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.

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

meta_non_terminal/1, predicate_property/2