57 lines
1.4 KiB
Plaintext
57 lines
1.4 KiB
Plaintext
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/* Logtalk meta-predicates accept not only goals but also closures
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as meta-arguments as illustrated in this example
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*/
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:- object(meta).
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% the meta_predicate/1 directive below changes the interpretation of meta-calls on apply/2
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% clauses; the integer argument ("1") implies that the first argument is a closure that will
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% be used to construct a goal by appending exactly one additional argument
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:- public(apply/2).
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:- mode(apply(+callable, ?term), zero_or_more).
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:- meta_predicate(apply(1, *)).
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apply(Closure, Arg) :- % the Logtalk compiler verifies that any closure which is a
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call(Closure, Arg). % meta-argument is used within a call/N method that honors the
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% meta-predicate directive (in this case, apply(1, *) => call/2)
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:- public(test_this/0). % simple predicate for testing calls to a local meta-predicate
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test_this :-
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apply(foo(X), Y),
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write((X, Y)), nl.
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foo(1, meta).
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:- end_object.
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:- object(desc,
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extends(meta)).
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:- public(test_self/0). % simple predicate for testing calls to a meta-predicate
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% defined in an ancestor object
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test_self :-
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::apply(foo(X), Y),
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write((X, Y)), nl.
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foo(2, desc).
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:- end_object.
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:- object(test).
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:- public(test_obj/0). % simple predicate for testing calls to a meta-predicate
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% defined in another object
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test_obj :-
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meta::apply(foo(X), Y),
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write((X, Y)), nl.
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foo(3, test).
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:- end_object.
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