses(lgtunit, [
         op(700, xfx, =~=), (=~=)/2
     ]).

Consult the ‘lgtunit’ object API documentation for more details on these
predicates.

5.13.29 Exporting test results in xUnit XML format
--------------------------------------------------

To output test results in the xUnit XML format (from JUnit; see e.g.
‘https://github.com/windyroad/JUnit-Schema’ or
‘https://llg.cubic.org/docs/junit/’), simply load the ‘xunit_output.lgt’
file before running the tests.  This file defines an object,
‘xunit_output’, that intercepts and rewrites unit test execution
messages, converting them to the xUnit XML format.

To export the test results to a file using the xUnit XML format, simply
load the ‘xunit_report.lgt’ file before running the tests.  A file named
‘xunit_report.xml’ will be created in the same directory as the object
defining the tests.  When running a set of test suites as a single
unified suite (using the ‘run_test_sets/1’ predicate), the single xUnit
report is created in the directory of the first test suite object in the
set.

To use in alternative the xUnit.net v2 XML format
(‘https://xunit.net/docs/format-xml-v2’), load either the
‘xunit_net_v2_output.lgt’ file or the ‘xunit_net_v2_report.lgt’ file.

When using the ‘logtalk_tester’ automation script, use either the ‘-f
xunit’ option or the ‘-f xunit_net_v2’ option to generate the
‘xunit_report.xml’ files on the test set directories.

There are several third-party xUnit report converters that can generate
HTML files for easy browsing.  For example:

   - ‘https://docs.qameta.io/allure-report/’ (supports multiple reports)

   - ‘https://github.com/Zir0-93/xunit-to-html’ (supports multiple test
     sets in a single report)

   - ‘https://www.npmjs.com/package/xunit-viewer’

   - ‘https://github.com/JatechUK/NUnit-HTML-Report-Generator’

   - ‘https://plugins.jenkins.io/xunit’

5.13.30 Exporting test results in the TAP output format
-------------------------------------------------------

To output test results in the TAP (Test Anything Protocol) format,
simply load the ‘tap_output.lgt’ file before running the tests.  This
file defines an object, ‘tap_output’, that intercepts and rewrites unit
test execution messages, converting them to the TAP output format.

To export the test results to a file using the TAP (Test Anything
Protocol) output format, load instead the ‘tap_report.lgt’ file before
running the tests.  A file named ‘tap_report.txt’ will be created in the
same directory as the object defining the tests.

When using the ‘logtalk_tester’ automation script, use the ‘-f tap’
option to generate the ‘tap_report.xml’ files on the test set
directories.

When using the ‘test/3’ dialect with the TAP format, a ‘note/1’ option
whose argument is an atom starting with a ‘TODO’ or ‘todo’ word results
in a test report with a TAP TODO directive.

When running a set of test suites as a single unified suite, the single
TAP report is created in the directory of the first test suite object in
the set.

There are several third-party TAP report converters that can generate
HTML files for easy browsing.  For example:

   - ‘https://github.com/Quobject/tap-to-html’

   - ‘https://plugins.jenkins.io/tap/’

5.13.31 Generating Allure reports
---------------------------------

A ‘logtalk_allure_report.pl’ Bash shell script and a
‘logtalk_allure_report.ps1’ PowerShell script are provided for
generating Allure reports(1).  This requires exporting test results in
xUnit XML format.  A simple usage example (assuming a current directory
containing tests):

     $ logtalk_tester -p gnu -f xunit
     $ logtalk_allure_report
     $ allure open

The ‘logtalk_allure_report’ script supports command-line options to pass
the tests directory (i.e.  the directory where the ‘logtalk_tester’
script was run), the directory where to collect all the xUnit report
files for generating the report, the directory where the report is to be
saved, and the report title (see the script man page or type
‘logtalk_allure_report -h’).  The script also supports saving the
history of past test runs.  In this case, a persistant location for both
the results and report directories must be used.

It’s also possible to use the script just to collect the xUnit report
files generated by ‘lgtunit’ and delegate the actual generation of the
report to e.g.  an Allure Docker container or to a Jenkins plug-in.  Two
examples are:

   - ‘https://github.com/fescobar/allure-docker-service’

   - ‘https://plugins.jenkins.io/allure-jenkins-plugin/’

In this case, we would use the ‘logtalk_allure_report’ script option to
only perform the preprocessing step:

     $ logtalk_allure_report -p

The scripts also supports passing 'environment pairs', which are
displayed in the generated Allure reports in the environment pane.  This
feature can be used to pass e.g.  the backend name and the backend
version or git commit hash.  The option syntax differs, however, between
the two scripts.  For example, using the Bash script:

     $ logtalk_allure_report -- Backend='GNU Prolog' Version=1.5.0

Or:

     $ logtalk_allure_report -- Project='Deep Thought' Commit=`git rev-parse --short HEAD`

In the case of the PowerShell script, the pairs are passed comma
separated inside a string:

     PS> logtalk_allure_report -e "Backend='GNU Prolog',Version=1.5.0"

Or:

     PS> logtalk_allure_report -e "Project='Deep Thought',Commit=bf166b6"

To show tests run trends in the report (e.g.  when running the tests for
each application source code commit), save the processed test results
and the report data to permanent directories.  For example:

     $ logtalk_allure_report \
       -i "$HOME/my_project/allure-results" \
       -o "$HOME/my_project/allure-report"
     $ allure open "$HOME/my_project/allure-report"

Note that Allure cleans the report directory when generating a new
report.  Be careful to always specify a dedicated directory to prevent
accidental data loss.

When using the Allure 2.21.0 or a later version, it’s possible to
generate reports with links to the tests source code.  This requires
using the ‘logtalk_tester’ shell script option that allows passing the
base URL for those links.  This option needs to be used together with
the option to suppress the tests directory prefix so that the links can
be constructed by appending the tests file relative path to the base
URL. For example, assuming that you want to generate a report for the
tests included in the Logtalk distribution when using the GNU Prolog
backend:

     $ cd $LOGTALKUSER
     $ logtalk_tester \
       -p gnu \
       -f xunit \
       -s "$LOGTALKUSER" \
       -u "https://github.com/LogtalkDotOrg/logtalk3/tree/3e4ea295986fb09d0d4aade1f3b4968e29ef594e"

The use of a git hash in the base URL ensures that the generated links
will always show the exact versions of the tests that were run.  The
links include the line number for the tests in the tests files (assuming
that the git repo is stored in a BitBucket, GitHub, or GitLab server).
But note that not all supported backends provide accurate line numbers.

When using the Allure 2.24.0 or a later version, it’s possible to
generate single file reports.  For example:

     $ logtalk_allure_report -s -t "My Amazing Tests Report"

There are some caveats when generating Allure reports that users must be
aware.  First, Allure expects test names to be unique across different
tests sets.  If there are two test with the same name in two different
test sets, only one of them will be reported.  Second, when using the
‘xunit’ format, dates are reported as MM/DD/YYYY. Finally, when using
the ‘xunit_net_v2’ format, tests are reported in a random order instead
of their run order and dates are displayed as “unknown” in the overview
page.

   ---------- Footnotes ----------

   (1) https://docs.qameta.io/allure-report/

5.13.32 Exporting code coverage results in XML format
-----------------------------------------------------

To export code coverage results in XML format, load the
‘coverage_report.lgt’ file before running the tests.  A file named
‘coverage_report.xml’ will be created in the same directory as the
object defining the tests.

The XML file can be opened in most web browsers (with the notorious
exception of Google Chrome) by copying to the same directory the
‘coverage_report.dtd’ and ‘coverage_report.xsl’ files found in the
‘tools/lgtunit’ directory (when using the ‘logtalk_tester’ script, these
two files are copied automatically).  In alternative, an XSLT processor
can be used to generate an XHTML file instead of relying on a web
browser for the transformation.  For example, using the popular
‘xsltproc’ processor:

     $ xsltproc -o coverage_report.html coverage_report.xml

On Windows operating-systems, this processor can be installed using e.g.
Chocolatey.  On a POSIX operating-systems (e.g.  Linux, macOS, …) use
the system package manager to install it if necessary.

The coverage report can include links to the source code when hosted on
Bitbucket, GitHub, or GitLab.  This requires passing the base URL as the
value for the ‘url’ XSLT parameter.  The exact syntax depends on the
XSLT processor, however.  For example:

     $ xsltproc \
       --stringparam url https://github.com/LogtalkDotOrg/logtalk3/blob/master \
       -o coverage_report.html coverage_report.xml

Note that the base URL should preferably be a permanent link (i.e.  it
should include the commit SHA1) so that the links to source code files
and lines remain valid if the source code is later updated.  It’s also
necessary to suppress the local path prefix in the generated
‘coverage_report.xml’ file.  For example:

     $ logtalk_tester -c xml -s $HOME/logtalk/

Alternatively, you can pass the local path prefix to be suppressed to
the XSLT processor (note that the ‘logtalk_tester’ script suppresses the
‘$HOME’ prefix by default):

     $ xsltproc \
       --stringparam prefix logtalk/ \
       --stringparam url https://github.com/LogtalkDotOrg/logtalk3/blob/master \
       -o coverage_report.html coverage_report.xml

If you are using Bitbucket, GitHub, or GitLab hosted in your own
servers, the ‘url’ parameter may not contain a ‘bitbucket’, ‘github’, or
‘gitlab’ string.  In this case, you can use the XSLT parameter ‘host’ to
indicate which service are you running.

5.13.33 Automatically creating bug reports at issue trackers
------------------------------------------------------------

To automatically create bug report issues for failed tests in GitHub or
GitLab servers, see the ‘issue_creator’ tool.

5.13.34 Minimizing test results output
--------------------------------------

To minimize the test results output, simply load the
‘minimal_output.lgt’ file before running the tests.  This file defines
an object, ‘minimal_output’, that intercepts and summarizes the unit
test execution messages.

5.13.35 Known issues
--------------------

Deterministic unit tests are currently not available when using Quintus
Prolog as it lacks built-in support that cannot be sensibly defined in
Prolog.

Parameter variables (‘_VariableName_’) cannot currently be used in the
definition of the ‘condition/1’, ‘setup/1’, and ‘cleanup/1’ test options
when using the ‘test/3’ dialect.  For example, the following condition
will not work:

     test(some_id, true, [condition(_ParVar_ == 42)]) :-
         ...

The workaround is to define an auxiliary predicate called from those
options.  For example:

     test(check_xyz, true, [condition(xyz_condition)]) :-
         ...

     xyz_condition :-
         _ParVar_ == 42.

5.14 ‘linter’
=============

Logtalk provides a built-in linter tool that runs automatically when
compiling and loading source files.  The lint warnings are controlled by
a set of flags(1).  The default values for these flags are defined in
the backend Prolog compiler adapter files and can be overriden from a
settings file or from a source file (e.g.  a loader file).  These flags
can be set globally using the set_logtalk_flag/2(2) built-in predicate.
For (source file or entity) local scope, use instead the
set_logtalk_flag/2(3) directive.

Some lint checks are turned off by default, specially when
computationally expensive.  Still, it’s a good idea to turn them on to
check your code on a regular basis (e.g.  in CI/CD pipelines).

Note that, in some cases, the linter may generate false warnings due to
source code analysis limitations or special cases that, while valid when
intended, usually result from programming issues.  When a code rewrite
is not a sensible solution to avoid the warning, the workaround is to
turn off as locally as possible the flag that controls the warning.

   ---------- Footnotes ----------

   (1) ../userman/programming.html#programming-flags-lint

   (2) ../refman/predicates/set_logtalk_flag_2.html

   (3) ../refman/directives/set_logtalk_flag_2.html

5.14.1 Main linter checks
-------------------------

Lint checks include:

   - Missing directives (including scope, meta-predicate, dynamic,
     discontiguous, and multifile directives)

   - Duplicated directives, clauses, and grammar rules

   - Missing predicates (unknown messages plus calls to non-declared and
     non-defined predicates)

   - Calls to declared but not defined static predicates

   - Non-terminals called as predicates (instead of via the ‘phrase/2-3’
     built-in methods)

   - Predicates called as non-terminals (instead of via the ‘call//1’
     built-in method)

   - Non-portable predicate calls, predicate options, arithmetic
     function calls, directives, flags, and flag values

   - Missing arithmetic functions (with selected backends)

   - Suspicious calls (syntactically valid calls that are likely
     semantic errors; e.g.  float comparisons using the standard
     arithmetic comparison operators or comparing numbers using
     unification)

   - Deprecated directives, predicates, arithmetic functions, control
     constructs, and flags

   - References to unknown entities (objects, protocols, categories, or
     modules)

   - Top-level shortcuts used as directives

   - Unification goals that will succeed without binding any variables

   - Unification goals that will succeed creating a cyclic term

   - Goals that are always true or always false

   - Trivial goal fails (due to no matching predicate clause)

   - Redefined built-in predicates

   - Redefined standard operators

   - Lambda expression unclassified variables and mixed up variables

   - Lambda expression with parameter variables used elsewhere in a
     clause

   - Singleton variables

   - If-then-else and soft cut control constructs without an else part

   - If-then-else and soft cut control constructs where the test is a
     unification between a variable and a ground term

   - Missing parenthesis around if-then-else and disjunction control
     constructs in the presence of cuts in the first argument

   - Cuts in clauses for multifile predicates

   - Missing cut in repeat loops

   - Possible non-steadfast predicate definitions

   - Non-tail recursive predicate definitions

   - Redundant calls to control constructs and built-in predicates

   - Calls to all-solutions predicates with existentially qualified
     variables not occurring in the qualified goal

   - Calls to all-solutions predicates with no shared variables between
     template and goal

   - Calls to ‘bagof/3’ and ‘setof/3’ where the goal argument contains
     singleton variables

   - Calls to ‘findall/3’ used to backtrack over all solutions of a goal
     without collecting them

   - Calls to ‘catch/3’ that catch all exceptions

   - Calls to standard predicates that have more efficient alternatives

   - Unsound calls in grammar rules

   - File, entity, predicate, and variable names not following official
     coding guidelines

   - Variable names that differ only on case

   - Clauses whose body is a disjunction (and that can be rewritten as
     multiple clauses per coding guidelines)

   - Naked meta-variables in cut-transparent control constructs

   - Left-recursion in clauses and grammar rules

Additional lint checks are provided by the ‘lgtunit’, ‘lgtdoc’, ‘make’,
and ‘dead_code_scanner’ tools.  For large projects, the data generated
by the ‘code_metrics’ tool may also be relevant in accessing code
quality and suggesting code refactoring candidates.

5.14.2 Help on linter warnings
------------------------------

By loading the ‘tutor’ tool, most lint warnings are expanded with
explanations and suggestions on how to fix the reported issues.  See
also the coding guidelines(1) for additional explanations.

   ---------- Footnotes ----------

   (1) https://logtalk.org/coding_style_guidelines.html

5.14.3 Extending the linter
---------------------------

Experimental support for extending the linter with user-defined warnings
is available using the logtalk_linter_hook/7(1) multifile hook
predicate.  For example, the ‘format’ and ‘list’ library objects define
this hook predicate to lint calls to the ‘format/2-3’ and ‘append/3’
predicates for common errors and misuses.

   ---------- Footnotes ----------

   (1) ../refman/predicates/logtalk_linter_hook_7.html

5.14.4 Linting Prolog modules
-----------------------------

This tool can also be applied to Prolog modules that Logtalk is able to
compile as objects.  For example, if the Prolog module file is named
‘module.pl’, try:

     | ?- logtalk_load(module, [source_data(on)]).

Due to the lack of standardization of module systems and the abundance
of proprietary extensions, this solution is not expected to work for all
cases.

5.14.5 Linting plain Prolog files
---------------------------------

This tool can also be applied to plain Prolog code.  For example, if the
Prolog file is named ‘code.pl’, simply define an object including its
code:

     :- object(code).
         :- include('code.pl').
     :- end_object.

Save the object to an e.g.  ‘code.lgt’ file in the same directory as the
Prolog file and then load it:

     | ?- logtalk_load(code, [source_data(on)]).

In alternative, use the ‘object_wrapper_hook’ provided by the
‘hook_objects’ library:

     | ?- logtalk_load(hook_objects(loader)).
     ...

     | ?- logtalk_load(code, [hook(object_wrapper_hook), source_data(on)]).

With either wrapping solution, pay special attention to any compilation
warnings that may signal issues that could prevent the plain Prolog from
being fully checked when wrapped by an object.

5.15 ‘make’
===========

Logtalk provides a make tool supporting several targets using the
‘logtalk_make/0-1’ built-in predicates.  Top-level shortcuts for the
targets are also provided.

5.15.1 API documentation
------------------------

To consult the documentation of the ‘logtalk_make/0-1’ built-in
predicates, open in a web browser the links:

   - ‘../refman/predicates/logtalk_make_0.html’

   - ‘../refman/predicates/logtalk_make_1.html’

There is also a user-defined hook predicate that supports defining
additional actions for the make targets (e.g.  running tests
automatically on make check or regenerating API documentation on make
documentation):

   - ‘../refman/predicates/logtalk_make_target_action_1.html’

5.16 ‘packs’
============

This tool provides predicates for downloading, installing, upgrading,
and uninstalling third-party libraries and applications, generically
known as 'packs'.  Collections of pack specifications are made available
using 'registries'.  Registries can be local to a system, publicly
shared, or private to a company (e.g.  only available in a VPN). There
is no concept of a central registry.  Users decide which registries they
trust and want to use and add them using their published URLs.  The tool
supports both pack checksums and signatures and takes several steps to
sanitize registry and pack specifications.  As other Logtalk developer
tools, portability is a main goal.  This tool can be used with any
supported Prolog backend and run in both POSIX and Windows systems.
Moreover, this tool can be used not only for handling Logtalk packs but
also Prolog only packs, thus providing a solution for sharing portable
resources between multiple systems.

A list of public Logtalk and Prolog pack registries is available at:

‘https://github.com/LogtalkDotOrg/pack-registries’

This tool is the beta stage of development.  Feedback is most welcome.

5.16.1 Requirements
-------------------

On POSIX systems (Linux, macOS, …), the following shell commands are
required:

   - ‘sha256sum’ (provided by GNU ‘coreutils’)

   - ‘curl’

   - ‘bsdtar’ (provided by ‘libarchive’ or ‘libarchive-tools’)

   - ‘gpg’ (provided by ‘gnupg2’)

   - ‘git’

   - ‘direnv’ (when using virtual environments)

The tool uses ‘bsdtar’ instead of GNU ‘tar’ so that it can uncompress
‘.zip’ archives (‘unzip’ doesn’t provide the desired options that allows
a simple and reliable solution for ignoring the non-predictable name of
the wrapper directory).

On Windows systems, the following shell commands are required:

   - ‘certutil.exe’

   - ‘curl.exe’

   - ‘tar.exe’

   - ‘gpg.exe’

   - ‘git.exe’

   - ‘Set-PsEnv’ (when using virtual environments)

In recent Windows 10 builds, only ‘gpg’, ‘git’, and ‘Set-PsEnv’ should
require installation.  You can download the GnuPG software from:

‘https://www.gnupg.org/’

You can download Git from:

‘https://gitforwindows.org’

To install Set-PsEnv(1) from the PowerShell Gallery:

     PS> Install-Module -Name Set-PsEnv

On macOS systems, Apple bundles both ‘curl’ and BSD ‘tar’ (under the
name ‘tar’; you can simply create a ‘bsdtar’ alias or install a more
recent version).  The required commands can be easily installed using
MacPorts:

     $ sudo port install coreutils libarchive gnupg2 git direnv

Or using Homebrew:

     $ brew install coreutils libarchive gnupg2 git direnv

On Linux systems, use the distribution own package manager to install
any missing command.  For example, in recent Ubuntu versions:

     $ sudo apt update
     $ sudo apt install coreutils curl libarchive-tools gnupg2 git direnv

   ---------- Footnotes ----------

   (1) https://github.com/rajivharris/Set-PsEnv

5.16.2 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#packs’

5.16.3 Loading
--------------

This tool can be loaded using the query:

     | ?- logtalk_load(packs(loader)).

5.16.4 Testing
--------------

To run the tool tests, use the query:

     | ?- logtalk_load(packs(tester)).

The tests can be run without interfering with the user packs setup.

5.16.5 Usage
------------

The ‘packs’ tool loads at startup all the currently defined registry and
pack specifications (from the registries/packs storage directory; see
below).  When no registry/pack setup exists, a new one is automatically
created.

The tool provides two main objects, ‘registries’ and ‘packs’, for
handling, respectively, registries and packs.  Both objects accept a
‘help/0’ message that describes the most common queries.

5.16.6 Registries and packs storage
-----------------------------------

The tool uses a directory specified using the ‘logtalk_packs’ library
alias when defined (in a settings file or in a backend Prolog
initialization file).  When this library alias is not defined, the tool
uses the value of the ‘LOGTALKPACKS’ environment variable when defined.
Otherwise it defaults to the ‘~/logtalk_packs’ directory.  The actual
directory can be retrieved by the query:

     | ?- packs::logtalk_packs(Directory).
     ...

This directory holds sub-directories for registries, packs, and
archives.  These sub-directories are automatically created when loading
the ‘packs’ tool if they don’t exist .  Users shouldn’t manually modify
the contents of these directories.  Multiple and independent
registry/pack setups are possible using 'virtual environments' as
explained next.

Your registries and packs setup can be saved and restored (e.g.  in a
different system) by using the ‘packs::save/1-2’ and
‘packs::restore/1-2’ predicates, as explained in the next section about
virtual environments.  If necessary, before restoring, the
‘packs::reset/0’ predicate can be called to delete any defined
registries and installed packs.

5.16.7 Virtual environments
---------------------------

An application may require specific pack versions.  These requirements
may differ between applications.  Different applications may also have
conflicting requirements.  Therefore, a 'virtual environment' where an
application requirements are fulfilled may be required to develop and/or
run it.  A virtual environment is essentially a registries/packs storage
directory.

Defining the ‘logtalk_packs’ library alias in a settings file or
defining the ‘LOGTALKPACKS’ environment variable before starting Logtalk
allows easy creation and switching between virtual environments.  By
using a per application settings file (or a per application environment
variable definition) each application can thus use its own virtual
environment.  The ‘settings.lgt’ file can define the ‘logtalk_packs’
library alias using code such as:

     :- initialization((
         logtalk_load_context(directory, Directory),
         assertz(logtalk_library_path(logtalk_packs, Directory))
     )).

The definition of the ‘logtalk_packs’ library alias 'must' always be an
atom and thus never use library notation (i.e.  it must never depend on
other library aliases).

When a virtual environment also requires a specific Logtalk version
(e.g.  the version used to test and certify it), this can be installed
as a pack from the official talkshow(1) registry and used by
(re)defining the ‘LOGTALKHOME’ and ‘LOGTALKUSER’ environment variables
to point to its pack directory (which can be queried by using the
‘packs::directory/2’ message).

Experimental ‘lgtenv.sh’ and ‘lgtenv.ps1’ scripts are included to
simplify creating virtual environments.  For example:

     $ lgtenv -d ~/my_venv -c
     $ cd ~/my_venv/
     direnv: loading ~/my_venv/.envrc
     direnv: export +LOGTALKPACKS

These scripts require, respectively, direnv(2) and Set-PsEnv(3) to be
installed.  These utilities load and unload environment variables when
changing the current directory.  On Windows systems, when using the
‘lgtenv.ps1’ script, you also need to redefine the PowerShell prompt in
a profile file (e.g.  ‘$HOME\Documents\PowerShell\Profile.ps1’) to mimic
the functionality of ‘direnv’ of automatically loading an existing
‘.env’ file when changing to its directory.  For example:

     function prompt {
         Set-PsEnv
         'PS ' + $(Get-Location) + '> '
     }

A virtual environment setup (i.e.  the currently defined registries and
installed packs) can be saved into a file (e.g.  ‘requirements.lgt’)
using the ‘packs::save/1’ predicate:

     | ?- packs::save('requirements.lgt').
     ...

This query saves a listing of all the installed packs and their
registries.  Using the saved file, the virtual environment setup can
then be restored using the ‘packs::restore/1-2’ predicates.  The file
uses a simple format with ‘registry/2’, ‘pack/3’, ‘pinned_registry/1’,
and ‘pinned_pack/1’ facts (in this order) and can be manually created or
edited if necessary.  For example:

     registry(talkshow, 'https://github.com/LogtalkDotOrg/talkshow.git').
     pack(talkshow, logtalk, 3:45:0).
     pack(talkshow, lflat, 2:1:0).

These files can be distributed with applications so that users can
easily fulfill application requirements by running once the query:

     | ?- packs::restore('requirements.lgt').

After, the application ‘loader.lgt’ file can then load the required
packs using their loader files:

     :- initialization((
         % load required packs
         logtalk_load(foo(loader)),
         logtalk_load(bar(loader)),
         ...
         % load application files
         ...
     )).

   ---------- Footnotes ----------

   (1) https://github.com/LogtalkDotOrg/talkshow

   (2) https://github.com/direnv/direnv

   (3) https://github.com/rajivharris/Set-PsEnv

5.16.8 Registry specification
-----------------------------

A registry is a git remote repo that can be cloned, a downloadable
archive, or a local directory containing a Logtalk loader file that
loads source files defining the registry itself and the packs it
provides.  The registry name is ideally a valid unquoted atom.  The
registry directory must contain at least two Logtalk source files:

   - A file defining an object named after the registry with a
     ‘_registry’ suffix, implementing the ‘registry_protocol’.  This
     naming convention helps preventing name conflicts.

   - A loader file (named ‘loader.lgt’ or ‘loader.logtalk’) that loads
     the registry object file and all pack object files.

An example of a registry specification object would be:

     :- object(jdoe_awesome_packs_registry,
         implements(registry_protocol)).

         :- info([
             version is 1:0:0,
             author is 'John Doe',
             date is 2021-10-18,
             comment is 'John Doe awesome packs registry spec.'
         ]).

         name(jdoe_awesome_packs).

         description('John Doe awesome packs').

         home('https://example.com/jdoe_awesome_packs').

         clone('https://github.com/jdoe/jdoe_awesome_packs.git').

         archive('https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip').

     :- end_object.

Optionally, the registry object can also define a ‘note(Action, Note)’
predicate.  The ‘Action’ argument is an atom: ‘add’, ‘update’, or
‘delete’.  The ‘Note’ argument is also an atom.  The tool will print any
available notes when executing one of the registry actions.  See the
‘registry_protocol’ documentation for more details.

The registry directory should also contain ‘LICENSE’ and ‘README.md’
files (individual packs can use a different license, however).  The path
to the ‘README.md’ file is printed when the registry is added.  It can
also be queried using the ‘registries::directory/2’ predicate.  The
‘NOTES.md’ file name can also be used in alternative to the recommended
‘README.md’ file name.

Summarizing the required directory structure using the above example
(note that the registry and pack specification files are named after the
objects):

     jdoe_awesome_packs
         LICENSE
         README.md
         jdoe_awesome_packs_registry.lgt
         loader.lgt
         foo_pack.lgt
         bar_pack.lgt
         ...

With the contents of the ‘loader.lgt’ file being:

     :- initialization((
         logtalk_load(jdoe_awesome_packs_registry),
         logtalk_load(foo_pack),
         logtalk_load(bar_pack),
         ...
     )).

It would be of course possible to have all objects in a single source
file.  But having a file per object and a loader file helps maintenance
and it’s also a tool requirement for applying safety procedures to the
source file contents and thus successfully loding the registry and pack
specs.

As registries are git repos in the most common case and thus adding them
performs a git repo cloning, they should only contain the strictly
required files.

5.16.9 Registry handling
------------------------

Registries can be added using the ‘registries::add/1-3’ predicates,
which take a registry URL. Using the example above:

     | ?- registries::add('https://github.com/jdoe/jdoe_awesome_packs.git').

HTTPS URLs must end with either a ‘.git’ extension or a an archive
extension.  Git cloning URLs are preferred but a registry can also be
made available via a local directory (using a ‘file://’ URL) or a
downloadable archive (using a ‘https://’ URL).

For registries made available using an archive, the
‘registries::add/2-3’ predicates 'must' be used as the registry name
cannot in general be inferred from the URL basename or from the archived
directory name.  The registry argument must also be the declared
registry name in the registry specification object.  For example:

     | ?- registries::add(
             jdoe_awesome_packs,
             'https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip'
          ).

When a registry may be already defined, you can use the ‘update(true)’
option to ensure that the registry will be updated to its latest
definition:

     | ?- registries::add(
             jdoe_awesome_packs,
             'https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip',
             [update(true)]
          ).

The added registries can be listed using the ‘registries::list/0’
predicate:

     | ?- registries::list.

     % Defined registries:
     %   jdoe_awesome_packs (git)
     %   ...

The ‘registries::describe/1’ predicate can be used to print the details
of a registry:

     | ?- registries::describe(jdoe_awesome_packs).

     % Registry:    jdoe_awesome_packs
     % Description: John Doe awesome packs
     % Home:        https://example.com/jdoe_awesome_packs
     % Cloning URL: https://github.com/jdoe/jdoe_awesome_packs.git
     % Archive URL: https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip

To update all registries, use the ‘registries::update/0’ predicate.  To
update a single registry, use the ‘registries::update/1-2’ predicates.
After updating, you can use the ‘packs::outdated/0-1’ predicates to list
any outdated packs.

Registries can also be deleted using the ‘registries::delete/1-2’
predicate.  By default, any registries with installed packs cannot be
deleted.  If you force deletion (by using the ‘force(true)’ option), you
can use the ‘packs::orphaned/0’ predicate to list any orphaned packs
that are installed.

See the tool API documentation on the registries(1) object for other
useful predicates.

   ---------- Footnotes ----------

   (1) ../../docs/registries_0.html

5.16.10 Registry development
----------------------------

To simplify registry development and testing, use a local directory and
a ‘file://’ URL when calling the ‘registries::add/1’ predicate.  For
example:

     | ?- registries::add('file:///home/jdoe/work/my_pack_collection').

If the directory is a git repo, the tool will clone it when adding it.
Otherwise, the files in the directory are copied to the registry
definition directory.  This allows the registry to be added and deleted
without consequences for the original registry source files.

To check your registry specifications, use the ‘registries::lint/0-1’
predicates after adding the registry.

5.16.11 Pack specification
--------------------------

A pack is specified using a Logtalk source file defining an object that
implements the ‘pack_protocol’.  The source file should be named after
the pack with a ‘_pack’ suffix.  This naming convention helps preventing
name conflicts, notably with the pack own objects.  The file must be
available from a declared pack registry (by having the registry loader
file loading it).  The pack name is preferably a valid unquoted atom.
An example of a pack specification object would be:

     :- object(lflat_pack,
         implements(pack_protocol)).

         :- info([
             version is 1:0:0,
             author is 'Paulo Moura',
             date is 2021-10-18,
             comment is 'L-FLAT - Logtalk Formal Language and Automata Toolkit pack spec.'
         ]).

         name(lflat).

         description('L-FLAT - Logtalk Formal Language and Automata Toolkit').

         license('MIT').

         home('https://github.com/l-flat/lflat').

         version(
             2:1:0,
             stable,
             'https://github.com/l-flat/lflat/archive/refs/tags/v2.1.0.tar.gz',
             sha256 - '9c298c2a08c4e2a1972c14720ef1498e7f116c7cd8bf7702c8d22d8ff549b6a1',
             [logtalk @>= 3:42:0],
             all
         ).

         version(
             2:0:2,
             stable,
             'https://github.com/l-flat/lflat/archive/refs/tags/v2.0.2.tar.gz',
             sha256 - '8774b3863efc03bb6c284935885dcf34f69f115656d2496a33a446b6199f3e19',
             [logtalk @>= 3:36:0],
             all
         ).

     :- end_object.

The ‘license/1’ argument must be an atom and should whenever possible be
a license identifier as specified in the SPDX standard(1).

Optionally, the pack object can also define a ‘note(Action, Version,
Note)’ predicate.  The ‘Action’ argument is an atom: ‘install’,
‘update’, or ‘uninstall’.  The ‘Note’ argument is also an atom.  The
tool will print any available notes when executing one of the registry
actions.  See the ‘pack_protocol’ documentation for more details.

The pack sources must be available either as a local directory (when
using a ‘file://’ URL) or for downloading as a supported archive.  The
checksum for the archive must use the SHA-256 hash algorithm (‘sha256’).
The pack may optionally be signed.  Supported archive formats and
extensions are:

   - ‘.zip’

   - ‘.tgz’, ‘.tar.gz’

   - ‘.tbz2’, ‘.tar.bz2’

The pack sources should contain ‘LICENSE’, ‘README.md’ (or ‘NOTES.md’),
and ‘loader.lgt’ (or ‘loader.logtalk’) files.  Ideally, it should also
contain a ‘tester.lgt’ (‘tester.logtalk’) file.  The path to the
‘README.md’ file is printed when the pack is installed or updated.  It
can also be queried using the ‘packs::directory/2’ predicate.

   ---------- Footnotes ----------

   (1) https://spdx.org/licenses/

5.16.12 Pack URLs and Single Sign-On
------------------------------------

Typically, pack archive download URLs are HTTPS URLs and handled using
‘curl’.  It’s also possible to use ‘git archive’ to download pack
archives, provided that the server supports it (as of this writing,
Bitbucket and GitLab public hosting services support it but not GitHub).
Using ‘git archive’ is specially useful when the packs registry in
hosted in a server using Single Sign-On (SSO) for authentication.  In
this case, HTTPS URLs can only be handled by ‘curl’ by passing a token
(see below for an example).  When the user have setup SSH keys to
authenticate to the packs registry server, ‘git archive’ simplifies pack
installation, providing a better user experience.  For example:

     version(
         1:0:1,
         stable,
         'git@gitlab.com:me/foo.git/v1.0.1.zip',
         sha256 - '0894c7cdb8968b6bbcf00e3673c1c16cfa98232573af30ceddda207b20a7a207',
         [logtalk @>= 3:36:0],
         all
     ).

The pseudo-URL must be the concatenation of the SSH repo cloning URL
with the archive name.  The archive name must be the concatenation of a
valid tag with a supported archive extension.  SSH repo cloning URLs use
the format:

     git@<hostname>:path/to/project.git

They can usually be easily copied from the hosting service repo webpage.
To compute the checksum, you must first download the archive.  For
example:

     $ git archive --output=foo-v1.0.1.zip --remote=git@gitlab.com:me/foo.git v1.0.1
     $ openssl sha256 foo-v1.0.1.zip

Be sure to use a format that is supported by both the ‘packs’ tool and
the ‘git archive’ command (the format is inferred from the ‘--output’
option).  Do not download the archive from the web interface of the git
hosting service in order to compute the checksum.  Different
implementations of the archiving and compressing algorithms may be used
resulting in mismatched checksums.

5.16.13 Multiple pack versions
------------------------------

A pack may specify multiple versions.  Each version is described using a
‘version/6’ predicate clause as illustrated in the example above.  The
versions must be listed ordered from newest to oldest.  For details, see
the ‘pack_protocol’ API documentation.

Listing multiple versions allows the pack specification to be updated
(by updating its registry) without forcing existing users into
installing (or updating to) the latest version of the pack.

5.16.14 Pack dependencies
-------------------------

Pack dependencies on other packs can be specified using a list of
‘Registry::Pack Operator Version’ terms where ‘Operator’ is a standard
term comparison operator:

   - ‘Registry::Pack @>= Version’ - the pack requires a dependency with
     version equal or above the specified one.  For example, ‘logtalk
     @>= 3:36:0’ means that the pack requires Logtalk 3.36.0 or later
     version.

   - ‘Registry::Pack @=< Version’ - the pack requires a dependency with
     version up to the specified one.  For example, ‘common::bits @=<
     2:1’ means that the pack requires a ‘common::bits’ pack up to 2.1.
     This includes all previous versions and also all patches for
     version 2.1 (e.g.  2.1.7, 2.1.8, …) but not version 2.2 or newer.

   - ‘Registry::Pack @< Version’ - the pack requires a dependency with
     version older than the specified one.  For example, ‘common::bits
     @< 3’ means that the pack requires a ‘common::bits’ 2.x or older
     version.

   - ‘Registry::Pack @> Version’ - the pack requires a dependency with
     version newer than the specified one.  For example, ‘common::bits
     @> 2:4’ means that the pack requires a ‘common::bits’ 2.5 or newer
     version.

   - ‘Registry::Pack == Version’ - the pack requires a dependency with a
     specific version.  For example, ‘common::bits == 2:1’ means that
     the pack requires a ‘common::bits’ pack version 2.1.x (thus, from
     version 2.1.0 to the latest patch for version 2.1).

   - ‘Registry::Pack \== Version’ - the pack requires a dependency with
     any version other than then the one specified.  For example,
     ‘common::bits \== 2.1’ means that the pack requires a
     ‘common::bits’ pack version other than any 2.1.x version.

To specify 'range' dependencies by using two consecutive elements with
the lower bound followed by the upper bound.  For example, ‘common::bits
@>= 2, common::bits @< 3’ means all ‘common::bits’ 2.x versions but not
older or newer major versions.

It’s also possible to specify 'alternative' dependencies using the
‘(;)/2’ operator.  For example, ‘(common::bits == 1:9; common::bits @>=
2:3)’ means either ‘common::bits’ 1.9.x versions or 2.3.x and later
versions.  Alternatives should be listed in decreasing order of
preference.

When a pack also depends on a Logtalk or backend version, the name
‘logtalk’ or the backend identifier atom can be used in place of
‘Registry::Pack’ (see below for the table of backend specifiers).  For
example, ‘logtalk @>= 3.36.0’.

When a pack also depends on an operating-system version (e.g.  a pack
containing shared libraries with executable code), the
‘os(Name,Machine)’ compound term can also be used in place of
‘Registry::Pack’.  For example, ‘os('Darwin',x86_64) @>= '23.0.0'’.
Note that, in this case, the release is an atom.  The operating-system
data (name, machine, and release) is queried using the corresponding
‘os’ library predicates (see the library documentation for details).

5.16.15 Pack portability
------------------------

Ideally, packs are fully portable and can be used with all Logtalk
supported Prolog backends.  This can be declared by using the atom ‘all’
in the last argument of the ‘version/6’ predicate (see example above).

When a pack can only be used with a subset of the Prolog backends, the
last argument of the ‘version/6’ predicate is a list of backend
identifiers (atoms):

   - B-Prolog: ‘b’

   - Ciao Prolog: ‘ciao’

   - CxProlog: ‘cx’

   - ECLiPSe: ‘eclipse’

   - GNU Prolog: ‘gnu’

   - JIProlog: ‘ji’

   - LVM: ‘lvm’

   - Quintus Prolog: ‘quintus’

   - SICStus Prolog: ‘sicstus’

   - SWI-Prolog: ‘swi’

   - Tau Prolog: ‘tau’

   - Trealla Prolog: ‘trealla’

   - XSB: ‘xsb’

   - YAP: ‘yap’

5.16.16 Pack development
------------------------

To simplify pack development and testing, define a local registry and
add to it a pack specification with the development version available
from a local directory.  For example:

     version(
         0:11:0,
         beta,
         'file:///home/jdoe/work/my_awesome_library',
         none,
         [],
         all
     ).

If the directory is a git repo, the tool will clone it when installing
the pack.  Otherwise, the files in the directory are copied to the pack
installation directory.  This allows the pack to be installed, updated,
and uninstalled without consequences for the pack source files.

Packs that are expected to be fully portable should always be checked by
loading them with the ‘portability’ flag set to ‘warning’.

To check your packs specifications, use the ‘packs::lint/0-2’ predicates
after adding the registry that provides the packs.

5.16.17 Pack handling
---------------------

Packs must be available from a defined registry.  To list all packs that
are available for installation, use the ‘packs::available/0’ predicate:

     | ?- packs::available.

To list all installed packs, call the ‘packs::installed/0’ predicate:

     | ?- packs::installed.

To list only the installed packs from a specific registry, call instead
the ‘packs::installed/1’ predicate.  For example:

     | ?- packs::installed(talkshow).

To know more about a specific pack, use the ‘packs::describe/1-2’
predicates.  For example:

     | ?- packs::describe(bar).

The ‘packs::describe/2’ predicate can be used when two or more
registries provide packs with the same name.  For example:

     | ?- packs::describe(reg, bar).

To install the latest version of a pack, we can use the
‘packs::install/1-4’ predicates.  In the most simple case, when a pack
name is unique among registries, we can use the ‘packs::install/1’
predicate.  For example:

     | ?- packs::install(bar).

Any pack dependencies are also checked and installed or updated if
necessary.  Other install predicates are available to disambiguate
between registries and to install a specific pack version.

Packs becomes available for loading immediately after successful
installation (no restarting of the Logtalk session is required).  For
example, after the pack ‘bar’ is installed, you can load it at the
top-level by typing:

     | ?- {bar(loader)}.

or load it from a loader file using the goal
‘logtalk_load(bar(loader))’.

After updating the defined registries, outdated packs can be listed
using the ‘packs::outdated/0’ predicate.  You can update all outdated
packs by calling the ‘packs::update/0’ predicate or update a single pack
using the ‘packs::update/1-2’ predicates.  For example:

     | ?- packs::update(bar).

By default, updating a pack fails if it would break any dependent pack
(the ‘force(true)’ option, described below, can be used to force
updating in this case).

The tool provides versions of the pack install, update, and uninstall
predicates that accept a list of options:

   - ‘verbose(Boolean)’ (default is ‘false’)

   - ‘clean(Boolean)’ (default is ‘false’)

   - ‘update(Boolean)’ (default is ‘false’)

   - ‘force(Boolean)’ (default is ‘false’)

   - ‘compatible(Boolean)’ (default is ‘true’)

   - ‘checksum(Boolean)’ (default is ‘true’)

   - ‘checksig(Boolean)’ (default is ‘false’)

   - ‘git(Atom)’ (extra command-line options; default is ‘''’)

   - ‘curl(Atom)’ (extra command-line options; default is ‘''’)

   - ‘gpg(Atom)’ (extra command-line options; default is ‘''’)

   - ‘tar(Atom)’ (extra command-line options; default is ‘''’)

Note that, by default, only compatible packs can be installed.  To
install a pack that is incompatible with the current Logtalk version,
backend version, or operating-system version, use the ‘install/4’ or
‘update/3’ predicates with the option ‘compatible(false)’.

When a pack may be already installed, you can use the ‘update(true)’
option to ensure that the installation will by updated to the specified
version:

     | ?- packs::install(reg, bar, 1:1:2, [update(true)]).

When using a ‘checksig(true)’ option to check a pack signature, is
strongly advised that you also use the ‘verbose(true)’ option.  For
example:

     | ?- packs::install(reg, bar, 1:1:2, [verbose(true), checksig(true)]).

Note that the public key used to sign the pack archive must be already
present in your local system.

Downloading pack archives may require passing extra command-line options
to ‘curl’ for authentication.  A common solution is to use a personal
access token.  The details depend on the server software.  An example
when using GitHub:

     | ?- packs::install(reg, bar, 1:1:2, [curl('--header "Authorization: token foo42"')]).

Another example when using GitLab:

     | ?- packs::install(reg, bar, 1:1:2, [curl('--header "PRIVATE-TOKEN: foo42"')]).

Pack archives may be encrypted, requiring passing the decryption
passphrase when installing or updating a pack.  For example:

     | ?- packs::install(reg, bar, 1:1:2, [tar('--passphrase test123')]).

In this case, you should be careful to not leak your passphrase in e.g.
the query history.

To uninstall a pack that you no longer need, use the
‘packs::uninstall/1-2’ predicates.  By default, only packs with no
dependent packs can be uninstalled.  You can print or get a list of the
packs that depend on a given pack by using the ‘packs::dependents/1-3’
predicates.  For example:

     | ?- packs::dependents(reg, bar, Dependents).

See the tool API documentation on the packs(1) object for other useful
predicates.

   ---------- Footnotes ----------

   (1) ../../docs/packs_0.html

5.16.18 Pack documentation
--------------------------

The path to the pack ‘README.md’ file is printed when the pack is
installed or updated.  It can also be retrieved at any time by using the
‘readme/2’ predicate.  For example:

     | ?- packs::readme(lflat, Path).

Additional documentation may also be available from the pack home page,
which can be printed by using the ‘describe/1-2’ predicates.  For
example:

     | ?- packs::describe(lflat).

     % Registry:    ...
     % Pack:        lflat
     % Description: L-FLAT - Logtalk Formal Language and Automata Toolkit
     % License:     MIT
     % Home:        https://github.com/l-flat/lflat
     % Versions:
     ...

The pack API documentation can be generated using the ‘lgtdoc’ tool
library and directory predicates (depending on the pack source files
organization).  For example:

     | ?- {lflat(loader)},
          {lgtdoc(loader)},
          logtalk::expand_library_path(lflat, Path),
          lgtdoc::rdirectory(Path).
     ...

This query creates a ‘xml_docs’ directory in the current directory.  The
XML documentation files can then be converted into a final format, e.g.
HTML, using one of the ‘lgtdoc’ tool provided scripts.  For example:

     $ cd xml_docs
     $ lgt2html

For more details and alternatives, see the ‘lgtdoc’ tool documentation.

It is also possible to add API documentation and diagrams for all the
installed packs to the Logtalk distribution API documentation and
diagrams by calling the ‘update_html_docs’ and ‘update_svg_diagrams’
scripts with the ‘-i’ option.  See the scripts documentation for more
details.

5.16.19 Pinning registries and packs
------------------------------------

Registries and packs can be 'pinned' after installation to prevent
accidental updating or deleting, e.g.  when using the batch ‘update/0’
predicate.  This is useful when your application requires a specific
version or for security considerations (see below).  For example, if we
want the ‘bar’ pack to stay at its current installed version:

     | ?- packs::pin(bar).
     yes

After, any attempt to update or uninstall the pack will fail with an
error message:

     | ?- packs::update(bar).
     !     Cannot update pinned pack: bar
     no

     | ?- packs::uninstall(bar).
     !     Cannot uninstall pinned pack: bar
     no

To enable the pack to be updated ou uninstalled, the pack must first be
unpinned.  Alternatively, the ‘force(true)’ option can be used.  Note
that if you force update a pinned pack, the new version will be
unpinned.

It’s also possible to pin (or unpin) all defined registries or installed
packs at once by using the ‘pin/0’ (or ‘unpin/0’) predicates.  But note
that registries added after or packs installed after will not be
automatically pinned.

5.16.20 Testing packs
---------------------

Logtalk packs (as most Logtalk libraries, tools, and examples) are
expected to have a ‘tester.lgt’ or ‘tester.logtalk’ tests driver file at
the root of their directory, which can be used for both automated and
manual testing.  For example, after installing the ‘foo’ pack:

     | ?- {foo(tester)}.

To test all installed packs, you can use the ‘logtalk_tester’ automation
script from the installed packs directory, which you can query using the
goal:

     | ?- packs::prefix(Directory).

Note that running the packs tests, like simply loading the pack, can
result in calling arbitrary code, which can potentially harm your
system.  Always take into account the security considerations discussed
below.

5.16.21 Security considerations
-------------------------------

New pack registries should be examined before being added, specially if
public and from a previously unknown source.  The same precautions
should be taken when adding or updating a pack.  Note that a registry
can always index third-party packs.

Pack checksums are checked by default.  But pack signatures are only
checked if requested as packs are often unsigned.  Care should be taken
when adding public keys for pack signers to your local system.  Detached
signature files are assumed and expected to share the name of the
archive and use a ‘.asc’ extension.  When the ‘checksig(true)’ option is
used, the signature file is automatically downloaded using a URL
constructed from the pack archive URL.

Registry and pack spec files plus the registry loader file are compiled
by term-expanding them so that only expected terms are actually loaded
and only expected ‘logtalk_load/2’ goals with expected relative file
paths are allowed.  Predicates defining URLs are discarded if the URLs
are neither ‘https://’ nor ‘file://’ URLs or if they contain non-allowed
characters (currently, only alpha-numeric ASCII characters plus the
ASCII ‘/’, ‘.’, ‘-’, and ‘_’ characters are accepted).  But note that
this tool makes no attempt to audit pack source files themselves.

Registries and packs can always be pinned so that they are not
accidentally updated to a version that you may not had the chance to
audit.

5.16.22 Best practices
----------------------

   - Make available a new pack registry as a git repo.  This simplifies
     updating the registry and rolling back to a previous version.

   - Use registry and pack names that are valid unquoted atoms, thus
     simplifying usage.  Use descriptive names with underscores if
     necessary to link words.

   - Name the registry and pack specification objects after their names
     with a ‘_registry’ or ‘_pack’ suffix.  Save the objects in files
     named after the objects.

   - Create new pack versions from git tags.

   - If the sources of a pack are available from a git repo, consider
     using signed commits and signed tags for increased security.

   - When a new pack version breaks backwards compatibility, list both
     the old and the new versions on the pack specification file.

   - Pin registries and packs when specific versions are critical for
     your work so that you can still easily batch update the remaining
     packs and registries.

   - Include the ‘$LOGTALKPACKS’ directory (or the default
     ‘~/logtalk_packs’ directory) on your regular backups.

5.16.23 Installing Prolog packs
-------------------------------

This tool can also be used to install Prolog packs that don’t use
Logtalk.  After installing a ‘pl_pack’ Prolog pack from a ‘pl_reg’
registry, it can be found in the ‘$LOGTALKPACKS/packs/pl_reg/pl_pack’
directory.  When the ‘LOGTALKPACKS’ environment variable is not defined,
the pack directory is by default ‘~/logtalk_packs/packs/pl_reg/pl_pack’.

Different Prolog systems provide different solutions for locating Prolog
code.  For example, several Prolog systems adopted the Quintus Prolog
‘file_search_path/2’ hook predicate.  For these systems, a solution
could be to add a fact to this predicate for each installed Prolog pack.
For example, assuming a ‘pl_pack’ Prolog pack:

     :- multifile(file_search_path/2).
     :- dynamic(file_search_path/2).

     file_search_path(library, '$LOGTALKPACKS/packs/pl_pack').

If the Prolog system also supports reading an initialization file at
startup, the above definition could be added there.

5.16.24 Known issues
--------------------

Using the ‘verbose(true)’ option on Windows systems may not provide the
shell commands output depending on the backend.

On Windows systems, the reset, delete, and uninstall predicates may fail
to delete all affected folders and files due to a operating-system bug.
Depending on the backend, this bug may cause some of the tests to fail.
For details on this bug, see:

‘https://github.com/microsoft/terminal/issues/309’

The workaround is to use the Windows File Explorer to delete the
left-over folders and files.

When using Ciao Prolog 1.20.0, a workaround is used for this system
non-standard support for multifile predicates.

When using GNU Prolog 1.5.0 as the backend on Windows, you may get an
error on ‘directory_files/2’ calls.  For details and a workaround, see:

‘https://github.com/didoudiaz/gprolog/issues/4’

This issue is fixed in the GNU Prolog 1.5.1 version.

Using SICStus Prolog as the backend on Windows doesn’t currently work in
version 4.7.0 and earlier versions.  The underlying issues are fixed in
the SICStus Prolog 4.7.1 version.

XSB have an odd bug (likely in its parser) when reading files that may
cause a pack installed version to be reported as the ‘end_of_file’ atom.

Some tests fail on Windows when using ECLiPSe or XSB due to file path
representation issues.

5.17 ‘ports_profiler’
=====================

This tool counts and reports the number of times each port in the
'procedure box model' is traversed during the execution of queries.  It
can also report the number of times each clause (or grammar rule) is
used.  It is inspired by the ECLiPSe ‘port_profiler’ tool.

The procedure box model is the same used in the debugger tool.  This is
an extended version of the original Byrd’s four port model.  Besides the
standard ‘call’, ‘exit’, ‘fail’, and ‘redo’ ports, Logtalk also defines
two post-unification ports, ‘fact’ and ‘rule’, and an ‘exception’ port.
This tool can also distinguishes between deterministic exits (reported
in the ‘exit’ column in the profiling result tables) and exits that
leave choice-points (reported in the ‘*exit’ column).

5.17.1 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#ports-profiler’

For sample queries, please see the ‘SCRIPT.txt’ file in the tool
directory.

5.17.2 Loading
--------------

     | ?- logtalk_load(ports_profiler(loader)).

5.17.3 Testing
--------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(ports_profiler(tester)).

5.17.4 Compiling source files for port profiling
------------------------------------------------

To compile source files for port profiling, simply compile them in debug
mode and with the ‘source_data’ flag turned on.  For example:

     | ?- logtalk_load(my_source_file, [debug(on), source_data(on)]).

Alternatively, you can also simply turn on the ‘debug’ and ‘source_data’
flags globally before compiling your source files:

     | ?- set_logtalk_flag(debug, on), set_logtalk_flag(source_data, on).

Be aware, however, that loader files (e.g.  library loader files) may
override default flag values and thus loaded files may not be compiled
in debug mode.  In this case, you will need to modify the loader files
themselves.

5.17.5 Generating profiling data
--------------------------------

After loading this tool and compiling the source files that you want to
profile in debug mode, simply call the ‘ports_profiler::start’ goal
followed by the goals to be profiled.  Use the ‘ports_profiler::stop’
goal to stop profiling.

Note that the ‘ports_profiler::start/0’ predicate implicitly selects the
‘ports_profiler’ tool as the active debug handler.  If you have
additional debug handlers loaded (e.g.  the ‘debugger’ tool), those
would no longer be active (there can be only one active debug handler at
any given time).

5.17.6 Printing profiling data reports
--------------------------------------

After calling the goals that you want to profile, you can print a table
with all profile data by typing:

     | ?- ports_profiler::data.

To print a table with data for a single entity, use the query:

     | ?- ports_profiler::data(Entity).

To print a table with data for a single entity predicate, use the query:

     | ?- ports_profiler::data(Entity, Predicate).

In this case, the second argument must be either a predicate indicator,
‘Name/Arity’, or a non-terminal indicator, ‘Name//Arity’.

The profiling data can be reset using the query:

     | ?- ports_profiler::reset.

To reset only the data about a specific entity, use the query:

     | ?- ports_profiler::reset(Entity).

To illustrate the tool output, consider the ‘family’ example in the
Logtalk distribution:

     | ?- {ports_profiler(loader)}.
     ...
     yes

     | ?- set_logtalk_flag(debug, on).
     yes

     | ?- logtalk_load(family(loader)).
     ...
     yes

     | ?- ports_profiler::start.
     yes

     | ?- addams::sister(Sister, Sibling).
     Sister = wednesday,
     Sibling = pubert ;
     Sister = wednesday,
     Sibling = pugsley ;
     Sister = wednesday,
     Sibling = pubert ;
     Sister = wednesday,
     Sibling = pugsley ;
     no

     | ?- ports_profiler::data.
     ----------------------------------------------------------------------
     Entity     Predicate    Fact  Rule  Call  Exit *Exit  Fail  Redo Error
     ----------------------------------------------------------------------
     addams     female/1        2     0     1     1     1     0     1     0
     addams     parent/2        8     0     4     3     5     1     5     0
     relations  sister/2        0     1     1     0     4     1     4     0
     ----------------------------------------------------------------------
     yes

     | ?- ports_profiler::data(addams).
     -----------------------------------------------------------
     Predicate    Fact  Rule  Call  Exit *Exit  Fail  Redo Error
     -----------------------------------------------------------
     female/1        2     0     1     1     1     0     1     0
     parent/2        8     0     4     3     5     1     5     0
     -----------------------------------------------------------
     yes

     | ?- ports_profiler::data(addams, parent/2).
     -------------
     Clause  Count
     -------------
          1      1
          2      1
          3      2
          4      1
          5      1
          6      2
     -------------
     yes

5.17.7 Interpreting profiling data
----------------------------------

Some useful information that can be inferred from the profiling data
include:

   - which predicates are called more often (from the ‘call’ port)

   - unexpected failures (from the ‘fail’ port)

   - unwanted non-determinism (from the ‘*exit’ port)

   - performance issues due to backtracking (from the ‘*exit’ and ‘redo’
     ports)

   - predicates acting like a generator of possible solutions (from the
     ‘*exit’ and ‘redo’ ports)

   - inefficient indexing of predicate clauses (from the ‘fact’, ‘rule’,
     and ‘call’ ports)

   - clauses that are never used or seldom used

The profiling data should be analyzed taking into account the expected
behavior for the profiled predicates.

5.17.8 Profiling Prolog modules
-------------------------------

This tool can also be applied to Prolog modules that Logtalk is able to
compile as objects.  For example, if the Prolog module file is named
‘module.pl’, try:

     | ?- logtalk_load(module, [debug(on), source_data(on)]).

Due to the lack of standardization of module systems and the abundance
of proprietary extensions, this solution is not expected to work for all
cases.

5.17.9 Profiling plain Prolog code
----------------------------------

This tool can also be applied to plain Prolog code.  For example, if the
Prolog file is named ‘code.pl’, simply define an object including its
code and declaring as public any predicates that you want to use as
messages to the object.  For example:

     :- object(code).

         :- public(foo/2).
         :- include('code.pl').

     :- end_object.

Save the object to an e.g.  ‘code.lgt’ file in the same directory as the
Prolog file and then load it in debug mode:

     | ?- logtalk_load(code, [debug(on), source_data(on)]).

In alternative, use the ‘object_wrapper_hook’ provided by the
‘hook_objects’ library:

     | ?- logtalk_load(hook_objects(loader)).
     ...

     | ?- logtalk_load(
              code,
              [hook(object_wrapper_hook), debug(on),
               source_data(on), context_switching_calls(allow)]
          ).

In this second alternative, you can then use the ‘(<<)/2’ context switch
control construct to call the wrapped predicates.  E.g.

     | ?- code<<foo(X, Y).

With either wrapping solution, pay special attention to any compilation
warnings that may signal issues that could prevent the plain Prolog code
of working as-is when wrapped by an object.  Often any required changes
are straight-forward (e.g.  adding ‘use_module/2’ directives for called
module library predicates).

5.17.10 Known issues
--------------------

Determinism information is currently not available when using Quintus
Prolog as the backend compiler.

5.18 ‘profiler’
===============

This tool contains simple wrappers for selected Prolog profiler tools.

5.18.1 Loading
--------------

This tool can be loaded using the query:

     ?- logtalk_load(profiler(loader)).

For sample queries, please see the ‘SCRIPT.txt’ file in the tool
directory.

5.18.2 Testing
--------------

To test this tool, load the ‘tester.lgt’ file:

     | ?- logtalk_load(profiler(tester)).

5.18.3 Supported backend Prolog compilers
-----------------------------------------

Currently, this tool supports the profilers provided with SICStus Prolog
4, SWI-Prolog, and YAP. The tool includes two files:

   -
          ‘yap_profiler.lgt’
          simple wrapper for the YAP count profiler

   -
          ‘sicstus_profiler.lgt’
          simple wrapper for the SICStus Prolog 4 profiler

Logtalk also supports the YAP tick profiler (using the latest YAP
development version) and the SWI-Prolog XPCE profiler.  When using the
XPCE profiler, you can avoid profiling the Logtalk compiler (which is
invoked e.g.  when you use the ‘(::)/2’ message-sending operator at the
top-level interpreter) by compiling your code with the ‘optimize’ flag
turned on:

     ?- set_logtalk_flag(optimize, on).
     true.

     ?- use_module(library(statistics)).
     true.

     ?- profile(... :: ...).
     ...

Given that ‘prolog_statistics:profile/1’ is a meta-predicate, Logtalk
will compile its argument before calling it thanks to the
‘goal_expansion/2’ hook predicate definitions in the adapter file.
Without this hook definition, you would need to use instead (to avoid
profiling the compiler itself):

     ?- logtalk << (prolog_statistics:profile(... :: ...)).
     ...

In either case, Don’t forget, however, to load the ‘prolog_statistics’
module 'before' using or compiling calls to the ‘profile/1’ to allow the
Logtalk compiler to access its meta-predicate template.

The profiler support attempts to conceal internal Logtalk
compiler/runtime predicates and the generated entity predicates that
implement predicate inheritance.  Calls to internal compiler and runtime
predicates have functors starting with ‘$lgt_’.  Calls to predicates
with functors such as ‘_def’, ‘_dcl’, or ‘_super’, used to implement
inheritance, may still be listed in a few cases.  Note that the time and
the number of calls/redos of concealed predicates is added to the caller
predicates.

5.18.4 Compiling source code for profiling
------------------------------------------

In order to get user-level object and predicate names instead of
compiler generated internal names when using the SWI-Prolog and YAP
profilers, you must set ‘code_prefix’ flag to a character other than the
default ‘$’ before compiling your source code.  For example:

     ?- set_logtalk_flag(code_prefix, '.').

See also the ‘settings-sample.lgt’ file for automating the necessary
setup at Logtalk startup.

5.19 ‘tutor’
============

This tool adds explanations and suggestions to selected compiler warning
and error messages.  It’s specially useful for new users not yet
familiar with the compiler and runtime warning and error messages.

5.19.1 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#tutor’

5.19.2 Loading
--------------

This tool can be loaded using the query:

     | ?- logtalk_load(tutor(loader)).

5.19.3 Usage
------------

Simply load the tool at startup (e.g.  from a settings file).  As an
example, with this tool loaded, instead of terse compiler warnings such
as:

     *     No matching clause for goal: baz(a)
     *       while compiling object main_include_compiler_warning
     *       in file logtalk/examples/errors/include_compiler_warning.lgt between lines 37-38
     *
     *     Duplicated clause: b(one)
     *       first found at or above line 45
     *       while compiling object main_include_compiler_warning
     *       in file logtalk/examples/errors/include_compiler_warning.lgt at or above line 48

the user will get:

     *     No matching clause for goal: baz(a)
     *       while compiling object main_include_compiler_warning
     *       in file logtalk/examples/errors/include_compiler_warning.lgt between lines 37-38
     *     Calls to locally defined predicates without a clause with a matching head
     *     fail. Typo in a predicate argument? Predicate definition incomplete?
     *
     *     Duplicated clause: b(one)
     *       first found at or above line 45
     *       while compiling object main_include_compiler_warning
     *       in file logtalk/examples/errors/include_compiler_warning.lgt at or above line 48
     *     Duplicated clauses are usually a source code editing error and can
     *     result in spurious choice-points, degrading performance. Delete or
     *     correct the duplicated clause to fix this warning.

5.20 ‘wrapper’
==============

This is a prototype tool to help port a plain Prolog application to
Logtalk.  It can also be used to enable applying other Logtalk developer
tools, such as the documenting and diagramming tools, to plain Prolog
code.

The tool takes a directory of Prolog files or a list of Prolog files,
loads and wraps the code in each file using an object wrapper, and
advises on missing directives to be added to those objects by using the
compiler lint checker and the reflection API. The user can then either
save the generated wrapper objects or copy and paste the printed advice
into the Prolog files (updating them to Logtalk files by adding the
object opening and closing directives to the Prolog files).  The wrapper
objects use ‘include/1’ directives to include the Prolog files and can
be loaded for testing and for use with other tools.  The wrapped Prolog
files are not modified and thus require only read permission.

5.20.1 API documentation
------------------------

This tool API documentation is available at:

‘../../docs/library_index.html#wrapper’

5.20.2 Loading
--------------

This tool can be loaded using the query:

     | ?- logtalk_load(wrapper(loader)).

5.20.3 Workflows
----------------

The typical porting workflow is simply:

     | ?- wrapper::rdirectory(root_directory_of_prolog_code).
     ...
     | ?- wrapper::save.
     ...

See the next section on how to customize the API calls for more flexible
processing.

5.20.4 Customization
--------------------

The tool can be customized by extending the ‘wrapper’ object.  A common
scenario is when wrapping plain Prolog code just to take advantage, for
example, of the documenting tool or for generating cross-referencing
diagrams.  In this case, we can workaround any compiler errors by
specializing the inherited definitions for the ‘term_expansion/2’ and
‘goal_expansion/2’ predicates and then load the wrapper objects for
further processing by using the ‘include_wrapped_files(false)’ option
described below.

The API predicates also accept a set of options for customization:

   -
          ‘prolog_extensions(Extensions)’
          list of file name extensions used to recognize Prolog source files (default is ‘['.pl', '.pro', '.prolog']’)

   -
          ‘logtalk_extension(Extension)’
          Logtalk file name extension to be used for the generated wrapper files (default is ‘'.lgt'’)

   -
          ‘exclude_files(Files)’
          list of Prolog source files to exclude (default is ‘[]’)

   -
          ‘exclude_directories(Files)’
          list of sub-directories to exclude (default is ‘[]’)

   -
          ‘include_wrapped_files(Boolean)’
          generate ‘include/1’ directives for the wrapped Prolog source files (default is ‘true’)

The use, by default, of ‘include/1’ directives to wrap the code in
Prolog source files facilitates running in parallel the Logtalk port and
the original Prolog code.  This is specially useful when the Prolog code
being ported lacks a comprehensive set of tests that could be adapted to
verify the Logtalk port.  The generated Logtalk files can also take
advantage of ‘uses/2’ directives, including predicate aliases and
predicate shorthands to minimize the changes to the original Prolog code
and help verify if replacement calls to Logtalk library predicates
provide the same semantics as the original calls.

5.20.5 Current limitations
--------------------------

   - The tool cannot deal with syntax errors in the Prolog files.  These
     errors usually occur when using a backend Prolog system different
     from the one used to compile the original plain Prolog code.  A
     common cause of syntax errors are operator definitions.  These can
     often be solved by defining those operators for the Prolog backend
     used to run Logtalk and this tool.  An alternative is to preload
     the Prolog files where those operators are declared.  Preloading
     the plain Prolog application can also help in wrapping it by
     ensuring that its dependencies are also loaded.

   - The tool assumes that all files to be wrapped have different names
     (even if found in different directories).  If that is not the case,
     the name conflicts must be manually solved before using the tool.

   - There’s only preliminary support for dealing with meta-predicates
     and advise on missing meta-predicate directives.

6 Libraries
***********

The documentation of each library can also be found in the library
directory in the ‘NOTES.md’ file.

6.1 Overview
============

This folder contains libraries of useful objects, categories, and
protocols.  Specific notes about individual libraries can be found in
the corresponding library directory ‘NOTES.md’ files.

A plain Prolog version of the Unicode 6.2 standard is also included in
the ‘unicode_data’ folder.  See its ‘README.md’ file for details.

A ‘parallel_logtalk_processes_setup.pl’ Prolog file is also provided
with sample code for selected backend Prolog compilers for initializing
Logtalk processes such that each process uses a unique scratch directory
therefore allowing parallel process execution (e.g.  for usage at
continuous integration servers).  Starting with Logtalk 3.48.0, this
setup is only required in general when running with the ‘clean’ flag
turned off.  See the comments in the file itself for usage instructions.

6.1.1 Library documentation
---------------------------

Specific notes about each library can be found in the corresponding
‘NOTES.md’ files.  HTML documentation for each library API can be found
on the ‘docs’ directory (open the ‘../docs/index.html’ file with your
web browser).  The documentation for these libraries can be regenerated
using the shell scripts ‘../scripts/update_html_docs.sh’ and
‘../scripts/update_svg_diagrams.sh’.

6.1.2 Loading libraries
-----------------------

All the individual libraries can be loaded using the ‘<library
name>(loader)’ notation as argument for the compiling and loading
predicates.  For example:

     | ?- logtalk_load(random(loader)).

There is also a file named ‘all_loader.lgt’ that will load all
libraries.  Simply type the goal:

     | ?- logtalk_load(library(all_loader)).

As a general rule, always use the corresponding loader file to load a
library.  Most library entities are part of small hierarchies or depend
on other libraries and thus cannot be loaded and compiled separately
(e.g.  the ‘list’ object implements the ‘listp’ protocol and is part of
a basic types hierarchy).  Using the loader files takes care of all
dependencies and also ensures compilation in optimized mode.

6.1.3 Testing libraries
-----------------------

Most of the libraries include unit tests in their directory, together
with a ‘tester.lgt’ file for running them.  For example, to run the
tests for the ‘random’ library, we can use the goal:

     | ?- logtalk_load(random(tester)).

To run all libraries tests, we can use the ‘logtalk_tester’ automation
script from the ‘library’ directory at the root of the Logtalk
distribution.  For example, assuming the Logtalk user directory is
‘~/logtalk’ and that we want to run the tests using ECLiPSe as the
backend Prolog compiler:

     $ cd ~/logtalk/library
     $ logtalk_tester -p eclipse

6.1.4 Credits
-------------

Some code in this library is based on public domain Prolog code, in
particular, code adopted from the Edinburgh Prolog library.  The
definition of predicate ‘reverse/2’ in object list is from Richard
O’Keefe and can be found in its book “The Craft of Prolog”.

Some elements of this library are inspired by Richard O’Keefe library
proposal available at:

‘http://www.cs.otago.ac.nz/staffpriv/ok/pllib.htm’

Some libraries, or part of libraries, are either ports of Prolog system
libraries or inspired by Prolog system libraries.  See the individual
library notes for details.  See also the ‘NOTICE.txt’ file at the root
of the Logtalk distribution for copyright information on third-party
source code.

6.1.5 Other notes
-----------------

Some files contained in this directory represent work in progress and
are not loaded by default by any loader utility file.

6.2 ‘arbitrary’
===============

The ‘arbitrary’ library defines an ‘arbitrary’ category providing
predicates for generating random values for selected types to the ‘type’
object, complementing its type checking predicates.  Both the object and
the category predicates can be extended by the user with definitions for
new types by defining clauses for multifile predicates.  This library is
notably used in the QuickCheck implementation by the ‘lgtunit’ tool.
See also the documentation of the ‘mutations’ library for related
functionality.

6.2.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#arbitrary’ link in a web
browser.

6.2.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(arbitrary(loader)).

6.2.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(arbitrary(tester)).

Several of provided tests are generic and verify correct behavior of all
pre-defined and loaded user-defined generators and shrinkers for all
ground types.

6.2.4 Pre-defined types
-----------------------

This library defines random generators for most common Logtalk and
Prolog types.  See the API documentation(1) for a listing of all the
pre-defined types.

   ---------- Footnotes ----------

   (1) ../../docs/library_index.html#arbitrary

6.2.5 Usage
-----------

The ‘arbitrary’ category complements the ‘type’ object and thus its
predicates are accessed via this object.  For example:

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = -816
     yes

6.2.6 Defining new generators and shrinkers
-------------------------------------------

To define a generator of arbitrary values for a type, define a clause
for the ‘arbitrary::arbitrary/1’ multifile predicate specifying the type
and a clause for the ‘arbitrary::arbitrary/2’ multifile predicate
generating an arbitrary term of the specified type.  As a simple
example, assume that we want to define an “odd integer type”.  We start
by defining both the type checker and the arbitrary generator:

     :- multifile(type::type/1).
     type::type(odd).

     :- multifile(type::check/2).
     type::check(odd, Term) :-
         (   var(Term) ->
             throw(instantiation_error)
         ;   integer(Term),
             Term mod 2 =:= 1 ->
             true
         ;   throw(type_error(odd, Term))
         ).

     :- multifile(arbitrary::arbitrary/1).
     arbitrary::arbitrary(odd).

     :- multifile(arbitrary::arbitrary/2).
     arbitrary::arbitrary(odd, Arbitrary) :-
         type::arbitrary(integer, Arbitrary0),
         (   Arbitrary0 mod 2 =:= 1 ->
             Arbitrary = Arbitrary0
         ;   Arbitrary is Arbitrary0 + 1
         ).

The ‘arbitrary’ library also provides 'meta-types' that can simplify the
definition of new generators.  For example, the ‘odd’ type above can
also be defined using the ‘constrain/2’ meta-type to only generate
values that satisfy a closure:

     arbitrary::arbitrary(odd, Arbitrary) :-
         arbitrary(constrain(integer, [Arbitrary]>>(Arbitrary mod 2 =:= 1)), Arbitrary).

Another example is using the ‘transform/2’ meta-type to transform
generated values for a base type using a closure.  Assuming that we want
to generate sorted list of random integers, we can write:

     arbitrary::arbitrary(sorted_integer_list, Arbitrary) :-
         arbitrary(transform(list(integer), sort), Arbitrary).

We can also define a clause for the ‘arbitrary::shrinker/1’ multifile
predicate to declare a new shrinker and a ‘arbitrary::shrink/3’
multifile predicate for shrinking arbitrary values for QuickCheck usage:

     :- multifile(arbitrary::shrinker/1).
     arbitrary::shrinker(odd).

     :- multifile(arbitrary::shrink/3).
     arbitrary::shrink(odd, Large, Small) :-
         integer(Large),
         (   Large < -1 ->
             Small is Large + 2
         ;   Large > 1,
             Small is Large - 2
         ).

Definitions for the ‘shrink/3’ predicate should either succeed or fail
but never throw an exception.  The ‘shrink_sequence/3’ predicate can be
used to help testing that shrinking a value results in a finite sequence
of values.

It is also possible to define edge cases for a given type for use with
QuickCheck implementations.  For example:

     :- multifile(arbitrary::edge_case/2).
     arbitrary::edge_case(odd,  1).
     arbitrary::edge_case(odd, -1).

Edge cases are tried before resorting to generating arbitrary values for
a type.

A more complex example is generating arbitrary values for a recursive
type.  A simple example of a recursive type is a binary tree.  Assuming
that we are working with a binary tree holding integers where each node
is represented by a ‘node(Left, Right)’ compound term, we can define a
‘node(Depth)’ type where ‘Depth’ is the maximum depth of the tree.  This
argument allows us to prevent excessively deep trees:

     :- category(binary_tree).

         :- multifile(type::type/1).
         type::type(node(_)).

         :- multifile(type::check/2).
         type::check(node(_), Term) :-
             (   check(Term) ->
                 true
             ;   var(Term) ->
                 throw(instantiation_error)
             ;   throw(type_error(node(_), Term))
             ).

         check(Term) :-
             (   integer(Term) ->
                 true
             ;   compound(Term),
                 Term = node(Left, Right),
                 check(Left),
                 check(Right)
             ).

         :- multifile(arbitrary::arbitrary/1).
         arbitrary::arbitrary(node(_)).

         :- multifile(arbitrary::arbitrary/2).
         arbitrary::arbitrary(node(Depth), Arbitrary) :-
         (   Depth > 1 ->
             NewDepth is Depth - 1,
             type::arbitrary(
                 types_frequency([
                     integer - 1,
                     compound(
                         node,
                         [
                             types([node(NewDepth), integer]),
                             types([node(NewDepth), integer])
                         ]
                     ) - 3
                 ]),
                 Arbitrary
             )
         ;   type::arbitrary(
         integer, Arbitrary)
         ).

     :- end_category.

In this second example, we use some of the pre-defined types provided by
the library.  The ‘types_frequency(Pairs)’ type supports generating
random terms for a type in the ‘Type-Frequency’ pairs list where the
type ie randomly chosen after the types relative frequency.  The
‘compound(Name, Types)’ type supports generating compound term with a
given name and random arguments after the given types:

     | ?- type::arbitrary(node(4), Arbitrary).
     Arbitrary = 907
     yes

     | ?- type::arbitrary(node(4), Arbitrary).
     Arbitrary = node(node(node(522, 509), node(83, 453)), node(454, -197))
     yes

     | ?- type::arbitrary(node(4), Arbitrary).
     Arbitrary = node(node(-875, -866), -254)
     yes

     | ?- type::arbitrary(node(4), Arbitrary).
     Arbitrary = node(-133, -831)
     yes

The source code of these examples can be found in the ‘test_files’
directory.  Other examples of arbitrary term generators can be found in
the implementation of the ‘optionals’ and ‘expecteds’ libraries.

6.2.7 Scoped generators and shrinkers
-------------------------------------

Declaring a new generator and possibly a shrinker for a custom type
rises the possibility of a conflict with third-party defined generators
and shrinkers.  An alternative is to use the ‘(::)/2’ meta-type to
define scoped generators and shrinkers.  For example:

     :- object(scoped).

         % the same predicate is used for both generating and validating
         :- public(custom/1).
         custom(Term) :-
             (   var(Term) ->
                 % assume predicate used as a generator
                 random::random(Term)
             ;   % assume predicate used as a validator
                 float(Term)
             ).

         % a predicate with the same name is used for shrinking
         :- public(custom/2).
         custom(Larger, Small) :-
             Small is Larger / 2.

     :- end_object.

Some sample calls:

     | ?- type::arbitrary(scoped::custom, Arbitrary).
     Arbitrary = 0.5788130906607927
     yes

     | ?- type::valid(scoped::custom, foo).
     no

     | ?- type::check(scoped::custom, _).
     ERROR: type_error(instantiation_error)

     | ?- type::check(scoped::custom, foo).
     ERROR: type_error(scoped::custom, foo)

     | ?- type::shrink(scoped::custom, 0.42, Smaller).
     Smaller = 0.21
     yes

The source code of this example can be found in the ‘test_files’
directory.

6.2.8 Reproducing sequences of arbitrary terms
----------------------------------------------

The ‘arbitrary’ category provides access to the pseudo-random generator
it uses via the ‘get_seed/1’ and ‘set_seed/1’.  This allows sequences of
arbitrary values to be reproduced.  For example:

     | ?- type::get_seed(Seed).
     Seed = seed(3172, 9814, 20125)
     yes

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = -816
     yes

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = -113
     yes

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = 446

     | ?- type::set_seed(seed(3172, 9814, 20125)).
     yes

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = -816
     yes

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = -113
     yes

     | ?- type::arbitrary(integer, Arbitrary).
     Arbitrary = 446
     yes

The seed should be regarded as an opaque term and handled using the
‘get_seed/1’ and ‘set_seed/1’ predicates.  These predicates are notably
used in the QuickCheck implementation provided by the ‘lgtunit’ tool.

6.2.9 Default size of generated terms
-------------------------------------

The library uses the value 42 for the default size of generated terms
for types where size is meaningful and implicit.  To override this
default value, define a clause for the ‘arbitrary::max_size/1’ multifile
predicate.  The new default size must be a positive integer.  For
example:

     :- multifile(arbitrary::max_size/1).
     arbitrary::max_size(7).

When multiple definitions exist, the first valid one found is used.
When no definition is valid, the default value of 42 is used.

6.2.10 Known issues
-------------------

Some Prolog systems either don’t support the null character or provide
buggy results when calling ‘char_code/2’ with a code of zero.  When
that’s the case, the null character is excluded when generating
arbitrary characters or character codes.

Generating arbitrary Unicode characters (instead of Unicode codepoints)
is inherently problematic as the process first generates codepoints and
then tries to use the standard ‘char_code/2’ to convert them to
characters.  But, depending on the backend Prolog system and its
internal (if any) Unicode normalization, it may not be possible to
convert a codepoint to a single character.

6.3 ‘assignvars’
================

The ‘assignvarsp’ protocol declares the predicates used for logical
assignment of Prolog terms developed by Nobukuni Kino.

The ‘assignvars’ object provides a declarative implementation of the
‘assignvarsp’ protocol.  It can be used with any backend Prolog
compiler.

The ‘nd_assignvars’ object provides a non-declarative but faster
implementation of the ‘assignvarsp’ protocol.  It can be used with the
following backend Prolog compilers: B-Prolog, CxProlog, ECLiPSe, GNU
Prolog, SICStus Prolog, SWI-Prolog, and YAP.

For more information on assignvars, please consult the URL:

‘https://web.archive.org/web/20160818050049/http://www.kprolog.com/en/logical_assignment/’

The representation of assignable terms should be regarded as an opaque
term and only accessed using the library predicates.

6.3.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#assignvars’ link in a web
browser.

6.3.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(assignvars(loader)).

6.3.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(assignvars(tester)).

6.4 ‘base64’
============

The ‘base64’ library provides predicates for parsing and generating data
in the Base64 and Base64URL formats as per the specification found at:

‘https://www.rfc-editor.org/rfc/rfc4648’

6.4.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#base64’ link in a web browser.

6.4.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(base64(loader)).

6.4.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(base64(tester)).

6.4.4 Encoding
--------------

Encoding a list of bytes in Base64 format is accomplished by the
‘base64::generate/2’ predicate.  For example:

     | ?- atom_codes('Hello world!', Bytes),
          base64::generate(atom(Base64), Bytes).
     Base64 = 'SGVsbG8gd29ybGQh'
     Bytes = [72,101,108,108,111,32,119,111,114,108,100,33]
     yes

     | ?- atom_codes('Hello world!', Bytes),
          base64::generate(codes(Base64), Bytes).
     Base64 = [83,71,86,115,98,71,56,103,100,50,57,121,98,71,81,104]
     Bytes = [72,101,108,108,111,32,119,111,114,108,100,33]
     yes

The Base64 result can also be represented using a list of chars, written
to a file or to a stream.  See the API documentation for details.

For safe encoding of URLs, use instead the Base64URL format.  For
example:

     | ?- base64url::generate(atom(Base64URL), 'https://logtalk.org').
     Base64URL == 'aHR0cHM6Ly9sb2d0YWxrLm9yZw'
     yes

The Base64URL can also be represented using a list of chars or a list of
codes.  The input URL should be in the same format.

6.4.5 Decoding
--------------

Decoding of Base64 data is accomplished using the ‘base64::parse/2’
predicate.  For example:

     | ?- base64::parse(atom('SGVsbG8gd29ybGQh'), Bytes),
          atom_codes(Atom, Bytes).
     Atom = 'Hello world!'
     Bytes = [72,101,108,108,111,32,119,111,114,108,100,33]
     yes

     | ?- base64::parse(chars(['S','G','V',s,b,'G','8',g,d,'2','9',y,b,'G','Q',h]), Bytes),
          atom_codes(Atom, Bytes).
     Atom = 'Hello world!'
     Bytes = [72,101,108,108,111,32,119,111,114,108,100,33]
     yes

The ‘base64::parse/2’ predicate accepts other input source such as a
file or a stream.  See the API documentation for details.

For decoding of URLs in the Base64URL format, use the
‘base64url::parse/2’ predicate.  For example:

     | ?- base64url::parse(atom('aHR0cHM6Ly9sb2d0YWxrLm9yZw'), URL).
     URL == 'https://logtalk.org'
     yes

The ‘base64url::parse/2’ predicate also accepts a list of chars or a
list of codes as input.  See the API documentation for details.

6.5 ‘basic_types’
=================

The ‘basic_types’ library is a virtual library that loads only basic
types from the ‘types’ library:

   - ‘comparingp’

   - ‘termp’, ‘term’

   - ‘atomic’, ‘atom’, ‘number’, ‘float’, ‘integer’

   - ‘compound’, ‘listp’, ‘list’

   - ‘type’

6.5.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#types’ link in a web browser.

6.5.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(basic_types(loader)).

6.5.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file for the
‘types’ library:

     | ?- logtalk_load(types(tester)).

6.6 ‘coroutining’
=================

The ‘coroutining’ object provides a portable abstraction over how common
coroutining predicates are made available by the supported backend
Prolog systems (ECLiPSe, LVM, SICStus Prolog, SWI-Prolog, Trealla
Prolog, and YAP) that provide them.  Partial support for XSB is provided
(the predicate ‘frozen/2’ is not available and calls to it fail).

Calls to the library predicates are inlined when compiled with the
‘optimize’ flag turned on.  In this case, there is no overhead compared
with calling the abstracted predicates directly.

See also the ‘dif’ library.

6.6.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#coroutining’ link in a web
browser.

6.6.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(coroutining(loader)).

6.6.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(coroutining(tester)).

6.6.4 Usage
-----------

Load this library from your application loader file.  To call the
coroutining predicates using implicit message sending, add the following
directive to any object or category calling the predicates (adjust the
list to the predicates actually called):

     :- uses(coroutining, [
         dif/2, dif/1, freeze/2, frozen/2, when/2
     ]).

6.7 ‘cbor’
==========

The ‘cbor’ library implements predicates for importing and exporting
data in the Concise Binary Object Representation (CBOR) format:

   - ‘https://www.rfc-editor.org/rfc/rfc8949.html’

   - ‘http://cbor.io/’

This library is a work-in-progress.  Currently it requires a backend
supporting unbounded integer arithmetic.

6.7.1 Representation
--------------------

   - Maps are represented using curly-bracketed terms, ‘{Pairs}’, where
     each pair uses the representation ‘Key-Value’.

   - Arrays are represented using lists.

   - Byte strings uses ‘bytes(List)’ compound terms.

   - Text strings can be represented as atoms, ‘chars(List)’, or
     ‘codes(List)’.  The default when decoding is to use atoms when
     using the ‘cbor’ object.  To decode text strings into lists of
     chars or code, use the ‘cbor/1’ with the parameter bound to ‘chars’
     or ‘codes’.  For example:

          | ?- cbor::parse([0x65,0x68,0x65,0x6c,0x6c,0x6f], Term).
          Term = hello
          yes

          | ?- cbor(atom)::parse([0x65,0x68,0x65,0x6c,0x6c,0x6f], Term).
          Term = hello
          yes

          | ?- cbor(chars)::parse([0x65,0x68,0x65,0x6c,0x6c,0x6f], Term).
          Term = chars([h,e,l,l,o])
          yes

          | ?- cbor(codes)::parse([0x65,0x68,0x65,0x6c,0x6c,0x6f], Term).
          Term = codes([104,101,108,108,111])
          yes

   - Tagged data uses ‘tag(Tag, Data)’ compound terms.

   - Simple values can be represented using ‘simple(Simple)’ compound
     terms.

   - The CBOR elements ‘false’, ‘true’, ‘null’, and ‘undefined’ are
     represented by, respectively, the ‘@false’, ‘@true’, ‘@null’, and
     ‘@undefined’ compound terms.

   - The compound terms ‘@infinity’, ‘@negative_infinity’, and
     ‘@not_a_number’ are used to represent the corresponding CBOR
     elements.

   - Only some backends distinguish between positive zero and negative
     zero.  The compound terms ‘@zero’ and ‘@negative_zero’ can be used
     as an alternative for encoding.  The decoder, however, produces the
     ‘0.0’ and ‘-0.0’ floats.

6.7.2 Encoding
--------------

Encoding is accomplished using the ‘generate/2’ predicate.  For example:

     | ?- cbor::generate([a,{b-c}], Encoding).
     Encoding = [0x9f,0x61,0x61,0xbf,0x61,0x62,0x61,0x63,0xff,0xff]
     yes

The encoding of arrays and maps uses indefinite-length encoding.  All
floats are currently encoded using decimal fractions.  Encoding
indicators and big floats are not currently supported.

6.7.3 Decoding
--------------

Decoding is accomplished using the ‘parse/2’ predicate.  For example:

     | ?- cbor::parse([0x9f,0x61,0x61,0xbf,0x61,0x62,0x61,0x63,0xff,0xff], Term).
     Term = [a,{b-c}]
     yes

6.7.4 API documentation
-----------------------

Open the ‘../../docs/library_index.html#cbor’ link in a web browser.

6.7.5 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(cbor(loader)).

6.7.6 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(cbor(tester)).

6.8 ‘core’
==========

This library consists of built-in entities.

6.8.1 API documentation
-----------------------

See this Handbook “Objects”, “Protocols”, and “Categories” sections.

6.8.2 Loading
-------------

All entities in this library are automatically loaded at Logtalk
startup.

6.8.3 Testing
-------------

Tests for this library can be found in the ‘tests/logtalk/entities’
directory.

6.9 ‘csv’
=========

The ‘csv’ library provides predicates for reading and writing CSV files
and streams:

‘https://www.rfc-editor.org/rfc/rfc4180.txt’

The main object, ‘csv/3’, is a parametric object allowing passing
options for the handling of the header of the file, the fields
separator, and the handling of double-quoted fields.  The ‘cvs’ object
extends the ‘csv/3’ parametric object using default option values.

The library also include predicates to guess the separator and guess the
number of columns in a given CSV file.

Files and streams can be read into a list of rows (with each row being
represented by a list of fields) or asserted using a user-defined
dynamic predicate.  Reading can be done by first loading the whole file
(using the ‘read_file/2-3’ predicates) into memory or line by line
(using the ‘read_file_by_line/2-3’ predicates).  Reading line by line is
usually the best option for parsing large CSV files.

Data can be saved to a CSV file or stream by providing the object and
predicate for accessing the data plus the name of the destination file
or the stream handle or alias.

6.9.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#csv’ link in a web browser.

6.9.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(csv(loader)).

6.9.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(csv(tester)).

6.9.4 Usage
-----------

The ‘csv(Header, Separator, IgnoreQuotes)’ parametric object allows
passing the following options:

  1. ‘Header’: possible values are ‘missing’, ‘skip’, and ‘keep’.

  2. ‘Separator’: possible values are ‘comma’, ‘tab’, ‘semicolon’, and
     ‘colon’.

  3. ‘IgnoreQuotes’: possible values are ‘true’ to ignore double quotes
     surrounding field data and ‘false’ to preserve the double quotes.

The ‘csv’ object uses the default values ‘keep’, ‘comma’, and ‘false’.

When writing CSV files or streams, set the quoted fields option to
‘false’ to write all non-numeric fields double-quoted (i.e.  escaped).

The library objects can also be used to guess the separator used in a
CSV file if necessary.  For example:

     | ?- csv::guess_separator('test_files/crlf_ending.csv', Separator).
     Is this the proper reading of a line of this file (y/n)? [aaa,bb,ccc]
     |> y.

     Separator = comma ?

This information can then be used to read the CSV file returning a list
of rows:

     | ?- csv(keep, comma, true)::read_file('test_files/crlf_ending.csv', Rows).

     Rows = [[aaa,bbb,ccc],[zzz,yyy,xxx]] ?

Alternatively, The CSV data can be saved using a public and dynamic
object predicate (that must be previously declared).  For example:

     | ?- assertz(p(_,_,_)), retractall(p(_,_,_)).
     yes

     | ?- csv(keep, comma, true)::read_file('test_files/crlf_ending.csv', user, p/3).
     yes

     | ?-  p(A,B,C).

     A = aaa
     B = bbb
     C = ccc ? ;

     A = zzz
     B = yyy
     C = xxx

Given a predicate representing a table, the predicate data can be
written to a file or stream.  For example:

     | ?- csv(keep, comma, true)::write_file('output.csv', user, p/3).
     yes

leaving the content just as the original file thanks to the use of
‘true’ for the ‘IgnoreQuotes’ option:

     aaa,bbb,ccc
     zzz,yyy,xxx

Otherwise:

     | ?- csv(keep, comma, false)::write_file('output.csv', user, p/3).
     yes

results in the following file content:

     "aaa","bbb","ccc"
     "zzz","yyy","xxx"

The ‘guess_arity/2’ method, to identify the arity, i.  e.  the number of
fields or columns per record in a given CSV file, for example:

     | ?- csv(keep, comma, false)::guess_arity('test_files/crlf_ending.csv', Arity).
     Is this the proper reading of a line of this file (y/n)? [aaa,bbb,ccc]
     |> y.

     Arity = 3

6.10 ‘dates’
============

The ‘date’ object implements some useful calendar date predicates.

The ‘time’ object implements some useful time predicates.

Please note that the functionality of these objects depends on the
chosen Prolog support for accessing the operating system time and date.

6.10.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#dates’ link in a web browser.

6.10.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(dates(loader)).

6.11 ‘dependents’
=================

The ‘observer’ and ‘subject’ categories implement the Smalltalk
dependents handling mechanism.  This mechanism can be used as an
alternative to Logtalk system-wide support for event-driven programming.

6.11.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#dependents’ link in a web
browser.

6.11.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(dependents(loader)).

6.12 ‘dictionaries’
===================

This library provides a dictionary (also know as associative array, map,
or symbol table) protocol and binary tree, AVL tree, and Red–Black tree
implementations.  The different representations of a dictionary should
be regarded as opaque terms and only accessed using the library
predicates.

6.12.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#dictionaries’ link in a web
browser.

6.12.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(dictionaries(loader)).

6.12.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(dictionaries(tester)).

6.12.4 Usage
------------

First, select the dictionary implementation that you want to use.  For
cases where the number of elements is relatively small and performance
is not critical, ‘bintree’ can be a good choice.  For other cases,
‘avltree’ or ‘rbtree’ are likely better choices.  If you want to compare
the performance of the implementations, either define an object alias or
use a ‘uses/2’ directive so that you can switch between implementations
by simply changing the alias definition or the first argument of the
directive.  Note that you can switch between implementations at runtime
without code changes by using a parameter variable in the first argument
of a ‘uses/2’ directive.

Dictionary keys should preferable be ground terms.  If the keys contain
variables, the user must ensure that any instantiation of those
variables when calling this library predicates will not affect the key
ordering.

To create a new dictionary, you can use the ‘new/1’ predicate.  For
example:

     | ?- avltree::new(Dictionary).
     Dictionary = ...
     yes

You can also create a new dictionary from a list of key-value pairs by
using the ‘as_dictionary/2’ predicate.  For example:

     | ?- avltree::as_dictionary([a-1,c-3,b-2], Dictionary).
     Dictionary = ...
     yes

Several predicates are provided for inserting key-value pairs, lookup
key-value pairs updating the value associated with a key, and deleting
key-value pairs.  For example:

     | ?- avltree::(
             new(Dictionary0),
             insert(Dictionary0, a, 1, Dictionary1),
             update(Dictionary1, a, 2, Dictionary2),
             lookup(a, Value, Dictionary2)
         ).
     Dictionary0 = ...,
     Dictionary1 = ...,
     Dictionary2 = ...,
     Value = 2
     yes

For details on these and other provided predicates, consult the library
API documentation.

6.12.5 Credits
--------------

The AVL tree implementation is an adaptation to Logtalk of the ‘assoc’
SWI-Prolog library authored by R.A.O’Keefe, L.Damas, V.S.Costa, Glenn
Burgess, Jiri Spitz, and Jan Wielemaker.  Additional predicates authored
by Paulo Moura.

The Red–Black tree implementation is an adaptation to Logtalk of the
‘rbtrees’ Prolog library authored by Vitor Santos Costa.

6.13 ‘dif’
==========

The ‘dif’ object provides a portable abstraction over how the ‘dif/2’
predicate is made available by the supported backend Prolog systems that
implement it (B-Prolog, ECLiPSe, LVM, SICStus Prolog, SWI-Prolog,
Trealla Prolog, XSB, and YAP).

Calls to the library predicates are inlined when compiled with the
‘optimize’ flag turned on.  In this case, there is no overhead compared
with calling the abstracted predicate directly.

See also the ‘coroutining’ library.

6.13.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#dif’ link in a web browser.

6.13.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(dif(loader)).

6.13.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(dif(tester)).

6.13.4 Usage
------------

Load this library from your application loader file.  To call the
‘dif/1-2’ predicates using implicit message sending, add the following
directive to any object or category calling the predicates:

     :- uses(dif, [
         dif/2, dif/1
     ]).

6.14 ‘edcg’
===========

This library provides a Logtalk port of the Peter Van Roy’s extended DCG
implementation.  For full documentation on EDCGs, see:

‘https://www.info.ucl.ac.be/~pvr/edcg.html’

This Logtalk version defines a hook object, ‘edcg’.  Source files
defining EDCGs must be compiled using the compiler option ‘hook(edcg)’:

     | ?- logtalk_load(source, [hook(edcg)]).

Alternatively, the following directive can be added at the beginning of
the source file containing the EDCGs:

     :- set_logtalk_flag(hook, edcg).

The hook object automatically adds the EDCGs ‘-->>’ infix operator
scoped to the source file.

This port has simplified by copying and then modifying Michael
Hendricks’s ‘edcg’ repo at:

‘https://github.com/mndrix/edcg’

A notable difference is that Michael’s version declares Peter’s original
predicates for declaring accumulators and predicates using the hidden
arguments as multifile predicates.  But this is risky as two independent
EDCGs may use e.g.  the same accumulator names and introduce conflicts.
The Logtalk version uses instead the ‘edcg’ hook object internal state
to temporarily save those predicates in order to parse the corresponding
EDCGs.

6.14.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#edcg’ link in a web browser.

6.14.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(edcg(loader)).

6.14.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file of the
‘edcgs’ example:

     | ?- logtalk_load(edcgs(tester)).

6.14.4 Usage
------------

Follows the usage documentation written by Michael Hendricks (with a
contribution from Peter Ludemann), used here with permission, with the
necessary changes for the Logtalk port.

     % declare accumulators
     acc_info(adder, X, In, Out, integer::plus(X,In,Out)).

     % declare predicates using these hidden arguments
     pred_info(len,0,[adder,dcg]).
     pred_info(increment,0,[adder]).

     increment -->>
         % add one to the accumulator
         [1]:adder.

     len(Xs,N) :-
         len(0,N,Xs,[]).

     len -->>
         % 'dcg' accumulator has an element
         [_],
         !,
         % increment the 'adder' accumulator
         increment,
         len.
     len -->>
         [].

6.14.5 Introduction
-------------------

DCG notation gives us a single, hidden accumulator.  Extended DCG
notation (implemented by this library) lets predicates have arbitrarily
many hidden accumulators.  As demonstrated by the synopsis above, those
accumulators can be implemented with arbitrary goals (like
‘integer::plus/3’).

Benefits of this library:

   - avoid tedium and errors from manually threading accumulators
     through your predicates

   - add or remove accumulators with a single declaration

   - change accumulator implementation with a single declaration (ex,
     switching from ordsets to rbtrees)

6.14.6 Syntax
-------------

Extended DCG syntax is very similar to DCG notation.  An EDCG is created
with clauses whose neck is the ‘-->>’ operator.  The following syntax is
supported inside an EDCG clause:

   - ‘{Goal}’ - don’t expand any hidden arguments of ‘Goal’

   - ‘Goal’ - expand all hidden arguments of Goal that are also in the
     head.  Those hidden arguments not in the head are given default
     values.

   - ‘Goal:L’ - If ‘Goal’ has no hidden arguments then force the
     expansion of all arguments in ‘L’ in the order given.  If ‘Goal’
     has hidden arguments then expand all of them, using the contents of
     ‘L’ to override the expansion.  ‘L’ is either a term of the form
     ‘Acc’, ‘Acc(Left,Right)’, ‘Pass’, ‘Pass(Value)’, or a list of such
     terms.  When present, the arguments ‘Left’, ‘Right’, and ‘Value’
     override the default values of arguments not in the head.

   - ‘List:Acc’ - Accumulate a list of terms in the accumulator ‘Acc’

   - ‘List’ - Accumulate a list of terms in the accumulator ‘dcg’

   - ‘X/Acc’ - Unify ‘X’ with the left term for the accumulator ‘Acc’

   - ‘Acc/X’ - Unify ‘X’ with the right term for the accumulator ‘Acc’

   - ‘X/Acc/Y’ - Unify ‘X’ with the left and ‘Y’ with the right term for
     the accumulator ‘Acc’

   - ‘insert(X,Y):Acc’ - Insert the arguments ‘X’ and ‘Y’ into the chain
     implementing the accumulator ‘Acc’.  This is useful when the value
     of the accumulator changes radically because ‘X’ and ‘Y’ may be the
     arguments of an arbitrary relation

   - ‘insert(X,Y)’ - Insert the arguments ‘X’ and ‘Y’ into the chain
     implementing the accumulator ‘dcg’.  This inserts the difference
     list X-Y into the accumulated list

6.14.7 Declaration of Predicates
--------------------------------

Predicates are declared with facts of the following form:

     pred_info(Name, Arity, List).

The predicate ‘Name/Arity’ has the hidden parameters given in ‘List’.
The parameters are added in the order given by ‘List’ and their names
must be atoms.

6.14.8 Declaration of Accumulators
----------------------------------

Accumulators are declared with facts in one of two forms.  The short
form is:

     acc_info(Acc, Term, Left, Right, Joiner).

The long form is:

     acc_info(Acc, Term, Left, Right, Joiner, LStart, RStart).

In most cases the short form gives sufficient information.  It declares
the accumulator ‘Acc’, which must be an atom, along with the
accumulating function, ‘Joiner’, and its arguments ‘Term’, the term to
be accumulated, and ‘Left’ & ‘Right’, the variables used in chaining.

The long form of ‘acc_info’ is useful in more complex programs.  It
contains two additional arguments, ‘LStart’ and ‘RStart’, that are used
to give default starting values for an accumulator occurring in a body
goal that does not occur in the head.  The starting values are given to
the unused accumulator to ensure that it will execute correctly even
though its value is not used.  Care is needed to give correct values for
‘LStart’ and ‘RStart’.  For DCG-like list accumulation both may remain
unbound.

Two conventions are used for the two variables used in chaining
depending on which direction the accumulation is done.  For forward
accumulation, ‘Left’ is the input and ‘Right’ is the output.  For
reverse accumulation, ‘Right’ is the input and ‘Left’ is the output.

6.14.9 Declaration of Passed Arguments
--------------------------------------

Passed arguments are conceptually the same as accumulators with ‘=/2’ as
the joiner function.  Passed arguments are declared as facts in one of
two forms.  The short form is:

     pass_info(Pass).

The long form is:

     pass_info(Pass, PStart).

In most cases the short form is sufficient.  It declares a passed
argument ‘Pass’, that must be an atom.  The long form also contains the
starting value ‘PStart’ that is used to give a default value for a
passed argument in a body goal that does not occur in the head.  Most of
the time this situation does not occur.

6.14.10 Additional documentation
--------------------------------

Peter Van Roy’s page: Declarative Programming with State(1)

Technical Report UCB/CSD-90-583 Extended DCG Notation: A Tool for
Applicative Programming in Prolog(2) by Peter Van Roy

   - The Tech Report’s PDF is here(3)

A short Wikipedia article(4) on DCGs and extensions.

   ---------- Footnotes ----------

   (1) https://www.info.ucl.ac.be/~pvr/edcg.html

   (2) https://www2.eecs.berkeley.edu/Pubs/TechRpts/1990/5471.html

   (3) https://www2.eecs.berkeley.edu/Pubs/TechRpts/1990/CSD-90-583.pdf

   (4) https://en.wikipedia.org/wiki/Definite_clause_grammar#Extensions

6.15 ‘events’
=============

The objects ‘event_registry’, ‘before_event_registry’, and
‘after_event_registry’ implement convenient predicates for registering
before and after events.

The code makes use of the ‘monitoring’ built-in protocol, which declares
the two basic event handler predicates (‘before/3’ and ‘after/3’).  You
will need to refer to this protocol in your objects if you want to use
the super control structure ‘(^^/1)’ with these predicates.

The ‘monitor’ object implements more sophisticated event handling
predicates.

6.15.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#events’ link in a web browser.

6.15.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ loader file:

     | ?- logtalk_load(events(loader)).

6.16 ‘expand_library_alias_paths’
=================================

This library provides provides a hook object,
‘expand_library_alias_paths’, for expanding library alias paths in
‘logtalk_library_path/2 facts’ in source files.  It is mainly used when
embedding Logtalk and Logtalk applications.

6.16.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#expand-library-alias-paths’ link
in a web browser.

6.16.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(expand_library_alias_paths(loader)).

6.16.3 Usage
------------

Use the ‘hook/1’ option when compiling a source file:

     | ?- logtalk_load(my_source_file, [hook(expand_library_alias_paths)]).
     ...

Alternatively, assuming it is the only hook object you are using, you
can set it as thew default hook object:

     | ?- set_logtalk_flag(hook, expand_library_alias_paths).
     ...

6.17 ‘expecteds’
================

This library provides an implementation of 'expected terms' with an API
that is inspired by the ‘optional’ library and C++ standardization
proposals for an ‘Expected<T>’ type.  An expected term is an opaque
compound term that either contains an expected value or an error
informing why the expected value is not present.  Expected terms provide
an alternative to generating an exception (or a failure) when something
unexpected happens when asking for a value.  This allows e.g.
separating the code that constructs expected terms from the code that
processes them, which is then free to deal if necessary and at its
convenience with any unexpected events.

6.17.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#expecteds’ link in a web
browser.

6.17.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(expecteds(loader)).

6.17.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(expecteds(tester)).

6.17.4 Usage
------------

The ‘expected’ object provides constructors for expected terms.  For
example:

     | ?- expected::of_expected(1, Expected).
     ...

The created expected terms can then be passed as parameters to the
‘expected/1’ parametric object.  For example:

     | ?- expected::of_expected(1, Expected), expected(Expected)::or_else(Value, 0).
     Expected = expected(1),
     Value = 1
     yes

     | ?- expected::of_unexpected(-1, Expected), expected(Expected)::or_else(Value, 0).
     Expected = unexpected(-1),
     Value = 0
     yes

The ‘either’ object provides types and predicates for extended
type-checking and predicates for handling lists of expected terms.

6.17.5 See also
---------------

The ‘optionals’ library.

6.18 ‘format’
=============

The ‘format’ object provides a portable abstraction over how the de
facto standard ‘format/2-3’ predicates are made available by the
supported backend Prolog systems.  Some system provide these predicates
as built-in predicates while others make them available using a library
that must be explicitly loaded.

Calls to the library predicates are inlined when compiled with the
‘optimize’ flag turned on for most of the backends.  When that’s the
case, there is no overhead compared with calling the abstracted
predicates directly.

This library provides linter checks for calls to the ‘format/2-3’
predicates.  Given the differences between implementation of these
predicates among Prolog systems, the linter checks focus on detecting
common errors such as missing arguments and too many arguments.  The
linter warnings are printed when the ‘suspicious_calls’ flag is set to
‘warning’ (its usual default).

6.18.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#format’ link in a web browser.

6.18.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(format(loader)).

6.18.3 Testing
--------------

Minimal tests for this library predicates can be run by loading the
‘tester.lgt’ file:

     | ?- logtalk_load(format(tester)).

Detailed tests for the ‘format/2-3’ predicates are available in the
‘tests/prolog/predicates’ directory as part of the Prolog standards
conformance test suite.  Use those tests to confirm the portability of
the format specifiers that you want to use.

6.18.4 Usage
------------

Load this library from your application loader file.  To call the
‘format/2-3’ predicates using implicit message sending, add the
following directive to any object or category calling the predicates:

     :- uses(format, [
         format/2, format/3
     ]).

6.18.5 Portability
------------------

Some Prolog systems provide only a subset of the expected format
specifiers.  Notably, table related format specifiers are only fully
supported by a few systems.  See the section below on testing.

Only some of the supported Prolog backends provide implementations of
the ‘format/2-3’ predicates that allow using not only an atom or a list
of character codes for the format string (as de facto standard) but also
using a list of characters.  These currently include ECLiPSe, GNU
Prolog, LVM, SICStus Prolog, SWI-Prolog, Trealla Prolog, and YAP.
Therefore, when wide portability is sought, atoms must be used for the
format specifier argument as they bypass any dependency on the
‘double_quotes’ standard Prolog flag.  Some systems, like Tau Prolog,
only accept a list of characters for the format string.  In this case,
this library will convert the atom format string before calling these
systems native implementations.

6.19 ‘gensym’
=============

The ‘gensym’ library implements predicates for generating unique atoms.
The public predicates are declared synchronized to prevent race
conditions when using a backend Prolog compiler with multi-threading
support.

6.19.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#gensym’ link in a web browser.

6.19.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(gensym(loader)).

6.19.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(gensym(tester)).

6.19.4 Usage
------------

The ‘gensym_core’ category implements the library predicates.  This
category is imported by the default ‘gensym’ object to provide
application global generators.  To make the generators local and thus
minimize the potential for generator name clashes, the category can be
imported by one of more application objects.  Use protected or private
import to restrict the scope of the library predicates.  For example:

     :- object(foo,
         imports(private::gensym_core)).

         bar :-
             ^^gensym(p, S),
             ...

     :- end_object.

6.20 ‘genint’
=============

The ‘genint’ library implements predicates for generating positive
integers in increasing order.  The public predicates are declared
synchronized to prevent race conditions when using a backend Prolog
compiler with multi-threading support.

6.20.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#genint’ link in a web browser.

6.20.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(genint(loader)).

6.20.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(genint(tester)).

6.20.4 Usage
------------

The ‘genint_core’ category implements the library predicates.  This
category is imported by the default ‘genint’ object to provide
application global named counters.  To make the counters local and thus
minimize the potential for counter name clashes, the category can be
imported by one of more application objects.  Use protected or private
import to restrict the scope of the library predicates.

6.21 ‘git’
==========

This library provides access to a git project current branch and latest
commit data (e.g.  commit hash).  Support for using this library on
Windows operating-systems is experimental and may or may not work
depending on the used Prolog backend.

6.21.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#git’ link in a web browser.

6.21.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(git(loader)).

6.21.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(git(tester)).

6.21.4 Usage
------------

All predicates take as first argument a directory, which should be
either a git repository directory or a sub-directory of a git repository
directory.  The main predicate is ‘commit_log/3’.  It provides access to
the ‘git log’ command when called with the ‘--oneline -n 1
--pretty=format:’ options.  By passing as second argument the desired
format, it returns an atom with the formatted output.  For example:

     | ?- git::commit_log('/Users/pmoura/logtalk3', '%h%n%B', Output),
          write(Output), nl.

     eccaa1a2a
     Update SVG diagrams

     Output = 'eccaa1a2a\nUpdate SVG diagrams\n'
     yes

See e.g.  the official documentation on ‘git log’ pretty formats for
details:

‘https://git-scm.com/docs/pretty-formats’

Convenient predicates are also provided for common used formats such as
the commit author and the commit hash.  For example:

     | ?- git::commit_author('/Users/pmoura/Documents/Logtalk/logtalk3', Author).

     Author = 'Paulo Moura'
     yes

     | ?- git::commit_hash('/Users/pmoura/Documents/Logtalk/logtalk3', Hash).

     Hash = eccaa1a2a9495fef441915bbace84e0a4b0394a2
     yes

It’s also possible to get the name of the current local branch.  For
example:

     | ?- git::branch('/Users/pmoura/Documents/Logtalk/logtalk3', Branch).

     Branch = master
     yes

6.22 ‘grammars’
===============

This library provides Definite Clause Grammars (DCGs) for common parsing
tasks.  The DCGs support parsing both lists of characters (aka chars)
and lists of character codes (aka codes).

Currently, four groups of DCGs are available, each defined in its own
file:

   - blanks (‘blank_grammars.lgt’)

   - numbers (‘number_grammars.lgt’)

   - sequences (‘sequence_grammars.lgt’)

   - IP addresses (‘ip_grammars.lgt’; depends on ‘number_grammars.lgt’)

6.22.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#grammars’ link in a web browser.

6.22.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(grammars(loader)).

6.22.3 Testing
--------------

Minimal tests for this library predicates can be run by loading the
‘tester.lgt’ file:

     | ?- logtalk_load(grammars(tester)).

6.22.4 Usage
------------

The library uses (when necessary) parametric objects where the single
parameter can be either ‘chars’ or ‘codes’.  The parameter must be bound
when using the DCGs.  For example, when using implicit message sending:

     :- uses(blank_grammars(chars), [
         white_spaces//0, new_lines//0
     ]).

6.23 ‘heaps’
============

This library implements minimum and maximum heaps.

6.23.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#heaps’ link in a web browser.

6.23.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(heaps(loader)).

6.23.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(heaps(tester)).

6.23.4 Credits
--------------

Original code by Richard O’’Keefe.  Adapted to Logtalk by Paulo Moura
and Victor Lagerkvist.

6.24 ‘hierarchies’
==================

This library provides categories implementing reflection predicates over
class and prototype hierarchies.  These categories can be imported by
any object that requires reasoning about hierarchies.

6.24.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#hierarchies’ link in a web
browser.

6.24.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(hierarchies(loader)).

6.24.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(hierarchies(tester)).

6.25 ‘hook_flows’
=================

Hook objects (i.e.  objects that define term- and goal-expansion rules)
can be combined to define expansion 'workflows'.  While in some cases
the expansions are independent and thus can be applied in any order, in
other cases a specific order is required.  The ‘hook_pipeline’ and
‘hook_set’ parametric objects in this library implement the two most
common scenarios of combining multiple hook objects for the expansion of
source files.  These parametric hook objects can be combined to define
workflows of any complexity (e.g.  a pipeline where one of the steps is
set with an element that is a pipeline).  These two basic hook flows can
also used as examples of how to construct your own custom expansion
workflows.

6.25.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#hook-flows’ link in a web
browser.

6.25.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(hook_flows(loader)).

6.25.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(hook_flows(tester)).

6.25.4 Usage
------------

Pre-processing a source file with a hook object can be accomplished by
either compiling the source file using the option ‘hook(HookObject)’ or
by adding to the top of the file the directive:

     :- set_logtalk_flag(hook, HookObject).

Note that ‘set_logtalk_flag/2’ directives are local to a source file.

The ‘hook_pipeline(Pipeline)’ is a parametric object where the parameter
is a list of hook objects, interpreted as a pre-processing pipeline: the
results of a hook object are passed to the next hook object.  This
parametric object is used when the expansions must be applied in a
specific order.  It also allows overriding the default compiler
semantics where term-expansion rules are tried in sequence only until
one of them succeeds.

The ‘hook_set(Set)’ is a parametric object where the parameter is a list
of hook objects, interpreted as a set of hook objects.  For
term-expansion, hooks in the set are tried until one of them succeeds.
For goal-expansion, as the compiler expands a goal until a fixed-point
is reached, all the hooks objects in the set that are applicable at any
point will be used.  This parametric object is used when applying
multiple independent expansions.

When using a backend Prolog compiler that supports modules, it’s also
possible to use as parameter a list of hook modules as long as their
names do not coincide with the names of loaded objects.

6.26 ‘hook_objects’
===================

This library provides a set of convenient hook objects for defining
custom expansion workflows (using e.g.  the ‘hook_flows’ library) and
for debugging.  They are usable and useful as-is but should also be
regarded as term- and goal-expansion examples that you can learn from,
clone, and change to fit your application requirements.

6.26.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#hook-objects’ link in a web
browser.

6.26.2 Loading
--------------

To load all hook objects in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(hook_objects(loader)).

To load a specific hook object, e.g.  the ‘backend_adapter_hook’ object:

     | ?- logtalk_load(hook_objects(backend_adapter_hook)).

6.26.3 Testing
--------------

To test this library hook objects, load the ‘tester.lgt’ file:

     | ?- logtalk_load(hook_objects(tester)).

6.26.4 Usage
------------

The provided hook objects cover different expansion scenarios as
follows.

6.26.4.1 Using the Prolog backend adapter file expansion rules
..............................................................

Useful when defining a custom expansion workflow.  This can be
accomplished by loading the ‘backend_adapter_hook.lgt’ file, which
defines a ‘backend_adapter_hook’ hook object that can be used as a
workflow step.

6.26.4.2 Restoring the default compiler expansion workflow
..........................................................

In this case, load the ‘default_workflow_hook.lgt’ file, which defines a
‘default_workflow_hook’ hook object, and use the following 'goal' to set
the default hook object:

     | ?- set_logtalk_flag(hook, default_workflow_hook).

6.26.4.3 Preventing applying any (other) user-defined expansion rules
.....................................................................

When compiling a source file, we sometimes want to prevent applying
expansion rules.  This can be accomplished by simply loading the
‘identity_hook.lgt’ file, which defines the ‘identity_hook’ hook object,
whose expansion rules simply succeed without changing the terms and
goals, and setting it as the file specific hook object writing as the
first term in the file the 'directive':

     :- set_logtalk_flag(hook, identity_hook).

Note that the compiler will always convert any grammar rules defined in
the file into clauses.  Although this conversion can also be performed
as an expansion, grammar rules are part of the Logtalk language.  If you
to preserve the grammar rules, use the hook objects described below to
write them to a stream.

6.26.4.4 Expanding grammar rules into clauses independently of the compiler
...........................................................................

Load the ‘grammar_rules_hook.lgt’ and use the term-expansion rules in
the ‘grammar_rules_hook’ object.  For example:

     | ?- grammar_rules_hook::term_expansion((a --> [b],c), Clause).

     Clause = (a([b|T], C) :- c(T, C))
     yes

6.26.4.5 Using the expansion rules defined in a Prolog module
.............................................................

Load the ‘prolog_module_hook.lgt’, which defines the parametric hook
object ‘prolog_module_hook(Module)’.  To use this hook object, you need
to instantiate the parameter to the name of the module.  For example:

     :- set_logtalk_flag(hook, prolog_module_hook(user)).

6.26.4.6 Wrap the contents of a plain Prolog file as an object
..............................................................

Load the ‘object_wrapper_hook.lgt’, which defines the
‘object_wrapper_hook/0-2’ hook objects.  Use them to wrap the contents
of a plain Prolog file as an object named after the file (optionally
implementing a protocol) or an object with the given name and object
relations.  Can be used to apply Logtalk developer tools to plain Prolog
code or when porting Prolog application to Logtalk.  For example:

     | ?- logtalk_load('plain.pl', [hook(object_wrapper_hook)]).
     ...

     | ?- current_object(plain).
     yes

Or:

     | ?- logtalk_load('world_1.pl', [hook(object_wrapper_hook(some_protocol))]).
     ...

     | ?- current_object(world_1).
     yes

     | ?- implements_protocol(world_1, Protocol).
     Protocol = some_protocol
     yes

Or:

     | ?- logtalk_load('foo.pl', [hook(object_wrapper_hook(bar,[imports(some_category))]).
     ...

     | ?- current_object(bar).
     yes

     | ?- imports_category(bar, Category).
     Category = some_category
     yes

The ‘object_wrapper_hook’ object sets the ‘context_switching_calls’ flag
to ‘allow’ for the generated object.  This enables calling the
predicates using the ‘(<<)/2’ context-switching control construct.  But
it’s usually better to define a protocol for the predicates being
encapsulated and use instead the ‘object_wrapper_hook/1-2’ objects.

6.26.4.7 Outputting term-expansion results to a stream or a file
................................................................

Load the ‘write_to_stream_hook.lgt’ file and using the
‘write_to_stream_hook(Stream)’ or ‘write_to_stream_hook(Stream,
Options)’ hook objects.  Alternatively, you can load the
‘write_to_file_hook.lgt’ file and use the ‘write_to_file_hook(File)’ or
‘write_to_file_hook(File, Options)’ hook objects.  The terms are not
modified and thus these hook objects may be used at any point in an
expansion workflow.  The terms are written followed by a period and a
new line.

For example, assume that we want to expand all terms in a ‘input.pl’
source file, writing the resulting terms to a ‘output.pl’ file, using
the expansion rules defined in a ‘expansions’ hook object.  Taking
advantage of the ‘hook_flows’ library ‘hook_pipeline/1’ object, we can
write:

     | ?- logtalk_compile(
              'input.pl',
              [hook(hook_pipeline([
                  expansions,
                  write_to_file_hook('output.pl')
              ]))]
          ).

6.26.4.8 Printing entity predicate goals before or after calling them
.....................................................................

This is helpful for quick debugging.  Load the ‘print_goal_hook.lgt’
file and use the ‘print_goal_hook’ hook object.  For example, we can set
this hook object as the default hook:

     | ?- set_logtalk_flag(hook, print_goal_hook).

Then, edit the entity source code to print selected goals:

     foo :-
         - bar,   % print goal before calling it
         + baz,   % print goal after calling it
         * quux.  % print goal before and after calling it

6.26.4.9 Suppressing goals
..........................

The ‘suppress_goal_hook.lgt’ file provides the ‘suppress_goal_hook’ hook
object that supports suppressing a goal in a clause body by prefixing it
using the ‘--’ operator.  We can set this hook object as the default
hook using the goal:

     | ?- set_logtalk_flag(hook, suppress_goal_hook).

If the expansion is only to be used in a single file, use instead the
source file directive:

     :- set_logtalk_flag(hook, suppress_goal_hook).

Then, edit entity predicates to suppress goals.  For example:

     foo :-
         bar,
         -- baz,
         quux.

The suppressed goals are replaced by calls to ‘true/0’.

6.27 ‘html’
===========

This library provides predicates for generating HTML content using
either HTML 5 or XHTML 1.1 formats from a term representation.  The
library performs minimal validation, checking only that all elements are
valid.  No attempt is made to generate nicely indented output.

Normal elements are represented using a compound term with one argument
(the element content) or two arguments (the element attributes
represented by a list of ‘Key=Value’ or ‘Key-Value’ pairs and the
element content).  The element content can be another element or a list
of elements.  For example:

     ol([type-a], [li(foo), li(bar), li(baz)])

The two exceptions are the ‘pre’ or ‘code’ elements whose content is
never interpreted as an element or a list of elements.  For example, the
fragment:

     pre([foo,bar,baz])

is translated to:

     <pre>
     [foo,bar,baz]
     </pre>

Void elements are represented using a compound term with one argument,
the (possibly empty) list of attributes represented by a list of
‘Key=Value’ or ‘Key-Value’ pairs.  For example:

     hr([class-separator])

Atomic arguments of the compound terms are interpreted as element
content.  Non atomic element content can be represented as a quoted atom
or by using the ‘pre’ or ‘code’ elements as explained above.

This library is a work in progress.

6.27.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#html’ link in a web browser.

6.27.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(html(loader)).

6.27.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(html(tester)).

6.27.4 Generating a HTML document
---------------------------------

HTML documents can be generated from a compound term representation and
written to a file or a stream.  For example, assuming we want to
generate a HTML 5 file:

     | ?- html5::generate(
              file('hello.html'),
              html([lang=en], [head(title('Hello world!')), body(p('Bye!'))])
          ).

When the second argument is a ‘html/1’ or ‘html/2’ compound term, a
'doctype' is automatically written.  If we prefer instead e.g.  a XHTML
1.1 document, we use the ‘xhtml11’ object:

     | ?- xhtml11::generate(
              file('hello.html'),
              html([lang=en], [head(title('Hello world!')), body(p('Bye!'))])
          ).

6.27.5 Generating a HTML fragment
---------------------------------

It’s also possible to generate just a fragment of a (X)HTML document by
using a list of compound terms or a compound term for an element other
then ‘html’.  For example:

     | ?- current_output(Stream),
          html5::generate(stream(Stream), ul([li(foo), li(bar), li(baz)])).

     <ul>
     <li>
     foo</li>
     <li>
     bar</li>
     <li>
     baz</li>
     </ul>

     Stream = ...

6.27.6 Working with callbacks to generate content
-------------------------------------------------

Often we need to programmatically generate HTML content from queries.
In other cases, we may have fixed content that we don’t want to keep
repeating (e.g.  a navigation bar).  The library supports a ‘(::)/2’
pseudo-element that sends a message to an object to retrieve content.
As an example, assume the following predicate definition in ‘user’:

     content(strong('Hello world!')).

This predicate can then be called from the HTML term representation.
For example:

     | ?- current_output(Stream),
          html5::generate(stream(Stream), span(user::content)).

     <span><strong>Hello world!</strong></span>

     Stream = ...

Note that the callback always takes the form ‘Object::Closure’ where
‘Closure’ is extended with a single argument (to be bound to the
generated content).  More complex callbacks are possible by using lambda
expressions.

6.27.7 Working with custom elements
-----------------------------------

The ‘html5’ and ‘xhtml11’ objects recognize the same set of standard
HTML 5 normal and void elements and generate an error for non-standard
elements.  If you need to generate HTML content containing custom
elements, define a new object that extends one of the library objects.
For example:

     :- object(html5custom,
         extends(html5)).

         normal_element(foo, inline).
         normal_element(bar, block).
         normal_element(Name, Display) :-
             ^^normal_element(Name, Display).

     :- end_object.

6.28 ‘ids’
==========

This library generates random identifiers given the number of bytes of
randomness.  The identifiers are Base64 encoded.  By default, 20 bytes
(160 bits) are used.

The generation of random identifiers uses the ‘/dev/urandom’ random
number generator when available.  This includes macOS, Linux, *BSD, and
other POSIX operating-systems.  On Windows, a pseudo-random generator is
used but randomized using the current wall time.

Identifiers can be generated as atoms, lists of characters, or lists of
character codes.

See also the ‘uuid’ and ‘ulid’ libraries.

6.28.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#ids’ link in a web browser.

6.28.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(ids(loader)).

6.28.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(ids(tester)).

6.28.4 Usage
------------

To generate an identifier using the default 160 bits of randomness:

     | ?- ids::generate(Identifier).
     Identifier = '2gpMzqAFXBO5mYFIPX1qMkHxgGE='
     yes

To generate an identifier represented by an atom using 240 bits (30
bytes) of randomness:

     | ?- ids(atom, 30)::generate(Identifier).
     Identifier = 'ie/jYcLsqo8ZguCOF1ZNPFDRvJ03Ww5Qa9e0FxRB'
     yes

To generate an identifier represented by a list of characters using 64
bits (8 bytes) of randomness:

     | ?- ids(chars, 8)::generate(Identifier).
     Identifier = ['5','0','8','V',d,'S',c,y,n,o,'A',=]
     yes

To generate an identifier represented by a list of character codes using
64 bits (8 bytes) of randomness:

     | ?- ids(codes, 8)::generate(Identifier).
     Identifier = [111,81,86,55,99,79,70,77,65,74,103,61]
     yes

6.29 ‘intervals’
================

This library provides an ‘intervalp’ protocol and an ‘interval’ object
that implement basic temporal interval relations protocol (based on the
James F. Allen Interval Algebra work).

6.29.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#intervals’ link in a web
browser.

6.29.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(intervals(loader)).

6.29.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(intervals(tester)).

6.30 ‘java’
===========

The library Java entities define a minimal abstraction for calling Java
from Logtalk.  This abstraction makes use of Logtalk parametric objects
and allows creating Java object, accessing Java class fields, and
calling Java class and object methods using syntax closer to Logtalk.
It also gives access to some Java utility predicates.

This abstraction was developed primarily to work with LVM (with its
‘jni’ plug-in installed) or the JPL library bundled with SWI-Prolog and
YAP. However, it’s expected to be implementable with alternative Java
interfaces found in other backend Prolog compilers.  Currently, a
preliminary implementation is also available for JIProlog.

The main idea in this abstraction layer is to use parametric objects
where the first parameter holds the Java reference (usually to a class
or object) and an optional second parameter holds the return value.
Together with a forward message handler, this allows the use of Java
messages with the same functor and number of arguments as found in the
relevant JavaDocs.

6.30.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#java’ link in a web browser.

6.30.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(java(loader)).

6.30.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(java(tester)).

6.30.4 Usage
------------

The two main objects in this library are ‘java(Reference, ReturnValue)’
and ‘java(Reference)’.  Use the latter if you want to ignore the return
value or when calling a void Java method.

The ‘java’ object implements utility predicates.  For some backend Java
interfaces such as JPL (available in LVM, SWI-Prolog, and YAP) there is
also a ‘java_hook’ hook object for removing any overhead when using this
library abstraction.

For usage examples and additional tests, see the ‘clustering’,
‘document_converter’, ‘jpl’, and ‘neo4j’ examples.

6.30.5 Known issues
-------------------

When running Java GUI examples on the macOS Terminal application, you
may get a Java error saying that the AWT cannot be started.  In
alternative, try to run the example from within the SWI-Prolog macOS
application instead of using the shell integration script.  This issue
is due to a macOS Java issue that’s orthogonal to both Prolog backends
and Logtalk.

6.31 ‘json’
===========

The ‘json’ library provides predicates for parsing and generating data
in the JSON format based on the specification and standard found at:

   - ‘https://www.rfc-editor.org/rfc/rfc8259’

   -
     ‘https://www.ecma-international.org/publications-and-standards/standards/ecma-404/’

It includes parametric objects whose parameters allow selecting the
representation for parsed JSON objects (‘curly’ or ‘list’), JSON text
strings (‘atom’, ‘chars’, or ‘codes’) and JSON pairs (‘dash’, ‘equal’,
or ‘colon’).

6.31.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#json’ link in a web browser.

6.31.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(json(loader)).

6.31.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(json(tester)).

Some of the sample JSON test files are based on examples published at:

‘https://www.json.org/’

6.31.4 Representation
---------------------

The following choices of syntax has been made to represent JSON elements
as terms:

   - By default, JSON objects are represented using curly-bracketed
     terms, ‘{Pairs}’, where each pair uses the representation
     ‘Key-Value’ (see below for alternative representations).

   - Arrays are represented using lists.

   - Text strings can be represented as atoms, ‘chars(List)’, or
     ‘codes(List)’.  The default when decoding is to use atoms when
     using the ‘json’ object.  To decode text strings into lists of
     chars or codes, use the ‘json/1’ with the parameter bound to
     ‘chars’ or ‘codes’.  For example:

          | ?- json::parse(codes([34,104,101,108,108,111,34]), Term).
          Term = hello
          yes

          | ?- json(atom)::parse(codes([34,104,101,108,108,111,34]), Term).
          Term = hello
          yes

          | ?- json(chars)::parse(codes([34,104,101,108,108,111,34]), Term).
          Term = chars([h,e,l,l,o])
          yes

          | ?- json(codes)::parse(codes([34,104,101,108,108,111,34]), Term).
          Term = codes([104,101,108,108,111])
          yes

   - The JSON values ‘false’, ‘true’ and ‘null’ are represented by,
     respectively, the ‘@false’, ‘@true’ and ‘@null’ compound terms.

The following table exemplifies the term equivalents of JSON elements
using default representations for objects, pairs, and strings:

JSON                          term

------------------------------------------------------------

[1,2]                         [1,2]

true                          @true

false                         @false

null                          @null

-1                            -1

[1.2345]                      [1.2345]

[]                            []

[2147483647]                  [2147483647]

[0]                           [0]

[1234567890123456789]         [1234567890123456789]

[false]                       [@false]

[-2147483648]                 [-2147483648]

{“a”:null,”foo”:”bar”}        <{a-@null>, foo-bar}

[2.225073858507201e-308]      [2.225073858507201e-308]

[0,1]                         [0,1]

[2.2250738585072014e-308]     [2.2250738585072014e-308]

[1.7976931348623157e+308]     [1.7976931348623157e+308]

[0.0]                         [0.0]

[4294967295]                  [4294967295]

[-1234567890123456789]        [-1234567890123456789]

[“foo”]                       [foo]

[1]                           [1]

[null]                        [@null]

[-1.2345]                     [-1.2345]

[5.0e-324]                    [5.0e-324]

[-1]                          [-1]

[true]                        [@true]

[9223372036854775807]         [9223372036854775807]

For JSON objects that are two possible term representations:

JSON object               term (curly)

----------------------------------------------

{“a”:1, “b”:2, “c”:3}     {a-1, b-2, c-3}

{}                        {}

and:

JSON object               term (list)

----------------------------------------------------

{“a”:1, “b”:2, “c”:3}     json([a-1, b-2, c-3])

{}                        json([])

For JSON pairs that are three possible representations:

JSON object               term (dash)

----------------------------------------------

{“a”:1, “b”:2, “c”:3}     {a-1, b-2, c-3}

and:

JSON object               term (equal)

----------------------------------------------

{“a”:1, “b”:2, “c”:3}     {a=1, b=2, c=3}

and:

JSON object               term (colon)

----------------------------------------------

{“a”:1, “b”:2, “c”:3}     {a:1, b:2, c:3}

By default, the curly-term representation and the dash pair
representation are used.  The ‘json/3’ parametric object allows
selecting the desired representation choices.  For example:

     | ?- json(curly,dash,atom)::parse(atom('{"a":1, "b":2, "c":3}'), JSON).
     JSON = {a-1, b-2, c-3}
     yes

     | ?- json(list,equal,atom)::parse(atom('{"a":1, "b":2, "c":3}'), JSON).
     JSON = json([a=1, b=2, c=3])
     yes

     | ?- json(curly,colon,atom)::parse(atom('{"a":1, "b":2, "c":3}'), JSON).
     JSON = {a:1, b:2, c:3}
     yes

6.31.5 Encoding
---------------

Encoding is accomplished using the ‘generate/2’ predicate.  For example:

     | ?- json::generate(codes(Encoding), [a,{b-c}]).
     Encoding = [91,34,97,34,44,123,34,98,34,58,34,99,34,125,93]
     yes

Alternatively:

     | ?- json::generate(chars(Encoding), [a,{b-c}]).
     Encoding = ['[','"',a,'"',',','{','"',b,'"',:,'"',c,'"','}',']']
     Yes

     | ?- json::generate(atom(Encoding), [a,{b-c}]).
     Encoding = '["a",{"b":"c"}]'
     Yes

Notice that ‘generate/2’ takes, as second argument, a Prolog term that
corresponds to the JSON Syntax in Prolog and produces the corresponding
JSON output in the format specified in the first argument:
(‘codes(Variable)’, ‘stream(Stream)’, ‘file(File)’, ‘chars(Variable)’ or
‘atom(Variable)’).

6.31.6 Decoding
---------------

Decoding is accomplished using the ‘parse/2’ predicate.  For example, to
decode a given json file:

     | ?- json::parse(file('simple/roundtrip_array_obj_array.json'), Term).
     Term = [{a-[b]}]
     yes

The ‘parse/2’ predicate first argument must indicate the input source
(‘codes(Codes)’, ‘stream(Stream)’, ‘line(Stream)’, ‘file(Path)’,
‘chars(Chars)’ or ‘atom(Atom)’) containing a JSON payload to be decoded
into the Prolog term in the second argument.

6.31.7 Known issues
-------------------

Some tests, notably ‘parse_simple_valid_files’ and
‘roundtrip_hexadecimals’, fail on backends such as ECLiPSe and GNU
Prolog that don’t support Unicode.

6.32 ‘listing’
==============

This library provides support for listing object dynamic predicate
clauses.  The predicates must (also) be declared using a scope directive
to make them visible to the listing predicates.

6.32.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#listing’ link in a web browser.

6.32.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(listing(loader)).

6.32.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(listing(tester)).

6.32.4 Usage
------------

This library provides a ‘listing’ category that can be imported by any
number of objects.  The main predicates are declared public.  If you
want to restrict their scope, use protected or private import.  For
example:

     :- object(data_store,
         imports(private::listing)).

         debug :-
             ^^listing(data/4).
         ...

     :- end_object.

The ‘listing’ category provides a bare bones ‘portray_clause/1’
predicate implementation.  As this predicate is called (by the
‘listing/0-1’ predicates) using the ‘(::)/1’ control construct, the
object importing the category can easily override the inherited
definition with its own or with a call to the backend system native
implementation of the predicate.  For example, assuming a backend that
provides ‘portray_clause/1’ as a built-in predicate, we can write:

     :- object(thing,
         imports(listing)).

         :- uses(user, [portray_clause/1]).
         ...

     :- end_object.

The main predicate, ‘listing/1’, accepts as argument a predicate
indicator, a non-terminal indicator, or a clause head template (to list
only clauses with a matching head).

This library is often useful as a debugging helper.  For example,
assuming that the we want to list dynamic predicate clauses for an
object ‘data’ compiled (or created) with the ‘complements’ flag set to
‘allow’, we can hot patch it to add the ‘listing’ category:

     | ?- create_category(patch, [extends(listing),complements(data)], [], []).
     yes

     | ?- data::listing.
     ...

Another example:

     | ?- create_category(
              patch,
              [extends(listing),complements(data)],
              [public(debug/1)],
              [(debug(Key) :- ::listing(p(Key,Datum)))]
          ).
     yes

     | ?- data::debug(k42).
     ...

Note that the semantic of complementing categories require that we use
the ‘(::)/2’ control construct instead of the ‘(^^)/2’ control construct
as a 'super' call would be interpreted as made from the complemented
object.

6.33 ‘logging’
==============

This library provides support for logging events to files.

6.33.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#logging’ link in a web browser.

6.33.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(logging(loader)).

6.34 ‘loops’
============

This library provides implementations of several kinds of loops typical
of imperative languages.

6.34.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#loops’ link in a web browser.

6.34.2 Loading
--------------

To load the main entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(loops(loader)).

6.34.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(loops(tester)).

6.34.4 Usage
------------

See e.g.  the ‘searching’ example.

6.35 ‘meta’
===========

This library provides implementations of common meta-predicates.  The
‘meta’ object implements common meta-predicates like ‘map/3’ and
‘fold_left/4’.

See also the ‘meta_compiler’ library, which provides optimized
compilation of meta-predicate calls.

6.35.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#meta’ link in a web browser.

6.35.2 Loading
--------------

To load the main entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(meta(loader)).

6.35.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(meta(tester)).

6.35.4 Usage
------------

See e.g.  the ‘metapredicates’ example and unit tests.

6.36 ‘meta_compiler’
====================

This library supports implementations optimized compilation of
meta-calls for the predicates defined in the ‘meta’ library.

6.36.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#meta-compiler’ link in a web
browser.

6.36.2 Loading
--------------

To load the main entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(meta_compiler(loader)).

6.36.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(meta_compiler(tester)).

6.36.4 Usage
------------

If ‘meta_compiler’ is the only hook object you are using, you can set it
as the default hook object (but note that the optimizations are only
applied to entities compiled with the ‘optimize’ flag turned on):

     | ?- set_logtalk_flag(hook, meta_compiler).
     ...

Otherwise, use the ‘hook(meta_compiler)’ and ‘optimize(on)’ complier
options when compiling and loading the code that you want to optimize.
For example:

     | ?- logtalk_load(my_source_file, [hook(meta_compiler), optimize(on)]).
     ...

See also the ‘metapredicates_compiled’ example and unit tests.

6.37 ‘mutations’
================

Experimental library.  Should not be used in production code.  Details
can be changed without advance notice.

The ‘mutations’ library provides support for generating random mutations
of selected types.  The library defines default mutation algorithms for
the following basic types:

   - ‘atom’

   - ‘integer’

   - ‘float’

   - ‘compound’

   - ‘list’

The user can add additional mutation algorithms for these or other types
by defining objects or categories providing clauses for the ‘mutation/3’
predicate and expanding the entity source files using the
‘mutations_store’ object as the hook object.

This library is expected to eventually be used to support mutation-based
'fuzz testing'.

By default, loading this library loads a set of default mutation
algorithms.  These can be overriden by defining alternative mutations
and a custom loader file.

6.37.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#mutations’ link in a web
browser.

6.37.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(mutations(loader)).

6.37.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(mutations(tester)).

6.37.4 Usage
------------

The ‘mutations’ category complements the ‘type’ object and thus its
predicates are accessed via this object.  For example:

     | ?- type::mutation(integer, 123, M).
     M = 1293
     yes

     | type::mutation(integer, 123, M).
     M = 5123
     yes

     | type::mutation(integer, 123, M).
     M = -123
     yes

     | type::mutation(integer, 123, M).
     M = 23
     yes

When there are multiple mutation algorithms for a given type, the
predicate ‘type::mutation/3’ choses one of them randomly.  We can query
the number of mutation algorithms available per type using the
‘mutations_store::counter/2’ predicate:

     | ?- mutations_store::counter(Type, Count).
     Type = atom,
     Count = 6 ;
     Type = integer,
     Count = 7 ;
     ...

Loading this library also loads the ‘arbitrary’ library, which provides
‘get_seed/1’ and ‘set_seed/1’ predicates that can be used to control the
pseudo-random number generator.

6.38 ‘nested_dictionaries’
==========================

This library provides nested dictionary implementations based on private
extensions to the ‘dictionaries’ library objects.  The representations
of a nested dictionary should be regarded as opaque terms and only
accessed using the library predicates.

This library is experimental, a work in progress, and future versions
can introduce incompatible changes.

6.38.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#nested-dictionaries’ link in a
web browser.

6.38.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(nested_dictionaries(loader)).

6.38.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(nested_dictionaries(tester)).

6.38.4 Usage
------------

First, select the nested dictionary implementation that you want to use.
For cases where the number of elements is relatively small and
performance is not critical, ‘nbintree’ can be a good choice.  For other
cases, ‘navltree’ or ‘nrbtree’ are likely better choices.  If you want
to compare the performance of the implementations, either define an
object alias or use a ‘uses/2’ directive so that you can switch between
implementations by simply changing the alias definition or the first
argument of the directive.  Note that you can switch between
implementations at runtime without code changes by using a parameter
variable in the first argument of a ‘uses/2’ directive.

To create an empty nested dictionary, you can use the ‘new/1’ predicate.
For example:

     | ?- navltree::new(Dictionary).
     Dictionary = ...
     yes

You can also create a new nested dictionary from a 'curly bracketed term
representation' (see below) by using the predicate
‘as_nested_dictionary/2’.  For example:

     | ?- navltree::as_nested_dictionary(
             {a-1, b-{c-3, d-{e-7,f-8}}},
             Dictionary
          ).

     Dictionary = ...
     yes

Several predicates are provided to insert, lookup, update, and delete
key-value pairs given a list of keys interpreted as an 'access path' to
a nested dictionary.  For example:

     | ?- navltree::as_nested_dictionary(
             {a-1, b-{c-3, d-{e-7,f-8}}},
             Dictionary
          ),
          navltree::lookup_in([b,d,f], Value, Dictionary).

     Dictionary = ...
     Value = 8
     yes

For details on these and other provided predicates, consult the library
API documentation.

6.38.5 Curly term representation
--------------------------------

To simplify importing and exporting data into a nested dictionary, the
library provides ‘as_nested_dictionary/2’ and ‘as_curly_bracketed/2’
predicates that work with a 'curly term representation'.  This format is
based on the JSON data interchange format.

A dictionary is represented by the ‘{Pairs}’ term where Pairs is a
conjunction of ‘Key-Value’ or ‘Key:Value’ pairs and ‘Value’ can be a
nested dictionary or lists of pairs.  An empty dictionary is represented
using the ‘{}’ term.

6.39 ‘optionals’
================

This library provides an implementation of 'optional terms' with an API
modeled after the Java 8 ‘Optional’ class (originally due to requests by
users working in Logtalk/Java hybrid applications).  An optional term is
an opaque compound term that may or may not hold a value.  Optional
terms avoid forcing the user to define a representation for the absence
of a value by providing an API with predicates that depend on the
presence or absence of a value.  Optional terms also allow separating
the code that constructs optional terms from the code that processes
them, which is then free to deal if necessary and at its convenience
with any case where the values hold by optional terms are not present.

6.39.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#optionals’ link in a web
browser.

6.39.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(optionals(loader)).

6.39.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(optionals(tester)).

6.39.4 Usage
------------

The ‘optional’ object provides constructors for optional terms.  For
example:

     | ?- optional::of(1, Optional).
     ...

The created optional terms can then be passed as parameters to the
‘optional/1’ parametric object.  For example:

     | ?- optional::of(1, Optional), optional(Optional)::or_else(Term, 0).
     Optional = optional(1),
     Term = 1
     yes

     | ?- optional::empty(Optional), optional(Optional)::or_else(Term, 0).
     Optional = empty,
     Term = 0
     yes

The ‘maybe’ object provides types and predicates for type-checking of
the term hold by optional terms.  It also provides some predicates for
handling lists of optional terms.

6.39.5 See also
---------------

The ‘expecteds’ library.

6.40 ‘options’
==============

This library provides useful predicates for managing developer tool and
application options.

6.40.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#options’ file in a web browser.

6.40.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ utility
file:

     | ?- logtalk_load(options(loader)).

6.40.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(options(tester)).

6.40.4 Usage
------------

The ‘options’ category is usually imported by the root object of the
developer tool or application.  The importing object should define the
‘default_option/1’ predicate and, if option type-checking is required,
the ‘valid_option/1’ predicate must be defined for each option.  This
library requires options to be represented by compound terms where the
functor is the option name (e.g.  ‘trim(true)’ or (‘box(0,2)’)).  The
‘option/2-3’ can be used to get or test an option given a list of
options.  When an option appears multiple times in a list, the
‘option/2-3’ predicates get or test the first (leftmost) occurrence.

The library also supports a user-defined ‘fix_option/2’ predicate.  An
usage example is when an option value can be a relative file path that
should be expanded before used.  Another usage example would be
converting from a user-friendly option to a form more suitable for
internal processing.  When a call to the ‘fix_option/2’ predicate fails,
the option is used as-is.

A simple example:

     :- object(foo,
         imports(options)).

         :- uses(type, [
             valid/2
         ]).

         :- public(p/0).
         p :-
             % use default options
             p([]).

         :- public(p/1).
         p(UserOptions) :-
             ^^check_options(UserOptions),
             % construct the full set of options from
             % the user options and the default options
             ^^merge_options(UserOptions, Options),
             ...
             % query an option
             ^^option(baz(Boolean), Options),
             q(Boolean),
             ...

         default_option(baz(true)).
         ...

         valid_option(baz(Boolean)) :-
             valid(boolean, Boolean).
         ...

     :- end_object.

Note that you can use protected or private import of the ‘options’
category if you don’t want to add its public predicates to the object
protocol.

6.41 ‘os’
=========

This library entities define a 'portable' operating-system interface for
the supported backend Prolog compilers.

The ‘os_types’ category defines some useful operating-system types for
type-checking when using with the ‘type’ library object.

6.41.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#os’ link in a web browser.

6.41.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(os(loader)).

6.41.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(os(tester)).

6.41.4 Known issues
-------------------

Some predicates may only be supported by a subset of backend Prolog
compilers on a subset of operating-systems.  They should be used with
care and fully tested in your application domain as some backend Prolog
compilers have buggy and inconsistent interfaces, specially across
operating-systems.  See the remarks section in the ‘os’ object
documentation for details.

6.42 ‘queues’
=============

This library implements queues.  The queue representation should be
regarded as an opaque term and only accessed using the library
predicates.

6.42.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#queues’ link in a web browser.

6.42.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(queues(loader)).

6.42.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(queues(tester)).

6.42.4 Usage
------------

To create a new queue, use the ‘new/1’ predicate:

     | ?- queue::new(Queue).
     Queue = ...
     yes

Elements can be added to either the end of the queue or the front of the
queue using, respectively, the ‘join/3’ and ‘join_all/3’ predicates or
the ‘jump/3’ and ‘jump_all/3’.  For example:

     | ?- queue::(new(Queue0), join_all([1,2,3], Queue0, Queue1)).
     Queue0 = ...,
     Queue1 = ...
     yes

We can query the head of the queue or remove the head of the queue
using, respectively, the ‘head/2’ and ‘serve/3’ predicates.  For
example:

     | ?- queue::(new(Queue0), join(1, Queue0, Queue1), head(Queue1, Head)).
     Queue0 = ...,
     Queue1 = ...,
     Head = 1
     yes

For details on these and other provided predicates, consult the library
API documentation.

6.43 ‘random’
=============

This library provides portable random number generators and an
abstraction over native backend Prolog compiler random number generator
if available.

6.43.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#random’ link in a web browser.

6.43.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(random(loader)).

6.43.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(random(tester)).

6.43.4 Usage
------------

The ‘random’ object implements portable random number generator and
supports multiple random number generators, using different seeds, by
defining derived objects.  For example:

     :- object(my_random_generator_1,
         extends(random)).

         :- initialization(::reset_seed).

     :- end_object.

The ‘fast_random’ object also implements a portable random number
generator but does not support deriving multiple random number
generators, which makes it a bit faster than the ‘random’ object.

The ‘random’ and ‘fast_random’ objects manage the random number
generator seed using internal dynamic state.  The predicates that update
the seed are declared as synchronized (when running on Prolog backends
that support threads).  Still, care must be taken when using these
objects from multi-threaded applications as there is not portable
solution to protect seed updates from signals and prevent inconsistent
state when threads are canceled.

The ‘random’ and ‘fast_random’ objects always initialize the random
generator seed to the same value, thus providing a pseudo random number
generator.  The ‘randomize/1’ predicate can be used to initialize the
seed with a random value.  The argument should be a large positive
integer.  In alternative, when using a small integer argument, discard
the first dozen random values.

The ‘backend_random’ object abstracts the native backend Prolog compiler
random number generator for the basic ‘random/1’, ‘get_seed/1’, and
‘set_seed/1’ predicates providing a portable implementation for the
remaining predicates.  This makes the object stateless, which can allow
reliable use from multiple threads.  Consult the backend Prolog compiler
documentation for details on its random number generator properties.
Note that several of the supported backend Prolog systems, notably
B-Prolog, CxProlog, ECLiPSe, JIProlog, and Quintus Prolog, do not
provide implementations for both the ‘get_seed/1’ and ‘set_seed/1’
predicates and calling these predicates simply succeed without
performing any action.

6.44 ‘reader’
=============

The ‘reader’ object provides portable predicates for reading text file
and text stream contents to lists of terms, characters, or character
codes and for reading binary files to lists of bytes.  The text file API
is loosely based on the SWI-Prolog ‘readutil’ module.

6.44.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#reader’ link in a web browser.

6.44.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(reader(loader)).

6.44.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(reader(tester)).

6.45 ‘recorded_database’
========================

The ‘recorded_database’ library aims to help port Prolog code using the
legacy recorded database.  Ported applications should still consider
migrating to more standard solutions to handle dynamic data that must
survive backtracking.

6.45.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#recorded_database’ link in a web
browser.

6.45.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(recorded_database(loader)).

6.45.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(recorded_database(tester)).

6.45.4 Usage
------------

The ‘recorded_database_core’ category implements the library predicates.
This category is imported by the default ‘recorded_database’ object to
provide application global database.  To make the database local and
thus minimize potential record clashes, the category can be imported by
one of more application objects.  Use protected or private import to
restrict the scope of the library predicates.  For example:

     :- object(foo,
         imports(private::recorded_database_core)).

         bar :-
             ^^recorda(key, value(1)),
             ...

     :- end_object.

6.45.5 Known issues
-------------------

Currently, references are non-negative integers.  They still must be
regarded as opaque terms and subject to change without notice.  But
using integers can result in integer overflows when running on backends
with bounded integers in applications performing a large number of
database updates.

6.46 ‘redis’
============

Redis client library.  Supports GNU Prolog, LVM, SICStus Prolog,
SWI-Prolog, and XSB. Support for Ciao Prolog and ECLiPSe is also
included but requires fixes for issues in these systems.

For general information on Redis, including a list of the available
commands, visit:

     https://redis.io

6.46.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#redis’ link in a web browser.

6.46.2 Loading
--------------

To load this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(redis(loader)).

6.46.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(redis(tester)).

The tests assume a localhost Redis server running on the default port
(6379) if the ‘REDIS_HOST’ and ‘REDIS_PORT’ environment variables are
not defined.  If the server is not detected, the tests are skipped.

The unit tests were originally written by Sean Charles for his GNU
Prolog Redis client library:

     https://github.com/emacstheviking/gnuprolog-redisclient

The Logtalk version is a straight-forward port of the original tests
using the ‘test/1’ dialect of ‘lgtunit’.

6.46.4 Credits
--------------

This library is inspired by the Sean Charles GNU Prolog Redis client
library.

6.46.5 Known issues
-------------------

Recent version of macOS seem to disable the mapping of ‘localhost’ to
‘127.0.0.1’.  This issue may prevent running this library unit tests and
the ‘redis::connect/1’ from working.  This can be fixed either by
editing the ‘/etc/hosts file or by using in alternative the
predicate’redis::connect/3‘with’’127.0.0.1’' as first argument.

6.47 ‘sets’
===========

This library provides a set protocol and two implementations of this
protocol using 'ordered lists', one of them a parametric object that
takes the type of the set elements as a parameter.  Although
representing sets as ordered lists is a common representation, is best
practice to regard sets as opaque terms and only access them using the
library predicates.

6.47.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#sets’ link in a web browser.

6.47.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(sets(loader)).

6.47.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(sets(tester)).

6.47.4 Usage
------------

First, select a set implementation.  Use the ‘set(Type)’ object if you
want to type-check the set elements.  Otherwise, use the ‘set’ object.

To create a new set, you can use the ‘new/1’ predicate.  For example:

     | ?- set::new(Set).
     Set = []
     yes

You can also create a new set with all unique elements from a list of
terms by using the ‘as_set/2’ predicate.  For example:

     | ?- set::as_set([1,3,2,1,2], Set).
     Set = [1, 2, 3]
     yes

Predicates are provided for the most common set operations.  For
example:

     | ?- set::(
             as_set([1,3,2,1,2], Set1),
             as_set([7,4,2,5,1], Set2),
             intersection(Set1, Set2, Intersection),
             symdiff(Set1, Set2, Difference)
         ).
     Set1 = [1, 2, 3],
     Set2 = [1, 2, 4, 5, 7],
     Intersection = [1, 2],
     Difference = [3, 4, 5, 7]
     yes

When working with a custom type of set elements, the corresponding
object must implement the ‘comparingp’ protocol.  For example:

     :- object(rainbow_colors,
         implements(comparingp)).

         order(red,    1).
         order(orange, 2).
         order(yellow, 3).
         order(green,  4).
         order(blue,   5).
         order(indigo, 6).
         order(violet, 7).

         Color1 < Color2 :-
             order(Color1, N1),
             order(Color2, N2),
             {N1 < N2}.

         Color1 =< Color2 :-
             order(Color1, N1),
             order(Color2, N2),
             {N1 =< N2}.

         ...

     :- end_object.

We can then use this object with the ‘set/1’ parametric object.  For
example:

     | ?- set(rainbow_colors)::as_set([blue, yellow, violet], Set).
     Set = [yellow, blue, violet]
     yes

For details on these and other provided predicates, consult the library
API documentation.

6.47.5 Credits
--------------

Some predicates adapted from code authored by Richard O’Keefe.

6.48 ‘statistics’
=================

The entities in this group define some useful predicates for descriptive
statistics.  Data is represented as a list of numbers (integers or
floats).  Use the object ‘sample’ of your data represents a sample.  Use
the object ‘population’ if your data represents a population.

6.48.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#statistics’ link in a web
browser.

6.48.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(statistics(loader)).

6.48.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(statistics(tester)).

6.49 ‘term_io’
==============

This library implements predicates for reading/writing terms from/to
atoms, chars (lists of characters), and codes (lists of character
codes).  These predicates are implemented using a single temporary file
created when the library is loaded.  This temporary file is unique per
Logtalk process.  The predicates can be safely used in multi-threaded
applications.

6.49.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#term-io’ link in a web browser.

6.49.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(term_io(loader)).

6.49.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(term_io(tester)).

6.50 ‘timeout’
==============

The ‘timeout’ object provides a portable abstraction over calling a goal
deterministically with a time limit as made available in some form by
some of the supported backend Prolog systems (B-Prolog, ECLiPSe, LVM,
SICStus Prolog, SWI-Prolog, Trealla Prolog, XSB, and YAP).

For better performance, compile calls to this library meta-predicates
with the ‘optimize’ flag turned on so that the meta-arguments, i.e.  the
goals that you are timing, are also compiled.

6.50.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#timeout’ link in a web browser.

6.50.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(timeout(loader)).

6.50.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(timeout(tester)).

6.50.4 Known issues
-------------------

Two tests are currently skipped when using the SWI-Prolog backend as
they cannot be interrupted and generate the expected timeout exceptions
due to tests being run from an ‘initialization/1’ directive goal that,
in the SWI-Prolog, implementation of this directive, ignores signals.
The test goals do generate the expected timeout exception when not
called from an ‘initialization/1’ directive.

6.51 ‘tsv’
==========

The ‘tsv’ library provides predicates for reading and writing TSV files
and streams:

‘https://www.iana.org/assignments/media-types/text/tab-separated-values’

The main object, ‘tsv/1’, is a parametric object allowing passing a
single option for the handling of the header of the file (‘keep’ or
‘skip’).  The ‘tvs’ object extends the ‘tsv/1’ parametric object using
the default ‘keep’ option value.

Files and streams can be read into a list of rows (with each row being
represented by a list of fields) or asserted using a user-defined
dynamic predicate.  Reading can be done by first loading the whole file
(using the ‘read_file/2-3’ predicates) into memory or line by line
(using the ‘read_file_by_line/2-3’ predicates).  Reading line by line is
usually the best option for parsing large TSV files.

Data can be saved to a TSV file or stream by providing the object and
predicate for accessing the data plus the name of the destination file
or the stream handle or alias.

6.51.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#tsv’ link in a web browser.

6.51.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(tsv(loader)).

6.51.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(tsv(tester)).

6.51.4 Usage
------------

A TSV file can be read as a list of rows:

     | ?- tsv::read_file('test_files/data.tsv', Rows).

     Rows = [['Name','Age','Address'], ['Paul',23,'1115 W Franklin'], ['Bessy the Cow',5,'Big Farm Way'], ['Zeke,45,'W Main St']]
     yes

Alternatively, The TSV data can be saved using a public and dynamic
object predicate (that must be previously declared).  For example:

     | ?- assertz(p(_,_,_)), retractall(p(_,_,_)).
     yes

     | ?- tsv(skip)::read_file('test_files/data.tsv', user, p/3).
     yes

     | ?-  p(A,B,C).

     A = 'Paul', B = 23, C = '1115 W Franklin' ? ;
     ...

Given a predicate representing a table, the predicate data can be
written to a file or stream.  For example:

     | ?- tsv::write_file('output.tsv', user, p/3).
     yes

6.52 ‘types’
============

This library implements predicates over standard Prolog term types and
also terms representing common data structures such as lists and pairs.

It also includes a user-extensible ‘type’ object defining type checking
predicates over common Logtalk and Prolog term types.  The types define
a hierarchy with the Prolog type ‘term’ at the root (i.e.  type-checking
a predicate argument of type ‘term’ trivially succeeds).  Some types are
only meaningful for backend Prolog systems supporting non-universal
features (e.g.  ‘cyclic’ or ‘char(CharSet)’ with a Unicode character
set).  See the API documentation for a full list of the types defined by
default.

6.52.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#types’ link in a web browser.

6.52.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(types(loader)).

In case your code only requires the most basic types, you can load in
alternative the file:

     | ?- logtalk_load(basic_types(loader)).

See the notes on the ‘basic_types’ virtual library for details.

6.52.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(types(tester)).

6.52.4 Type-checking
--------------------

This library ‘type’ object can be used to type-check common Logtalk and
Prolog term types (see the object documentation for a listing of all the
pre-defined types).  The ‘valid/2’ predicate succeeds or fails if a term
is of a given type.  For example:

     | ?- type::valid(positive_integer, 42).
     yes

     | ?- type::valid(positive_integer, -13).
     no

The ‘check/2’ and ‘check/3’ predicates throw an exception if a term is
not of a given type.  For example:

     | ?- catch(type::check(integer, abc), Error, true).
     Error = type_error(integer, abc)
     yes

If we require a standard ‘error/2’ exception term, the ‘check/3’
predicate takes a 'context' argument.  For example:

     | ?- catch(type::check(integer, abc, foo/3), Error, true).
     Error = error(type_error(integer, abc), foo/3)
     yes

Typically, the context is provided by calling the built-in ‘context/1’
method.

6.52.5 Defining new types
-------------------------

To define a custom type, define clauses for the multifile predicates
‘type::type/1’ (to declare the type) and ‘type::check/2’ (to type-check
values).  For example:

     :- multifile(type::type/1).
     type::type(age).

     :- multifile(type::check/2).
     type::check(age, Term) :-
         type::check(between(non_negative_integer, 0, 150), Term).

Be careful to ensure that new type definitions don’t introduce spurious
choice-points for these predicates.  The unit tests of the ‘types’
library perform this check for pre-defined and loaded user-defined
ground types.

When defining a meta-type (i.e.  a type with arguments that are also
types), add also a clause for the ‘type::meta_type/3’ multifile
predicate.  For example:

     :- multifile(type::meta_type/3).
     type::meta_type(tuple(Type1, Type2, Type3), [Type1, Type2, Type3], []).

This predicate is called when checking if a type is a defined type.  For
meta-types, that check must extend to the sub-types.

6.52.6 Examples
---------------

See e.g.  the ‘os’ library implementation of custom types for files and
directories.  Or the ‘expecteds’ and ‘optionals’ libraries custom types.
See also the ‘my_types’ programming example.

6.53 ‘unicode_data’
===================

VivoMind Prolog Unicode Resources

6.53.1 Authors
--------------

     Arun Majumdar @ VivoMind LLC
     Paulo Moura @ VivoMind LLC

6.53.2 License
--------------

Creative Commons CC0 1.0 Universal (CC0 1.0) - Public Domain Dedication:

     https://creativecommons.org/publicdomain/zero/1.0/

We do appreciate acknowledgement if you use these resources, however,
and we also welcome contributions to improve them.

6.53.3 Website
--------------

The latest release of the VivoMind Prolog Unicode Resources is available
at the URL:

     https://github.com/VivoMind

At this address you can also find additional information about the
VivoMind Prolog Unicode Resources and submit your bug reports and
contributions.

6.53.4 Description
------------------

The VivoMind Prolog Unicode Resources are a set of files resulting from
the conversion of most (but not all) official UCD 6.1 files and updated
for the few changes in the 6.2 standard.  The original files can be
downloaded from:

     http://www.unicode.org

The conversion of the UCD files resulted in a large number of Prolog
tables and also a set of auxiliary predicates (described below) for
accessing these tables.  Other than the obvious conversion in the
provided predicate names, no attempt was made to convert the identifiers
used for properties and other data.

6.53.5 Requirements
-------------------

Most of the auxiliary predicates assume that the de facto Prolog
standard predicate ‘between/3’ is available.  Unicode code point values
are represented using the ISO Prolog standard notation for hexadecimal
integers.  In addition, the ISO Prolog standard directives ‘include/1’
and ‘ensure_loaded/1’ are used in some of the files to load auxiliary
files.

6.53.6 Usage
------------

Most applications only require some of the tables present in these
resources.  Most of these tables define properties for ranges of code
points and not for single code points but the provided auxiliary
predicates allow access for a single code point.  When increased
performance is required, consider using the existing tables and
auxiliary predicates to generated derived tables more fit for your
specific application.

6.53.7 Known issues
-------------------

In the file ‘unicode_unihan_variant.pl’, when there’s more than one
variant for a code point, only the first one (as listed in the original
UCD file) is returned.

The ‘include/1’ and ‘ensure_loaded/1’ directives are specified in the
ISO Prolog standard published in 1995.  But some Prolog compilers either
don’t implement one or both directives or have flawed implementations.
Thus, you may need to change how some of the files are loaded depending
on the chosen Prolog compiler.  Using conditional compilation directives
would help in some cases but it would also rise portability issues on
their own.

6.53.8 Acknowledgements
-----------------------

We thank Richard O’Keefe for helpful suggestions to improve the
usability of these resources.

6.53.9 Files and API Summary
----------------------------

The Prolog file names are derived from the original file names by
prefixing them with the ‘unicode_’ string, converting to lower case, and
replacing the camel case spelling with underscores.  There are, however,
two exceptions: the files and directories holding the code point
categories and names.

There’s also an utility file, ‘unicode_data.pl’, that can be used to
load all the files in these resources.  Is mostly used to test
portability of the code across Prolog compilers.  Also included is a
Logtalk version of this file, ‘unicode_data.lgt’, that uses Logtalk’s
own implementation of the ‘include/1’ directive and the ‘logtak_load/1’
predicate to load all files.  This file can be used to workaround Prolog
systems with buggy or missing implementations of the ‘ensure_loaded/1’
and ‘include/1’ directives.

An overview of the original file names and the code point properties can
be found at:

     http://www.unicode.org/reports/tr44/#Directory_Structure

     http://www.unicode.org/reports/tr44/#Property_Definitions

6.53.9.1 ‘unicode_arabic_shaping.pl’
....................................

   - Provides: ‘unicode_arabic_shaping/4’

   - Dependencies: (none)

6.53.9.2 ‘unicode_bidi_mirroring.pl’
....................................

   - Provides: ‘unicode_bidi_mirroring/2’

   - Dependencies: (none)

6.53.9.3 ‘unicode_blocks.pl’
............................

   - Provides: ‘unicode_block/2-3’

   - Dependencies: (none)

6.53.9.4 ‘unicode_case_folding.pl’
..................................

   - Provides: ‘unicode_case_folding/3’

   - Dependencies: (none)

6.53.9.5 ‘unicode_categories.pl’
................................

   - Provides: ‘unicode_category/2’

   - Dependencies: files in the ‘unicode_categories’ directory

6.53.9.6 ‘unicode_cjk_radicals.pl’
..................................

   - Provides: ‘unicode_cjk_radical/3’

   - Dependencies: (none)

6.53.9.7 ‘unicode_composition_exclusions.pl’
............................................

   - Provides: ‘unicode_composition_exclusion/1’

   - Dependencies: (none)

6.53.9.8 ‘unicode_core_properties.pl’
.....................................

   - Provides: ‘unicode_math/1-2’ ‘unicode_alphabetic/1-2’
     ‘unicode_range_alphabetic/2’ ‘unicode_lowercase/1-2’
     ‘unicode_uppercase/1-2’ ‘unicode_cased/1-2’
     ‘unicode_case_ignorable/1-2’ ‘unicode_changes_when_lowercased/1-2’
     ‘unicode_changes_when_uppercased/1-2’
     ‘unicode_changes_when_titlecased/1-2’
     ‘unicode_changes_when_casefolded/1-2’
     ‘unicode_changes_when_casemapped/1-2’ ‘unicode_id_start/1-2’
     ‘unicode_id_continue/1-2’ ‘unicode_xid_start/1-2’
     ‘unicode_xid_continue/1-2’ ‘unicode_default_ignorable/1-2’
     ‘unicode_grapheme_extend/1-2’ ‘unicode_grapheme_base/1-2’
     ‘unicode_grapheme_link/1-2’

   - Dependencies: files in the ‘unicode_core_properties’ directory

6.53.9.9 ‘unicode_decomposition_type.pl’
........................................

   - Provides: ‘unicode_canonical/1-2’ ‘unicode_compat/1-2’
     ‘unicode_font/1-2’ ‘unicode_nobreak/1-2’ ‘unicode_initial/1-2’
     ‘unicode_medial/1-2’ ‘unicode_final/1-2’ ‘unicode_isolated/1-2’
     ‘unicode_circle/1-2’ ‘unicode_super/1-2’ ‘unicode_sub/1-2’
     ‘unicode_vertical/1-2’ ‘unicode_wide/1-2’ ‘unicode_narrow/1-2’
     ‘unicode_small/1-2’ ‘unicode_square/1-2’ ‘unicode_fraction/1-2’

   - Dependencies: files in the ‘unicode_decomposition_type’ directory

6.53.9.10 ‘unicode_derived_age.pl’
..................................

   - Provides: ‘unicode_age/2-3’

   - Dependencies: (none)

6.53.9.11 ‘unicode_derived_bidi_class.pl’
.........................................

   - Provides: ‘unicode_bidi_class/2-3’

   - Dependencies: (none)

6.53.9.12 ‘unicode_derived_combining_class.pl’
..............................................

   - Provides: ‘unicode_combining_class/2-3’

   - Dependencies: (none)

6.53.9.13 ‘unicode_derived_core_properties.pl’
..............................................

   - Provides: ‘unicode_core_property/2-3’

   - Dependencies: (none)

6.53.9.14 ‘unicode_derived_decomposition_type.pl’
.................................................

   - Provides: ‘unicode_decomposition_type/2-3’

   - Dependencies: (none)

6.53.9.15 ‘unicode_derived_east_asian_width.pl’
...............................................

   - Provides: ‘unicode_east_asian_width/2-3’

   - Dependencies: (none)

6.53.9.16 ‘unicode_derived_joining_group.pl’
............................................

   - Provides: ‘unicode_joining_group/2-3’

   - Dependencies: (none)

6.53.9.17 ‘unicode_derived_joining_type.pl’
...........................................

   - Provides: ‘unicode_joining_type/2-3’

   - Dependencies: (none)

6.53.9.18 ‘unicode_derived_line_break.pl’
.........................................

   - Provides: ‘unicode_line_break/2-3’

   - Dependencies: (none)

6.53.9.19 ‘unicode_derived_normalization_props.pl’
..................................................

   - Provides: ‘unicode_fc_nfkc/2’ ‘unicode_nfkc_cf/2’
     ‘unicode_full_composition_exclusion/1-2’ ‘unicode_nfd_qc_no/1-2’
     ‘unicode_nfc_qc_no/1-2’ ‘unicode_nfc_qc_maybe/1-2’
     ‘unicode_nfkd_qc_no/1-2’ ‘unicode_nfkc_qc_no/1-2’
     ‘unicode_nfkc_qc_maybe/1-2’ ‘unicode_expands_on_nfd/1-2’
     ‘unicode_expands_on_nfc/1-2’ ‘unicode_expands_on_nfkd/1-2’
     ‘unicode_expands_on_nfkc/1-2’
     ‘unicode_changes_when_nfkc_casefolded/1-2’

   - Dependencies: files in the ‘unicode_derived_normalization_props’
     directory

6.53.9.20 ‘unicode_derived_numeric_type.pl’
...........................................

   - Provides: ‘unicode_numeric_type/2-3’

   - Dependencies: (none)

6.53.9.21 ‘unicode_derived_numeric_values.pl’
.............................................

   - Provides: ‘unicode_numerical_value/3’

   - Dependencies: (none)

6.53.9.22 ‘unicode_hangul_syllable_type.pl’
...........................................

   - Provides: ‘unicode_hangul_syllable_type/2-3’

   - Dependencies: (none)

6.53.9.23 ‘unicode_indic_matra_category.pl’
...........................................

   - Provides: ‘unicode_indic_matra_category/2-3’

   - Dependencies: (none)

6.53.9.24 ‘unicode_indic_syllabic_category.pl’
..............................................

   - Provides: ‘unicode_indic_syllabic_category/2-3’

   - Dependencies: (none)

6.53.9.25 ‘unicode_jamo.pl’
...........................

   - Provides: ‘unicode_jamo/2’

   - Dependencies: (none)

6.53.9.26 ‘unicode_name_aliases.pl’
...................................

   - Provides: ‘unicode_name_alias/3’

   - Dependencies: (none)

6.53.9.27 ‘unicode_names.pl’
............................

   - Provides: ‘unicode_name/2’

   - Dependencies: files in the ‘unicode_names’ directory

6.53.9.28 ‘unicode_prop_list.pl’
................................

   - Provides: ‘unicode_white_space/1-2’ ‘unicode_bidi_control/1-2’
     ‘unicode_join_control/1-2’ ‘unicode_dash/1-2’ ‘unicode_hyphen/1-2’
     ‘unicode_quotation_mark/1-2’ ‘unicode_terminal_punctuation/1-2’
     ‘unicode_other_math/1-2’ ‘unicode_hex_digit/1-2’
     ‘unicode_ascii_hex_digit/1-2’ ‘unicode_other_alphabetic/1-2’
     ‘unicode_ideographic/1-2’ ‘unicode_diacritic/1-2’
     ‘unicode_extender/1-2’ ‘unicode_other_lowercase/1-2’
     ‘unicode_other_uppercase/1-2’ ‘unicode_noncharacter_code_point/1-2’
     ‘unicode_other_grapheme_extend/1-2’
     ‘unicode_ids_binary_operator/1-2’
     ‘unicode_ids_trinary_operator/1-2’ ‘unicode_radical/1-2’
     ‘unicode_unified_ideograph/1-2’
     ‘unicode_other_default_ignorable/1-2’ ‘unicode_deprecated/1-2’
     ‘unicode_soft_dotted/1-2’ ‘unicode_logical_order_exception/1-2’
     ‘unicode_other_id_start/1-2’ ‘unicode_other_id_continue/1-2’
     ‘unicode_sterm/1-2’ ‘unicode_variation_selector/1-2’
     ‘unicode_pattern_white_space/1-2’ ‘unicode_pattern_syntax/1-2’

   - Dependencies: files in the ‘unicode_prop_list’ directory

6.53.9.29 ‘unicode_range_scripts.pl’
....................................

   - Provides: ‘unicode_range_script/3’ ‘unicode_script/2’

   - Dependencies: (none)

6.53.9.30 ‘unicode_script_extensions.pl’
........................................

   - Provides: ‘unicode_script_extension/2-3’

   - Dependencies: ‘unicode_scripts.pl’

6.53.9.31 ‘unicode_scripts.pl’
..............................

   - Provides: ‘unicode_script/6’ ‘unicode_script_category/3’

   - Dependencies: (none)

6.53.9.32 ‘unicode_special_casing.pl’
.....................................

   - Provides: ‘unicode_special_casing/5’

   - Dependencies: (none)

6.53.9.33 ‘unicode_unihan_variants.pl’
......................................

   - Provides: ‘unicode_unihan_variant/2-3’

   - Dependencies: (none)

6.53.9.34 ‘unicode_version.pl’
..............................

   - Provides: ‘unicode_version/3’

   - Dependencies: (none)

6.54 ‘ulid’
===========

This library implements a Universally Unique Lexicographically Sortable
Identifier (ULID) generator.

‘https://github.com/ulid/spec’

Note that most backends provide time stamps with lower granularity than
required (i.e.  seconds but not milliseconds).  Also note that, per
spec, within the same millisecond, monotonic sort order is not
guaranteed.

The generation of ULIDs uses the ‘/dev/urandom’ random number generator
when available.  This includes macOS, Linux, *BSD, and other POSIX
operating-systems.  On Windows, a pseudo-random generator is used but
randomized using the current wall time.

ULIDs can be generated as atoms, lists of characters, or lists of
character codes.

See also the ‘ids’ and ‘uuid’ libraries.

6.54.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#ulid’ link in a web browser.

6.54.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(ulid(loader)).

6.54.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(ulid(tester)).

6.54.4 Generating ULIDs
-----------------------

By default, ULIDs are generated as atoms.  For example:

     | ?- ulid::generate(ULID).
     ULID = '01H0J31SYQXHJZWPRAKHQ6YVYH'
     yes

To generate a ULID using a list of characters representation, use
instead the ‘ulid/1’ parametric object:

     | ?- ulid(chars)::generate(ULID).
     ULID = ['0','1','H','0','J','3','2','Y','V','5','V','S','P','K','5','P','4','5','G','G','0','9','8','8','M','2']
     yes

Similar to get a ULID using a list of character codes representation:

     | ?- ulid(codes)::generate(ULID).
     ULID = [48,49,72,48,74,51,52,66,54,48,55,57,54,49,67,82,70,65,67,51,67,67,86,82,48,66]
     yes

It’s also possible to generate ULIDs from a given timestamp, i.e.  the
number of milliseconds since the Unix epoch (00:00:00 UTC on January 1,
1970), using the ‘generate/2’ predicate.  The timestamp must be an
integer.  For example, assuming a backend Prolog system providing a
‘time_stamp/1’ predicate returning the Unix epoch in milliseconds:

     | ?- time_stamp(Milliseconds), ulid(atom)::generate(Seconds, ULID).
     Seconds = 1684245175344, ULID = '01H0JDBQ1GAWJF35C44Y5S97DX'
     yes

You can also use timestamp discrete components using the ‘generate/8’
predicate.  For example:

     | ?- ulid(atom)::generate(2023, 5, 17, 16, 23, 38, 591, ULID).
     ULID = '01H0N8CDAZK75C5H3BJSGS4VCQ'
     yes

To extract the timestamp from a given ULID, use the ‘timestamp/2’ and
‘timestamp/8’ predicates.  For example:

     | ?- ulid(atom)::timestamp('01H0JDBQ1GAWJF35C44Y5S97DX', Milliseconds).
     Milliseconds = 1684245175344
     yes

     | ?- ulid(atom)::timestamp('01H0N8CDAZK75C5H3BJSGS4VCQ', Year, Month, Day, Hours, Minutes, Seconds, Milliseconds).
     Year = 2023, Month = 5, Day = 17, Hours = 16, Minutes = 23, Seconds = 38, Milliseconds = 591
     yes

6.54.5 Type-checking ULIDs
--------------------------

This library also defines a ‘ulid(Representation)’ type for
type-checking ULIDs.  For example, with an atom containing invalid
characters:

     | ?- type::check(ulid(atom), '01BX5ZIKBKALTAV9OEVGEMMVRY').
     uncaught exception: domain_error(ulid,'01BX5ZIKBKALTAV9OEVGEMMVRY')

6.55 ‘union_find’
=================

This library implements a union-find data structure.  This structure
tracks a set of elements partitioned into a number of disjoint
(non-overlapping) subsets.  It provides fast operations to add new sets,
to merge existing sets, and to determine whether elements are in the
same set.  This implementation of the union-find algorithm provides the
following features:

   - Path compression: Path compression flattens the structure of the
     tree by making every node point to the root whenever a find
     predicate is used on it.

   - Union by rank: Union predicates always attach the shorter tree to
     the root of the taller tree.  Thus, the resulting tree is no taller
     than the originals unless they were of equal height, in which case
     the resulting tree is taller by one node.

For a general and extended discussion on this data structure, see e.g.

‘https://en.wikipedia.org/wiki/Disjoint-set_data_structure’

6.55.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#union-find’ link in a web
browser.

6.55.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(union_find(loader)).

6.55.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(union_find(tester)).

6.55.4 Usage
------------

An usage example is Kruskal’s algorithm, a minimum-spanning-tree
algorithm which finds an edge of the least possible weight that connects
any two trees in the forest.  It is a greedy algorithm in graph theory
as it finds a minimum spanning tree for a connected weighted graph
adding increasing cost arcs at each step.

     :- object(kruskal).

         :- public(kruskal/2).

         :- uses(union_find, [
             new/2, find/4, union/4
         ]).

         kruskal(g(Vertices-Edges), g(Vertices-Tree)) :-
             new(Vertices, UnionFind),
             keysort(Edges, Sorted),
             kruskal(UnionFind, Sorted, Tree).

         kruskal(_, [], []).
         kruskal(UnionFind0, [Edge| Edges], [Edge| Tree]) :-
             Edge = _-(Vertex1, Vertex2),
             find(UnionFind0, Vertex1, Root1, UnionFind1),
             find(UnionFind1, Vertex2, Root2, UnionFind2),
             Root1 \== Root2,
             !,
             union(UnionFind2, Vertex1, Vertex2, UnionFind3),
             kruskal(UnionFind3, Edges, Tree).
         kruskal(UnionFind, [_| Edges], Tree) :-
             kruskal(UnionFind, Edges, Tree).

     :- end_object.

Sample query:

     | ?- kruskal::kruskal(g([a,b,c,d,e,f,g]-[7-(a,b), 5-(a,d), 8-(b,c), 7-(b,e), 9-(b,d), 5-(c,e), 15-(d,e), 6-(d,f), 8-(e,f), 9-(e,g), 11-(f,g)]), Tree).

     Tree = g([a,b,c,d,e,f,g]-[5-(a,d),5-(c,e),6-(d,f),7-(a,b),7-(b,e),9-(e,g)])
     yes

6.56 ‘uuid’
===========

This library implements a Universally unique identifier (UUID)
generator.  Currently only version 1 and version 4 UUIDs are supported.
For reference material, see e.g.

‘https://en.wikipedia.org/wiki/Universally_unique_identifier’

Some backends provide time stamps with low granularity (e.g.  seconds
but not milliseconds or nanoseconds).  To compensate, the generation of
version 1 UUIDs uses 14 random bits for the clock sequence.

The generation of version 4 UUIDs uses the ‘/dev/urandom’ random number
generator when available.  This includes macOS, Linux, *BSD, and other
POSIX operating-systems.  On Windows, a pseudo-random generator is used
but randomized using the current wall time.

UUIDs can be generated as atoms, lists of characters, or lists of
character codes.

See also the ‘ids’ and ‘ulid’ libraries.

6.56.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#uuid’ link in a web browser.

6.56.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(uuid(loader)).

6.56.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(uuid(tester)).

6.56.4 Generating version 1 UUIDs
---------------------------------

By default, version 1 UUIDs are generated as atoms.  For example:

     | ?- uuid::uuid_v1([0xf2,0xd1,0x90,0x94,0xdc,0x4b], UUID).
     UUID = '00a66fc0-82cf-11eb-bc83-f2d19094dc4b'
     yes

To generate a UUID using a list of characters representation, use
instead the ‘uuid/1’ parametric object:

     | ?- uuid(chars)::uuid_v1([0xf2,0xd1,0x90,0x94,0xdc,0x4b], UUID).
     UUID = ['0','0',d,e,'9','0',c,'0',-,'8','2',c,f,-,'1','1',e,b,-,
             a,'9','8','5',-,f,'2',d,'1','9','0','9','4',d,c,'4',b]
     yes

Similar to get a UUID using a list of character codes representation:

     | ?- uuid(codes)::uuid_v1([0xf2,0xd1,0x90,0x94,0xdc,0x4b], UUID).
     UUID = [48,48,52,99,99,54,99,48,45,56,50,99,102,45,49,49,101,98,45,
             98,57,102,52,45,102,50,100,49,57,48,57,52,100,99,52,98]
     yes

6.56.5 Generating version 4 UUIDs
---------------------------------

By default, version 4 UUIDs are generated as atoms.  For example:

     | ?- uuid::uuid_v4(UUID).
     UUID = '1c652782-69c5-4252-88c8-09e576a44db5'
     yes

To generate a UUID using a list of characters representation, use
instead the ‘uuid/1’ parametric object:

     | ?- uuid(chars)::uuid_v4(UUID).
     UUID = [d,'3',d,'3','3','5','1','3',-,'8','1',e,c,-,'4',d,'2','6',-,
             '9',f,'2','2',-,e,d,'9','5',e,'0','0',e,'1','5','7','0']
     yes

Similar to get a UUID using a list of character codes representation:

     | ?- uuid(codes)::uuid_v4(UUID).
     UUID = [102,97,52,54,57,98,100,50,45,51,57,54,51,45,52,97,100,55,45,
             98,50,50,55,45,101,100,52,99,56,55,99,54,53,55,102,98]
     yes

6.56.6 Generating the null UUID
-------------------------------

A predicate is also provided that returns the null UUID:

     | ?- uuid::uuid_null(UUID).
     UUID = '00000000-0000-0000-0000-000000000000'
     yes

6.57 ‘zippers’
==============

This library implements zippers over lists.

6.57.1 API documentation
------------------------

Open the ‘../../docs/library_index.html#zippers’ link in a web browser.

6.57.2 Loading
--------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(zippers(loader)).

6.57.3 Testing
--------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(zippers(tester)).

7 Ports
*******

The documentation of each port can also be found in the port directory
in the ‘NOTES.md’ file.

7.1 ‘fcube’
===========

This folder contains a Logtalk port of FCube: An Efficient Prover for
Intuitionistic Propositional Logic available from:

     https://www.vidal-rosset.net/fCube/

The port includes portability changes (notably, operator names) plus
changes to use an ordered representation for sets.  Also some code
formatting changes for Logtalk coding guidelines.

The port tests are adapted from the examples available from the web page
above.

To load this port and for sample queries, please see the ‘SCRIPT.txt’
file.

For more information about FCube, see the following paper:

     @InProceedings{10.1007/978-3-642-16242-8_21,
         author="Ferrari, Mauro and Fiorentini, Camillo and Fiorino, Guido",
         editor="Ferm{\"u}ller, Christian G. and Voronkov, Andrei",
         title="fCube: An Efficient Prover for Intuitionistic Propositional Logic",
         booktitle="Logic for Programming, Artificial Intelligence, and Reasoning",
         year="2010",
         publisher="Springer Berlin Heidelberg",
         address="Berlin, Heidelberg",
         pages="294--301",
         isbn="978-3-642-16242-8"
     }

For sample queries, please see the ‘SCRIPT.txt’ file.

7.1.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#fcube’ link in a web browser.

7.1.2 Loading
-------------

To load all entities in this port, load the ‘loader.lgt’ file:

     | ?- logtalk_load(fcube(loader)).

7.1.3 Testing
-------------

To test this port predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(fcube(tester)).

7.2 ‘metagol’
=============

This folder contains a Logtalk port of ‘metagol’, an inductive logic
programming (ILP) system based on meta-interpretive learning available
from:

     https://github.com/metagol/metagol

See the original code git repo for details and bibliography on Metagol
and ILP.

The port allows any number of datasets to be loaded simultaneously with
per-dataset learning options.  A dataset is simply wrapped in an object
that extends and is expanded by the ‘metagol’ object as illustrated by
the ported examples.

Both the original code and the port requires the coroutining ‘when/2’
predicate, which is only available in some backend Prolog systems.  The
port currently supports ECLiPSe, LVM, SICStus Prolog, SWI-Prolog, and
YAP. It can be used on both POSIX and Windows operating-systems.

The examples are ported from the original Metagol distribution.  Some of
the examples are taken from the following paper (with the original
Prolog examples source code files made available by MystikNinja):

     @article{DBLP:journals/jair/EvansG18,
         author    = {Richard Evans and Edward Grefenstette},
         title     = {Learning Explanatory Rules from Noisy Data},
         journal   = {J. Artif. Intell. Res.},
         volume    = {61},
         pages     = {1--64},
         year      = {2018},
         url       = {https://doi.org/10.1613/jair.5714},
         doi       = {10.1613/jair.5714},
         timestamp = {Mon, 21 Jan 2019 15:01:17 +0100},
         biburl    = {https://dblp.org/rec/bib/journals/jair/EvansG18},
         bibsource = {dblp computer science bibliography, https://dblp.org}
     }

The paper can be downloaded at

     https://arxiv.org/pdf/1711.04574.pdf

For sample queries, please see the ‘SCRIPT.txt’ file.

7.2.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#metagol’ link in a web browser.

7.2.2 Loading
-------------

To load all entities in this port, load the ‘loader.lgt’ file:

     | ?- logtalk_load(metagol(loader)).

7.2.3 Testing
-------------

To test this port predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(metagol(tester)).

There are three lengthy tests that only run when the tests are being run
manually instead of automatically.

7.3 ‘toychr’
============

This folder contains a Logtalk port of ToyCHR, a reference
implementation of Constraint Handling Rules (CHR) available from:

     https://www.comp.nus.edu.sg/~gregory/toychr/

The port is work in progress and includes significant modifications to
the original code:

   - Instead of compiling ‘.chr’ files, it uses the term-expansion
     mechanism, by defining ‘toychrdb’ as a hook object, to support
     writing rules inside objects and categories.  As a consequence, the
     original ‘chr_compile/1’ is not available.

   - The port is portable and should run on all supported backends.

The port also includes examples ported from the SWI-Prolog CHR package
examples and documentation.  These examples are ported using the same
license of the original code (BSD-2-Clause).

For sample queries, please see the ‘SCRIPT.txt’ file.

7.3.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#toychr’ link in a web browser.

7.3.2 Loading
-------------

To load all entities in this port, load the ‘loader.lgt’ file:

     | ?- logtalk_load(toychr(loader)).

7.3.3 Testing
-------------

To test this port predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(toychr(tester)).

8 Contributions
***************

The documentation of contribution can also be found in the contribution
directory in the ‘NOTES.md’ file.

8.1 ‘flags’
===========

Flags was developed at Katholieke Universiteit Leuven

SPDX-FileCopyrightText: 2010 Katholieke Universiteit Leuven
SPDX-License-Identifier: Artistic-2.0

     Contributions to this file:
     Author: Theofrastos Mantadelis
     Suggestions: Paulo Moura
     Version: 1
     Date: 27/11/2010

8.1.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#flags’ link in a web browser.

8.1.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     ?- logtalk_load(flags(loader)).

8.1.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(flags(tester)).

8.2 ‘iso8601’
=============

ISO 8601 (and European civil calendar) compliant library of date and
time (clock) related predicates.  That is, an ISO 8601 handler.

Author: Daniel L. Dudley Created: 2004-02-18

Modified: 2014-09-26 (to use the library “os” object to get the current
date)

8.2.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#iso8601’ link in a web browser.

8.2.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(iso8601(loader)).

8.2.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(iso8601(tester)).

8.3 ‘pddl_parser’
=================

     PDDL 3.0 parser
     Release 1.0

Copyright (c) 2011 Robert Sasak.  All Rights Reserved.  This is free
software.  You can redistribute it and/or modify it under the terms of
the “Artistic License 2.0” as published by The Perl Foundation.  Consult
the “LICENSE.txt” file for details.

This PDDL 3.0 file parser converts PDDL files to Logtalk/Prolog friendly
syntax.  For example:

     PDDL         Prolog
     (on ?x ?y)   on(?x, ?y)

Syntax sugar: op(200, fy, ?).

For whole example check the “pddl.lgt” file for usage and example
output.

8.3.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#pddl_parser’ link in a web
browser.

8.3.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     ?- logtalk_load(pddl_parser(loader)).

8.3.3 Testing
-------------

The provided unit tests are based on a collection of problem set files
from International Planning Competition 2008.  In order to run all unit
tests from the parser’s directory type:

     ?- logtalk_load(tester).

Or from any directory by typing:

     ?- logtalk_load(pddl_parser(tester)).

Some of the unit tests fail in some Prolog compilers due to limitations
to the maximum arity of a term.

8.4 ‘verdi_neruda’
==================

     Verdi Neruda - Meta-interpreter collection for Prolog.
     Release 1.0

Copyright (c) 2010 Victor Lagerkvist.  All Rights Reserved.  Verdi
Neruda is free software.  You can redistribute it and/or modify it under
the terms of the simplified BSD license.

CONTENTS

  1. License

  2. About

  3. Verdi Neruda web site

  4. Installation and running

  5. Examples

  6. Authors

  7. LICENSE

Copyright 2010 Victor Lagerkvist.  All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

  1. Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.

  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the
     distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS ''AS IS’’ AND ANY
EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

The views and conclusions contained in the software and documentation
are those of the authors and should not be interpreted as representing
official policies, either expressed or implied, of the copyright
holders.

  2. ABOUT

Verdi Neruda is a meta-interpreter collection for Prolog.  Or, to be
more precise, for a Prolog like language.  Or, to be pedantically
precise to the point that you are annoying people, it’s not really a
meta-interpreter collection at all since the interpreters themselves
aren’t interpreting the language that they are written in.  Let’s just
say that it is a collection of interpreters for a logic programming
language very much like pure Prolog with negation as finite failure.

Verdi Neruda is written entirely in Logtalk and compatible with most
major Prolog systems.  The name is sadly not a subtle wordplay or an
acronym, but was generated by a computer with the help of a soundex
algorithm.  The purpose of the interpreter suite was to compare top-down
methods to bottom-up methods and how resolution tree search rules
affected performance and completeness.  In the top-down family we find
such interpreters as the long-time standing champion depth-first, its
slow but orderly brother breadth-first and the youngster iterative
deepening.  A best-first framework can be found a stone throw away.
With it it’s possible to define interpreters that use greedy best-first
search as well as A* search.

In the bottom-up camp we find an interpreter that uses a semi naive
fixpoint construction.  Since bottom-up interpreters by their very
nature are not goal oriented a transformation technique called magic
transformation is used on logic programs before any inferences are made.
This technique allows the interpreter to only generate the facts that a
top-down interpreter would have used on the same logic program.

A shell akin to a Prolog top loop is also included.  It has commands
both for proving goals with an interpreter of choice and for
benchmarking logical inferences.  If Verdi Neruda is run with a Prolog
system that supports statistics/2 it’s possible to obtain statistics
such as CPU-time as well.

  3. VERDI NERUDA WEB SITE

Visit the Verdi Neruda GitHub www-page at:

‘https://github.com/Joelbyte/verdi-neruda’

  4. INSTALLATION AND RUNNING

Verdi Neruda requires Logtalk 2.40.0 or a later version.

To use the snapshot of Verdi Neruda bundled with Logtalk:

   - Start Logtalk.

   - Type ‘{verdi_neruda(loader)}.’ (Including ‘.’).

To use the latest version of Verdi Neruda, fetch the latest source code,
either as an archive or from the git repository, extract it to a
directory of your choice, and:

   - Start Logtalk from that directory.

   - Type ‘{loader}.’ (Including ‘.’).  If everything went according to
     the plan you should be greeted by the welcoming message.  If you
     replace the bundled version with the new one, you can use in
     alternative the steps above.

  5. EXAMPLES

Follow the previous instructions to get everything up and running.
First we’re going to run some predefined programs in the included
databases.  Begin by typing ‘databases.’ from the shell - this should
print a list of the currently loaded databases.  The demo database
‘demodb’ should be included in the list.  Next type ‘listing(demodb).’
to print the contents of the database.  The output should look something
like:

     append([],A,A) if
         true.
     append([A|B],C,[A|D]) if
         append(B,C,D).
     .
     .
     .

Which means that the ‘append/3’ program is loaded and ready for action.
Next we need to decide which interpreter to use.  Fortunately the shell
does not leave much to the imagination - as might be expected, the
‘interpreters.’ command prints the currently loaded interpreters.  The
list should look like:

     dfs_interpreter
     bfs_interpreter
     iddfs_interpreter(A)
     bup_interpreter
     a_star_interpreter(A)

The variables means that the interpreters are parametric objects and
that additional information is needed in order to run them.  The
iddfs-interpreter needs to know the increment and the A*-interpreter
needs to know what weight should be used when calculating the cost of
nodes.  To start with let’s use the dfs-interpreter and do something
exciting, namely appending two lists!

     prove(dfs_interpreter, append([a,b], [c,d], Xs), demodb).

The prove command takes three arguments.  The first is a interpreter,
the second the goal that shall be proved and the last the database that
the clauses are derived from.

To accomplish the same thing with the iddfs-interpreter with an
increment of 1 we need only type

     prove(iddfs_interpreter(1), append([a,b], [c,d], Xs), demodb).

The shell also has support for counting logical inferences.  To compare
the dfs- and iddfs-interpreter with the append program we could write:

     benchmark(dfs_interpreter, append([a,b,c,d],[e,f], Xs), demodb).  ->
     dfs_interpreter inferences: 5

     benchmark(iddfs_interpreter(1), append([a,b,c,d],[e,f], Xs), demodb).
     -> iddfs_interpreter(1) inferences: 15

For more information regarding the built in shell commands consult the
‘help.’ command.

  6. AUTHORS

The bulk of Verdi Neruda was written by Victor Lagerkvist during his
bachelor thesis at Linköping university in the spring of 2010.  Paulo
Moura also helped a great deal during the later stages of development,
especially with regards to compatibility between various Prolog systems.

8.5 ‘xml_parser’
================

This folder contains a Logtalk version of John Fletcher’s Prolog XML
parser:

‘https://binding-time.co.uk/index.php/Parsing_XML_with_Prolog’

For a detailed description of this XML parser, please see the comments
in the ‘xml.lgt’ source file or convert the automatically generated
documentation to HTML or PDF. For sample queries, please see the
‘SCRIPT.txt’ file.

See the copyright and license information on the contributed files for
usage and distributions conditions.

8.5.1 API documentation
-----------------------

Open the ‘../../docs/library_index.html#xml_parser’ link in a web
browser.

8.5.2 Loading
-------------

To load all entities in this library, load the ‘loader.lgt’ file:

     | ?- logtalk_load(xml_parser(loader)).

8.5.3 Testing
-------------

To test this library predicates, load the ‘tester.lgt’ file:

     | ?- logtalk_load(xml_parser(tester)).

8.5.4 Known issues
------------------

When using GNU Prolog as the backend compiler, you may need to to use a
larger default global stack size (see the GNU Prolog documentation on
the environment variable ‘GLOBALSZ’).

9 Glossary
**********

abstract class

     A class that cannot be instantiated by sending it a
     message.  Usually used to contain common predicates that are
     inherited by other classes.

abstract method

     A method implementing an algorithm whose step
     corresponds to calls to methods defined in the descendants of the
     object (or category) containing it.

adapter file

     A Prolog source file defining a minimal abstraction layer between
     the Logtalk compiler/runtime and a specific backend Prolog compiler.

ancestor

     A class or a parent prototype that
     contributes (via inheritance) to the definition of an object.  For
     class-based hierarchies, the ancestors of an instance are its
     class(es) and all the superclasses of its class(es).  For
     prototype-based hierarchies, the ancestors of a prototype are its
     parent(s) and the ancestors of its parent(s).

backend Prolog compiler

     The Prolog compiler that is used to host and run Logtalk and that
     is called for compiling the intermediate Prolog code generated by
     the Logtalk compiler when compiling source files.

built-in method

     A predefined method that can be called from within any
     object or category.  I.e.  built-in methods are built-in
     object and category predicates.  Built-in methods cannot be
     redefined.

built-in predicate

     A predefined predicate that can be called from anywhere.  Built-in
     predicates can be redefined within objects and
     categories.

category

     A set of predicates directives and clauses that can be
     (virtually) imported by any object.  Categories support composing
     objects using fine-grained units of code reuse and also hot patching
     of existing objects.  A category should be
     functionally-cohesive, defining a single functionality.

class

     An object that specializes another object, interpreted
     as its superclass.  A class define the common predicates of a set
     of objects that instantiates it.  An object can also be interpreted
     as a class when it instantiates itself.

closed-world assumption

     The assumption that what cannot be proved true is false.
     Therefore, sending a message corresponding to a
     'declared' but not 'defined' predicate, or calling a declared
     predicate with no clauses, fails.  But messages or calls to
     undeclared predicates generate an error.

closure

     A callable term (i.e.  an atom or a compound term) passed to a
     meta-predicate call where it is extended with additional
     arguments to form a goal called by the meta-predicate.

coinductive predicate

     A predicate whose calls are proved using greatest fixed point
     semantics.  Coinductive predicates allows reasoning about infinite
     rational entities such as cyclic terms and ω-automata.

complementing category

     A category used for hot patching an existing object (or
     a set of objects).

component

     A unique atom or compound term template identifying a library,
     tool, application, or application sub-system.  Component names are
     notably used by the message printing and question asking
     mechanisms.  Compound terms are used instead of atoms when
     parameterization is required.

directive

     A source file term that affects the interpretation of source code.
     Directives use the ‘(:-)/1’ prefix operator as functor.

discontiguous predicate

     A predicate whose clauses are not contiguous in a source file.
     I.e.  a predicate whose clauses are mixed with clauses
     for other predicates.

doclet file

     A source file whose main purpose is to generate
     documentation for e.g.  a library or an application.

doclet object

     An object specifying the steps necessary to (re)generate the API
     documentation for a project.  See the doclet and
     lgtdoc tools for details.

dynamic binding

     Runtime lookup of a predicate declaration and
     predicate definition to verify the validity of a
     message. (or a super call) and find the predicate
     definition that will be used to answer the message (or the super
     call).  Also known as 'late binding'.  See also static binding.

dynamic entity

     See entity.

dynamic predicate

     A predicate whose clauses can be dynamically added or retracted at
     runtime.

early binding

     See static binding.

encapsulation

     The hiding of an object implementation.  This promotes software
     reuse by isolating the object clients from its implementation
     details.  Encapsulation is enforced in Logtalk by using
     predicate scope directives.

entity

     Generic name for Logtalk compilation units: objects,
     categories, and protocols.  Entities share a
     single namespace (i.e.  entity identifiers must be
     unique) and can be static (the default) or dynamic.  Static
     entities are defined in source files.  Dynamic entities can also be
     defined in source files but are usually created and abolished at
     runtime using the language built-in predicates.

entity alias

     An alternative name for an entity.  Entity aliases can be defined
     using the uses/1 and use_module/1 directives.
     Entity aliases can be used to improve code clarity by using
     alternative names that are more meaningful in the calling context,
     to bind parametric entity parameters, and to simplify experimenting
     with alternative entities implementing the same protocol.

entity directive

     A directive that affects how Logtalk entities are used
     or compiled.

event

     The sending of a message to an object.  An event can be
     expressed as an ordered tuple: ‘(Event, Object, Message, Sender)’.
     Logtalk distinguish between the sending of a message — ‘before’
     event — and the return of control to the sender — ‘after’ event.

expansion workflow

     A sequence of term-expansion or goal-expansion steps where each
     step is usually defined using a hook object or a
     combination of hook objects.

grammar rule

     An alternative notation for predicates used to parse or generate
     sentences on some language.  This notation hides the arguments used
     to pass the sequences of tokens being processed, thus simplifying
     the representation of grammars.  Grammar rules are represented
     using as functor the infix operator ‘(-->)/2’ instead of the
     ‘(:-)/2’ operator used with predicate clauses.

grammar rule non-terminal

     A syntactic category of words or phrases.  A non-terminal is
     identified by its 'non-terminal indicator', i.e.  by its name and
     number of arguments using the notation ‘Name//Arity’.

grammar rule terminal

     A word or basic symbol of a language.

homoiconic

     A property of programming languages where 'code' and 'data' use the
     same representation.  Logtalk (and Prolog) are examples of
     homoiconic programming languages.  A Logtalk program is a set of
     terms (clauses and directives) that can be handled as 'data' by
     e.g.  the term-expansion mechanism.

hook object

     An object, implementing the expanding(1) built-in protocol,
     defining term- and goal-expansion predicates, used in the
     compilation of Logtalk or Prolog source files.  A hook object can
     be specified using the hook flag.  It can also be
     specified using a set_logtalk_flag/2 directive in the
     source files to be expanded.

hook predicate

     A predicate, usually declared multifile, that allows the
     user to customize another predicate or provide alternative
     definitions for a default predicate definition.

hot patching

     The act of fixing entity directives and predicates or
     adding new entity directives and predicates to loaded
     entities in a running application without requiring access to
     the entities source code or restarting the application.  Achieved
     using complementing categories.

identity

     Property of an entity that distinguishes it from every
     other entity.  The identifier of an entity is its functor (i.e.
     its name and arity), which must be unique.  Object and
     category identifiers can be atoms or compound terms.
     Protocol identities must be atoms.  All Logtalk entities (objects,
     protocols, and categories) share the same namespace.

inheritance

     An entity inherits predicates directives.
     and clauses from related entities.  In the particular case of
     objects, when an object extends other object, we have
     prototype-based inheritance.  When an object specializes or
     instantiates another object, we have class-based inheritance.  See
     also public inheritance, protected inheritance,
     and private inheritance.

instance

     An object that instantiates another object, interpreted
     as its class.  An object may instantiate multiple
     objects (also known as multiple instantiation).

instantiation

     The process of creating a new class. See instance.
     In Logtalk, this does not necessarily imply dynamic creation
     of an object at runtime; an instance may also be defined as a
     static object in a source file.

interface

     See protocol.

lambda expression

     A compound term that can be used in place of a goal or
     closure meta-argument and that abstracts a predicate definition
     by listing its variables and a callable term that
     implements the definition.  Lambda expressions help avoiding the
     need of naming and defining auxiliary predicates.

lambda free variable

     A variable that is global to a lambda expression.  All
     used global variables must be explicitly listed in a lambda
     expression for well-defined semantics.

lambda parameter

     A term (usually a variable or a non-ground compound term) that is
     local to a lambda expression.  All lambda parameters
     must be explicitly enumerated in a lambda expression for
     well-defined semantics.

late binding

     See dynamic binding.

library

     A directory containing source files.  See also library alias
     and library notation.

library alias

     An atom that can be used as an alias for a library full
     path.  Library aliases and their corresponding paths can be defined
     using the logtalk_library_path/2 predicate.  See also
     library notation.

library notation

     A compound term where the name is a library alias and
     the single argument is a source file relative path.
     Use of library notation simplifies compiling and loading source
     files and can make an application easily relocatable by defining an
     alias for the root directory of the application files.

loader file

     A source file whose main purpose is to load a set of
     source files (possibly with specific compiler flags) and any
     library dependencies.

local predicate

     A predicate that is defined in an object (or in a category)
     but that is not listed in a scope directive.
     These predicates behave like private predicates but are invisible
     to the reflection built-in methods.  Local predicates
     are usually auxiliary predicates and only relevant to the entity
     where they are defined.

message

     A query sent to an object.  In logical terms, a message can be
     interpreted as a request for proof construction using an object
     database and the databases of related entities.

message lookup

     Sending a message to an object requires a lookup for the
     predicate declaration, to check if the message is within the
     scope of the sender, and a lookup for the predicate definition
     that is going to be called to answer the message.
     Message lookup can occur at compile time or at
     runtime.

message to self

     A message sent to the object that received the original message
     under processing.  Messages to self require dynamic binding
     as the value of self is only know at runtime.

meta-argument

     A predicate argument that is called as a goal, used as a
     closure to construct a goal that will be called, or that is
     handled in a way that requires awareness of the predicate calling
     context.

meta-interpreter

     A program capable of running other programs written in the same
     language.

meta-predicate

     A predicate with one or more meta-arguments.  For
     example, see call/1-N and findall/3 are
     built-in meta-predicates.

meta-variable

     A variable in a meta-argument position that is expected
     to be unified with a goal or a closure at runtime.

metaclass

     The class of a class, when interpreted as an
     instance.  Metaclass instances are themselves classes.
     Metaclasses are optional, except for the root class, and can be
     shared by several classes.

method

     The predicate definition used to answer a message
     sent to an object.  Logtalk supports both static binding
     and dynamic binding to find which method to run to
     answer a message.

module

     A Prolog entity characterized by an identity and a set of
     predicates directives and clauses.  Prolog modules are
     usually static although some Prolog systems allow the creation of
     dynamic modules at runtime.  Prolog modules can be interpreted as
     prototypes.

monitor

     Any object, implementing the monitoring(2) built-in protocol, that
     is notified by the runtime when a spied event occurs.  The spied
     events can be set by the monitor itself or by any other
     object.

multifile predicate

     A predicate whose clauses can be defined in multiple
     entities and source files.  The object or category
     holding the directive without an entity prefix qualifying the
     predicate holds the multifile predicate 'primary declaration',
     which consists of both a scope directive and a
     multifile/1 directive for the predicate.

naked meta-variable

     A meta-variable used as the body of a predicate clause
     or grammar rule or used in a cut-transparent argument of a control
     construct.  The “naked” designation highlights that the
     meta-variable is not wrapped by ‘call/1’ or ‘phrase//1’ goals.

object

     An entity characterized by an identity and a set of
     predicates directives and clauses.  Logtalk objects can
     be either static or dynamic.  Logtalk objects can play the 'role'
     of classes, instances, or prototypes.  The role or roles an object
     plays are a function of its relations with other objects.

object database

     The set of predicates locally defined inside an object.

parameter

     An argument of a parametric object or a parametric category
     identifier.  Parameters are 'logical variables' implicitly shared
     by all the entity predicate clauses.

parameter variable

     A variable used as parameter in a parametric object or a parametric
     category using the syntax ‘_ParameterName_’ (i.e.  a variable whose
     name stars and ends with an underscore).  Parameter variables are
     'logical variables' shared by all entity terms.  Occurrences of
     parameter variables in entity directives and clauses
     are implicitly unified with the corresponding entity parameters.

parametric category

     See parametric entity.

parametric entity

     An object or category whose
     identifier is a compound term possibly containing free
     variables that can be used to parameterize the entity predicates.
     Parameters are 'logical variables' implicitly shared by all the
     entity clauses.  Note that the identifier of a parametric entity is
     its functor, irrespective of the possible values of its arguments
     (e.g.  ‘foo(bar)’ and ‘foo(baz)’ are different parameterizations of
     the same parametric entity, ‘foo/1’).

parametric object

     See parametric entity.

parametric object proxy

     A compound term (usually represented as a plain Prolog fact) with
     the same name and number of arguments as the identifier of a
     parametric object.

parent

     A prototype that is extended by another prototype.

polymorphism

     Different objects (and categories) can provide different
     implementations of the same predicate.  The predicate declaration
     can be inherited from a common ancestor, also known as 'subtype
     polymorphism'.  Logtalk implements 'single dispatch' on the
     receiver of a message, which can be described as 'single-argument
     polymorphism'.  As message lookup only uses the
     predicate functor, multiple predicate implementations for different
     types of arguments are possible, also known as 'ad hoc
     polymorphism'.  See Parametric objects and categories.
     enable implementation of 'parametric polymorphism' by using one of
     more parameters to pass object identifiers that can be used to
     parameterize generic predicate definitions.

predicate

     Predicates describe what is true about the application domain.  A
     predicate is identified by its 'predicate indicator', i.e.  by its
     name and number of arguments using the notation ‘Name/Arity’.  When
     predicates defined in objects or categories.
     they are also referred to as methods.

predicate alias

     An alternative functor (‘Name/Arity’) for a predicate.  Predicate
     aliases can be defined for any inherited predicate using the
     alias/2 directive and for predicates listed in uses/2
     and use_module/2 directives.  Predicate aliases can
     be used to solve inheritance conflicts and to improve code clarity
     by using alternative names that are more meaningful in the calling
     context.

predicate calling context

     The object or category from within a predicate is called (either
     directly or using a control construct such as a message sending
     control construct).

predicate declaration

     A predicate declaration is composed by a set of predicates directives,
     which must include ar least a scope directive.

predicate definition

     The set of clauses for a predicate, contained in an object or
     category.  Predicate definitions can be overriden or specialized in
     descendant entities.

predicate definition context

     The object or category that contains the definition (i.e.  clauses)
     for a predicate.

predicate directive

     A directive that specifies a predicate property that affects how
     predicates are called or compiled.

predicate execution context

     The implicit arguments (including sender, self,
     and this) required for the correct execution of a
     predicate call.

predicate scope container

     The object that inherits a predicate declaration from an
     imported category or an implemented protocol.

predicate scope directive

     A directive that declares a predicate by specifying its visibility
     as 'public', 'protected', or 'private'.

predicate shorthand

     A predicate alias that defines a call template, possibly
     using a different name, with a reduced number of arguments by
     hard-coding the value of the omitted arguments in the original call
     template.  Predicate shorthands can be defined using uses/2
     and use_module/2 directives.  They can be used to
     simplify predicate calls and to ensure consistent call patterns
     when some of the arguments always use the same fixed values in the
     calling context.

primary predicate declaration

     See multifile predicate.

private inheritance

     All public and protected predicates are inherited as private
     predicates.  See also public inheritance and
     protected inheritance.

private predicate

     A predicate that can only be called from the object that contains
     its scope directive.

profiler

     A program that collects data about other program performance.

protected inheritance

     All public predicates are inherited as protected.  No scope change
     for protected or private predicates.  See also public inheritance
     and private inheritance.

protected predicate

     A predicate that can only be called from the object containing its
     scope directive or from an object that inherits the
     predicate.

protocol

     An entity that contains predicate declarations.  A
     predicate is declared using a scope directive.  It may
     be further specified by additional predicate directives.  Protocols
     support the separation between interface and implementation, can be
     implemented by both objects and categories, and can be extended by
     other protocols.  A protocol should be functionally-cohesive,
     specifying a single functionality.  Also known as 'interface'.

prototype

     A self-describing object that may extend or be extended by other
     objects.  An object with no instantiation or specialization
     relations with other objects is always interpreted as a prototype.

public inheritance

     All inherited predicates maintain their declared scope.  See also
     protected inheritance and private inheritance.

public predicate

     A predicate that can be called from any object.

scratch directory

     The directory used to save the intermediate Prolog files generated
     by the compiler when compiling source files.

self

     The object that received the message under processing.

sender

     An object that sends a message to other object.  When a
     message is sent from within a category, the 'sender' is
     the object importing the category.

settings file

     A source file, compiled and loaded automatically by
     default at Logtalk startup, mainly defining default values for
     compiler flags that override the defaults found on the backend
     Prolog compiler adapter files.

singleton method

     A method defined in an instance itself.
     Singleton methods are supported in Logtalk and can also be found in
     other object-oriented programming languages.

source file

     A text file defining Logtalk and/or Prolog code.  Multiple Logtalk
     entities may be defined in a single source file.  Plain Prolog code
     may be intermixed with Logtalk entity definitions.  Depending on
     the used backend Prolog compiler, the text encoding may
     be specified using an encoding/1 directive as the first
     term in the first line in the file.

source file directive

     A directive that affects how a source file.
     is compiled.

specialization

     A class is specialized by defining a new class that
     inherits its predicates and possibly adds new ones.

static binding

     Compile time lookup of a predicate declaration and
     predicate definition when compiling a message.
     sending call (or a super call).  Dynamic binding is
     used whenever static binding is not possible (e.g.  due to the
     predicate being dynamic or due to lack of enough information at
     compilation time).  Also known as 'early binding'.  See also
     dynamic binding.

static entity

     See entity.

steadfastness

     A predicate definition is 'steadfast' when it still generates only
     correct answers when called with unexpected arguments (notably,
     bound output arguments).  Typically, a predicate may not be
     steadfast when output argument unifications can occur before a cut
     in a predicate clause body.

subclass

     A class that is a specialization, direct or indirectly,
     of another class.  A class may have multiple subclasses.

super call

     Call of an inherited (or imported) predicate definition.
     Mainly used when redefining an inherited (or imported) predicate to
     call the overridden definition while making additional calls.
     Super calls preserve self and may require dynamic binding
     if the predicate is dynamic.

superclass

     A class from which another class is a specialization
     (directly or indirectly via another class).  A class may have
     multiple superclasses.

synchronized predicate

     A synchronized predicate is protected by a mutex ensuring that, in
     a multi-threaded application, it can only be called by a single
     thread at a time.

template method

     See abstract method.

tester file

     A source file whose main purpose is to load and a run a
     set of unit tests.

this

     The object that contains the predicate clause under execution.
     When the predicate clause is contained in a category,
     'this' is a reference to the object importing the category for
     which the predicate clause is being executed.

threaded engine

     A computing thread running a goal whose solutions can be lazily and
     concurrently computed and retrieved.  A threaded engine also
     supports a term queue that allows passing arbitrary terms to the
     engine.  This queue can be used to pass e.g.  data and new goals to
     the engine.

top-level interpreter shorthand

     Aliases for frequently used built-in predicates such as
     logtalk_load/1 and logtalk_make/1.  These
     shorthands are 'not' part of the Logtalk language and must only be
     used at the top-level interpreter.

visible predicate

     A predicate that is within scope, a locally defined predicate, a
     built-in method, a Logtalk built-in predicate, or a
     Prolog built-in predicate.

(Alexiev93) Mutable Object State for Object-Oriented Logic Programming:
A Survey Alexiev, V. Technical Report TR 93-15, Department of Computing
Science, University of Alberta, Canada

(Belli_et_al_92) Object-oriented programming in Prolog: rationale and a
case study Belli, F., Jack, O., Naish, L. Technical Report 92/2,
Department of Electrical and Electronics Engineering, University of
Paderborn, Germany URL:
‘https://lee-naish.github.io/papers/oolp/index.html’

(Block89) An Extended Frame Language Block, F. P., Chan, N. C.
Proceedings OOPLSLA 89(10):151-157, ACM

(Bobrow_et_al_88) Common Lisp Object System Specification Bobrow, D. G.,
Michiel, L. G., Gabriel, R. P., Keene, S. E., Kiczales, G., Moon, D. A.
ACM SIGPLAN Notices(23)

(Bratko90) Prolog Programming for Artificial Intelligence Bratko, I.
Addison Wesley, 2º edition, 1990

(Champaux92) A comparative Study of Object-Oriented Analysis Methods
Champaux, D., Faure, P. Journal of Object-Oriented Programming, Vol.  5,
N.1, 1992

(Clocksin87) Programming in Prolog Clocksin, W.F., Mellish, C.S.
Springer-Verlag, New York, 1987

(Cointe87) Metaclasses are First Class: the ObjVlisp Model Cointe, P.
Proceedings OOPLSLA 87(10):156-167, ACM

(Cordes91) The Literate Programming Paradigm Cordes, D., Brown, M. IEEE
Computer, June 1991:52-61

(Covington94) ISO Prolog: A Summary of the Draft Proposed Standard
Covington, M. A. URL: ‘ftp://ai.uga.edu/pub/prolog.standard/’

(Cox86) Object-Oriented Programming: An Evolutionary Approach Cox, Brad
J. Addison-Wesley Publishing Company, Don Mills, Ontario

(Davison89) Polka: A Parlog Object oriented language Davison, A. Ph.D.
Thesis, Imperial College, London, 1989

(Davison92) A survey of logic programming-based object oriented
languages Davison, A. Tech Report 92/3, Dept.  of Computer Science,
University of Melbourne, Australia URL:
‘https://catalogue.nla.gov.au/Record/772526’

(Davison93) The deductive and object oriented features of BeBOP Davison,
A. Tech Report 93/6, Dept.  of Computer Science, University of
Melbourne, Australia URL: ‘https://catalogue.nla.gov.au/Record/1273317’

(Delzanno97) Logic and Object-Oriented Programming in Linear Logic
Delzanno, G. Ph.D. Thesis, University of Pisa, Italy URL:
‘https://opac.bncf.firenze.sbn.it/bncf-prod/resource?uri=BVE0136144’

(Dony90) Exception Handling and Object-Oriented Programming: Towards a
Synthesis Dony, C. Proceedings OOPLSLA 90:322-330, ACM

(Fornarino_et_al_89) An Original Object-Oriented Approach for Relation
Management Fornarino, M., Pinna, A.-M.,Trousse, B. Proceedings of the
4th Portuguese Conference on Artificial Intelligence Lecture Notes in
Artificial Intelligence, Springer-Verlag (390):13-26

(Fromherz93) OL(P): Object Layer for Prolog Fromherz, M. URL:
‘ftp://parcftp.xerox.com/ftp/pub/ol/’

(Fukunaga86) An Experience with a Prolog-based Object-Oriented Language
Fukunaga, K., Hirose, S. Proceedings OOPLSLA 86, 21(11):224-231, ACM

(Goldberg83) Smalltalk-80 The language and its implementation Goldberg,
A., Robson, D. Addison-Wesley Series in Computer Science

(Joy_et_al_00) The Java Language Specification, Second Edition Joy, B.,
Steele, G., Gosling, J., Bracha, G. Addison-Wesley, 2000

(ISO95) ISO/IEC DIS 13211-1 - Programming Language Prolog Part 1:
General Core Joint Technical Committee ISO/IEC JTC 1 URL:
‘https://www.iso.org/standard/21413.html’

(Knuth84) Literate Programming Knuth, D. E. Computer Journal, May 84,
27(2):97-111

(Lieberman86) Using Prototypical Objects to Implement Shared Behaviour
in Object Oriented Systems Lieberman, H. Proceedings OOPLSLA 86:189-214,
ACM

(Maes87) Concepts and Experiments in Computational Reflection Maes, P.
Proceedings OOPLSLA 87, ACM

(McCabe92) Logic and Objects McCabe, F. G. Prentice Hall Series in
Computer Science

(Moon86) Object-Oriented Programming in Flavors Moon, D. Proceedings
OOPLSLA 86:1-8, ACM

(Moss94) Prolog++ The Power of Object-Oriented and Logic Programming
Moss, C. Addison-Wesley International Series in Logic Programming, 1994

(Moura94) Logtalk: Programação Orientada para Objectos em Prolog Moura,
P., Costa, E. 2ª Conferência e Exposição Portuguesa de Tecnologia
Orientada por Objectos 3i Consultores, Lisboa

(Moura99) Porting Prolog: Notes on porting a Prolog program to 22 Prolog
compilers or the relevance of the ISO Prolog standard Moura, P. ALP
Newsletter, Vol.  12/2, May 1999

(Moura00) Logtalk 2.6 Documentation Moura, P. Technical Report DMI
2000/1 University of Beira Interior, Portugal

(Razek92) Combining Objects and Relations Razek, G. Comunications of the
ACM, 27(12):66-70

(Rumbaugh87) Relations as Semantic Constructs in an Object-Oriented
Language Rumbaugh, J. Proceedings OOPLSLA 87:466-481, ACM

(Rumbaugh88) Controlling Propagation of Operations using Attributes on
Relations Rumbaugh, J. Proceedings OOPLSLA 88:285-296, ACM

(Schachte95) Efficient Object-Oriented Programming in Prolog Schachte,
P., Saab, G. Logic Programming: Formal Methods and Pratical Applications
Studies in Computer Science and Artificial Intelligence, 11 Elsevier
Science B.V. North-Holland, Amsterdam, 1995

(SICStus95) SICStus Prolog Manual SICStus URL: ‘https://sicstus.sics.se’

(Shan_et_al_93) Is Multiple Inheritance Essential to OOP? (Panel) Shan,
Y., Cargill, T., Cox, B., Cook, W., Loomis, M., Snyder, A. Proceedings
OOPLSLA 93:360-363

(Stefik_et_al_86) Integrating Acess-Oriented Programming into a
Multiparadigm Environment Stefik, M. J., Bobrow, D. G. , Kahn, K. M.
IEEE Software, January 1986:10-18

(Stroustrup86) The C++ Programming Language Stroustrup, B.
Addison-Wesley Series in Computer Science

(Taenzer89) Problems in Object-Oriented Software Reuse Taenzer, D.,
Ganti, M., Podar, S. Proceedings of ECOOP 89 British Computer Society
Workshop Series, Cambridge University Press

(Tanzer95) Remarks on Object-Oriented Modeling of Associations Tanzer,
C. Journal of Object-Oriented Programming, February 1995, SIGS
Publications

(Tanenbaum87) Operating Systems - Design and Implementation Tanenbaum,
A. Prentice-Hall Software Series, 1987

(Welsch89) Reasoning Objects with Dynamic Knowledge Bases Welsch, C.,
Barth, G. Proceedings of the 4th Portuguese Conference on Artificial
Intelligence(390):257-268 Lecture Notes in Artificial Intelligence,
Springer-Verlag, 1989

   ---------- Footnotes ----------

   (1) ../docs/expanding_0.html#expanding-0

   (2) ../docs/monitoring_0.html#monitoring-0

10 Index
********