..
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   SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
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.. rst-class:: align-right

**control construct**

.. index:: pair: {}/1; Control construct
.. _control_external_call_1:

``{}/1``
========

Description
-----------

::

   {Goal}
   {Closure}
   {Term}

This control construct allows the programmer to bypass the Logtalk compiler
(including its linter but not its optimizer) in multiple contexts:

- Calling a goal as-is (from within an object or category) in the context of
  the :ref:`user <objects_user>` pseudo-object.

- Extending a closure as-is with the remaining arguments of a
  :ref:`call/2-N <methods_call_N>` call in order to construct a goal that will
  be called within the context of the ``user`` pseudo-object.

- Wrapping a source file term (either a clause or a directive) or a source file
  goal to bypass the :ref:`term-expansion mechanism <expansion_expansion>`.

- Using it in place of an object identifier when sending a message. In this
  case, its argument is proved as a goal within the context of the ``user``
  pseudo-object with the resulting term being used as an object identifier
  in the message-sending goal. This feature is mainly used with
  :term:`parametric objects <parametric object>` when their identifiers
  correspond to predicates defined in ``user``.

- Using it as a message to an object. This is mainly useful when the message
  is a :ref:`conjunction of messages <messages_broadcasting>` where some of
  goals are calls to Prolog built-in predicates.

.. note::

   This control construct is opaque to cuts when used to wrap a goal (thus
   ensuring the same semantics independently of the argument being bound at
   compile-time or at runtime).

Modes and number of proofs
--------------------------

::

   {+callable} - zero_or_more

Errors
------

| ``Goal`` is a variable:
|     ``instantiation_error``
| ``Goal`` is neither a variable nor a callable term:
|     ``type_error(callable, Goal)``
|
| ``Closure`` is a variable:
|     ``instantiation_error``
| ``Closure`` is neither a variable nor a callable term:
|     ``type_error(callable, Closure)``
|
| ``Term`` is a variable:
|     ``instantiation_error``
| ``Term`` is neither a variable nor a callable term:
|     ``type_error(callable, Term)``

Examples
--------

::

   % overload the standard (<)/2 operator by
   % calling its standard built-in definition:
   N1/D1 < N2/D2 :-
       {N1*D2 < N2*D1}.

   % call a closure in the context of "user":
   call_in_user(F, X, Y, Z) :-
       call({F}, X, Y, Z).

   % bypass the compiler for a proprietary backend directive:
   {:- load_foreign_resource(file)}.

   % use parametric object proxies:
   | ?- {circle(Id, Radius, Color)}::area(Area).
   ...

   % use Prolog built-in predicates as messages:
   | ?- logtalk::{write('hello world!'), nl}.
   hello world!
   yes
