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.. rst-class:: align-right

directive

.. index:: pair: meta_predicate/1; Directive .. _directives_meta_predicate_1:

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 :term:`meta-predicates <meta-predicate>`, i.e., predicates that have arguments interpreted as goals, arguments interpreted as :term:`closures <closure>` 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 :term:`calling context <predicate calling context>`, not in the meta-predicate :term:`definition context <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 :ref:methods_call_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, *, *)).

.. seealso::

:ref:directives_meta_non_terminal_1, :ref:methods_predicate_property_2