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roles

Logtalk provides objects, protocols, and categories as first-class entities. Relations between entities define patterns of code reuse and the roles played by the entities. For example, when an object instantiates another object, the first object plays the role of an instance and the second object plays the role of a class. An extends relation between two objects implies that both objects play the role of prototypes, with one of them extending the other, its parent prototype.

This simple example illustrates the different roles an object can play. See the comments in the roles.lgt file for detailed explanations.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Start by loading the example:

logtalk_load(roles(loader)).

Prototypes can declare and defined their own predicates:

prototype::foo(Foo).

<!-- Foo = 1. -->

Derived prototypes inherit predicates from their parents:

descendant::foo(Foo).

<!-- Foo = 2. -->

descendant::bar(X, Y).

<!-- X = 1, Y = 2. -->

A class that is its own metaclass can access its own public predicates using message-sending:

superclass::foo(Foo).

<!-- Foo = 1 -->

A class that doesn't have a metaclass cannot receive any message as the predicate declaration lookup to answer to validate the message would start in the metaclass:

\+ subclass::current_predicate(_).

<!-- true. -->

An instance can receive messages for predicates declared in its class(es) and in its class(es) superclass(es):

instance::current_predicate(P).

<!-- P = bar/2 ; P = foo/1 ; false. -->

instance::foo(Foo).

<!-- Foo = 2. -->

instance::bar(X, Y).

<!-- X = 1, Y = 2. -->

empty_instance::foo(Foo).

<!-- Foo = 1. -->