directive
meta_predicate/1
Description
meta_predicate(Template)
meta_predicate((Template, ...))
meta_predicate([Template, ...])
meta_predicate(Entity::Template)
meta_predicate((Entity::Template, ...))
meta_predicate([Entity::Template, ...])
meta_predicate(Module: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. Another case is when a meta-predicate directive is missing. The compiler requires access to correct and non-ambiguous meta-predicate templates to correctly compile calls to Prolog meta-predicates.
Warning
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
meta_predicate(+meta_predicate_template_term)
meta_predicate(+object_identifier::+meta_predicate_template_term)
meta_predicate(+category_identifier::+meta_predicate_template_term)
meta_predicate(+module_identifier:+meta_predicate_template_term)
Examples
% 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, *, *)).
See also