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Logtalk - Object oriented extension to Prolog
Release 2.15.2

Copyright (c) 1998-2003 Paulo Moura.  All Rights Reserved.
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% Sending to descendant the message p/1, returns the definition in root:

| ?- descendant::p(Value).

Value = root
yes


% Asserting a local definition for p/1 in descendant overrides the inherited 
% definition:

| ?- descendant::(assertz(p(descendant)), p(Value)).

Value = descendant
yes


% If we retract the local definition, again the inherited definition form root
% will be used:

| ?- descendant::(retractall(p(_)), p(Value)).

Value = root
yes


% class does not understand the message p1/1 (the predicate is declared only 
% for the class descendant instances):

| ?- class::p1(X).

error(existence_error(predicate_declaration, p1(_)), class::p1(_), user)


% the same message is valid for the class instances:

| ?- instance::p1(X).

X = class
yes


% If we assert a clause for a new predicate, p2/1, in the class
% (a side-effect being a dynamic declaration of the predicate):

| ?- class::assertz(p2(class)).

yes


% the new predicate, like p1/1, is not available for the class:

| ?- class::p2(Value).

error(existence_error(predicate_declaration, p2(_)), class::p2(_), user)


% but is available for the class instances, the same way as p1/1:

| ?- instance::p2(X).

X = class
yes


% Using a prototype, assert three new predicates (the method object_assert/0 
% asserts the predicate public_predicate/0 from outside the prototype; the 
% method self_assert/0 asserts the predicate protected_predicate/0 in self; 
% the method this_assert/0 asserts the predicate private_predicate/0 in this):

| ?- prototype::(object_assert, self_assert, this_assert).

yes


% and check the resulting scope of each predicate:

| ?- prototype::dynamic_predicates.

public_predicate/0 - public
protected_predicate/0 - protected
private_predicate/0 - private
yes