meta_predicate((Template, ...))
meta_predicate([Template, ...])

meta_predicate((Entity::Template, ...))
meta_predicate([Entity::Template, ...])

meta_predicate((Module:Template, ...))
meta_predicate([Module:Template, ...])

Declares meta-predicates, i.e., predicates that have arguments interpreted as goals, arguments interpreted as closures used to construct goals, or arguments with sub-terms that will be interpreted as goals or closures. Meta-arguments are always called in the meta-predicate calling context, not in the meta-predicate definition context.

Meta-arguments which are goals are represented by the integer 0. Meta-arguments which are closures are represented by a positive integer, N, representing the number of additional arguments that will be appended to the closure in order to construct the corresponding goal (typically by calling the call/1-N built-in method). Meta-arguments with sub-terms that will be interpreted as goals or closures are represented by ::. Meta-arguments that will be called using the bagof/3 or setof/3 predicates and that can thus be existentially-qualified are represented by the atom ^. Normal arguments are represented by the atom *.

Logtalk allows the use of this directive to override the original meta-predicate directive. This is sometimes necessary when calling Prolog built-in meta-predicates or Prolog module meta-predicates due to the lack of standardization of the syntax of the meta-predicate templates.


Some backend Prolog compilers declare the atom meta_predicate as an operator for a lighter syntax. But this makes the code non-portable and is therefore a practice best avoided.

Template and modes





% findall/3 second argument is interpreted as a goal:
:- meta_predicate(findall(*, 0, *)).

% both forall/2 arguments are interpreted as goals:
:- meta_predicate(forall(0, 0)).

% maplist/3 first argument is interpreted as a closure
% that will be expanded to a goal by appending two
% arguments:
:- meta_predicate(maplist(2, *, *)).