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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`
