Logtalk user manual
Nomenclature
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 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 an 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.
- 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. By default, Logtalk uses dynamic binding for locating both method declarations and method definitions. Moreover, methods that are declared but not defined simply fail when called.
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 sub-class.
- 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 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
- 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; methods may always be 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 (functor) and number of arguments (arity). Logtalk (and Prolog) are not typed languages in the same sense as Java.
- reflection
- Logtalk supports both structural reflection (using a set of built-in predicates and built-in methods) and behavioral reflection (using event-driven programming).
- 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, ^^/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 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.