Nomenclature

Depending on your 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 Smalltalk, arguably (and somehow sadly) not the most popular OOP language nowadays. In this section, we map nomenclatures from popular OOP languages such as Smalltalk, C++, and Java 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.

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.

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 current instance. 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.

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.

class method

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

class variable

True class variables may be implemented in Logtalk by using a metaclass for the class and defining the class variables in the class. I.e. class variables are simply instance variables of the class metaclass. Shared instance variables may be implemented by using the built-in database methods (which can be used to implement variable assignment) to access and updated a single occurrence of the variable stored in the class (there is no static keyword in Logtalk).

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 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 on class methods and class variables.

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 current instance. Logtalk also provides a this/1 method but for returning the class containing the predicate clause being executed. Why the name clashes? Well, the notion of self was inherited from Smalltalk, which predates Java.