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.

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).

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.

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, 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, 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).

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 (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 (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, 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, (^^)/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++.

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.