.. This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2024 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. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

.. 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