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 that will be called as goals. An argument may also be a closure instead of a goal if the meta-predicate uses the call/1-N Logtalk built-in methods to construct and call the actual goal from the closure and the additional arguments.

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 meta-call. Normal arguments are represented by the atom *. Meta-arguments are always called in the meta-predicate calling context, not in the meta-predicate definition context.

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

Warning

Some backend Prolog compilers declare meta_predicate as an operator for a lighter syntax. But this makes the code non-portable and is 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, *, *)).