root/trunk/examples/metapredicates/closures.lgt

/*  Logtalk meta-predicates accept both goals and closures
    as meta-arguments as illustrated in this example
*/


:- object(metapreds).

    % the meta_predicate/1 directive below changes the interpretation of meta-calls on apply/2
    % clauses; the integer argument ("1") implies that the first argument is a closure that will 
    % be used to construct a goal by appending exactly one additional argument

    :- public(apply/2).
    :- meta_predicate(apply(1, *)).
    :- mode(apply(+callable, ?term), zero_or_more).

    apply(Closure, Arg) :-      % the Logtalk compiler verifies that any closure which is a
        call(Closure, Arg).     % meta-argument is used within a call/N method that complies with
                                % the meta-predicate directive (in this case, apply(1, *) => call/2)

    :- public(test_this/0).     % simple predicate for testing calls to a local meta-predicate

    test_this :-
        apply(foo(X), Y),
        writeq((X, Y)), nl.

    foo(1, metapreds).

:- end_object.


:- object(descendant,
    extends(metapreds)).    

    :- public(test_self/0).     % simple predicate for testing calls to a meta-predicate
                                % defined in an ancestor object
    test_self :-
        ::apply(foo(X), Y),
        writeq((X, Y)), nl.

    foo(2, descendant).

:- end_object.


:- object(test).

    :- public(test_obj/0).      % simple predicate for testing calls to a meta-predicate
                                % defined in another object
    test_obj :-
        metapreds::apply(foo(X), Y),
        writeq((X, Y)), nl.

    foo(3, test).

:- end_object.