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 try to map nomenclatures from popular OOP languages such as C++ and Java to the Logtalk nomenclature.

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 a operational definition of abstract class: any class that does not inherit a method for creating new instances is 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 is often used 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 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.

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

Logtalk allows any predicate defined in a class to be overridden on a descendant class. There is no virtual keyword in Logtalk.

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 a 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 sub-class.

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 or prototype extension.

interface

Logtalk uses the term protocol with the same meaning.

callback method

Logtalk supports event-driven programming, the most common use 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

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. Moreover, 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; methods may always be redefined in subclasses (and instances!).

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 (functor) and number of arguments (arity). Logtalk (and Prolog) are not typed languages in the same sense as Java.

super

Instead of a super keyword, Logtalk provides a super operator, ^^/1, for calling overridden methods.

synchronized

Logtalk supports multi-threading programming in selected Prolog compilers, including a synchronized/1 predicate directive and a synchronized/0 object (and category) directive. Logtalk allows you to synchronize a predicate, a set of predicates, or all object predicates using per-predicate and per-object locks.

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 Java.