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Asked: 2026-06-13T00:27:42+00:00

How can I check if a predicate exists in a Prolog program? That would

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How can I check if a predicate exists in a Prolog program? That would be an exists/1, like:

?- exists(some_predicate).
false.

?- assert(some_predicate).
true.

?- exists(some_predicate).
true.
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  1. Editorial Team

    You can use current_predicate/1, current_predicate/2 or predicate_property/2 (for the last you will probably need functor/3):

    ?- current_predicate(a/1).
false.

?- functor(A,a,1),predicate_property(A,visible).
false.

?- functor(A,a,1),current_predicate(_,A).
false.

?- assert(a(42)).
true.

?- current_predicate(a/1).
true.

?- functor(A,a,1),predicate_property(A,visible).
A = a(_G136).

?- functor(A,a,1),current_predicate(_,A).
A = a(_G122).

    current_predicate/2 and predicate_property/2 (with visible) succeeds if the predicate can be autoloaded while currrent_predicate/1 fails

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