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   SPDX-FileCopyrightText: 1998-2023 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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       http://www.apache.org/licenses/LICENSE-2.0

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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: threaded_call/1-2; Built-in predicate
.. _predicates_threaded_call_1_2:

``threaded_call/1-2``
=====================

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

::

   threaded_call(Goal)
   threaded_call(Goal, Tag)

Proves ``Goal`` asynchronously using a new thread. The argument can be a
message sending goal. Calls to this predicate always succeeds and return
immediately. The results (success, failure, or exception) are sent back
to the message queue of the object containing the call (:term:`this`) and
can be retrieved by calling the
:ref:`threaded_exit/1 <predicates_threaded_exit_1_2>` predicate.

The ``threaded_call/2`` variant returns a threaded call identifier tag that
can be used with the :ref:`threaded_exit/2 <predicates_threaded_exit_1_2>`
and :ref:`predicates_threaded_cancel_1` predicates. Tags shall be regarded
as opaque terms; users shall not rely on its type.

.. note::

   This predicate requires a :term:`backend Prolog compiler` providing
   compatible multi-threading primitives. The value of the read-only
   :ref:`threads <flag_threads>` flag is set to ``supported`` when that
   is the case.

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

::

   threaded_call(@callable) - one
   threaded_call(@callable, --nonvar) - one

Errors
------

| ``Goal`` is a variable:
|     ``instantiation_error``
| ``Goal`` is neither a variable nor a callable term:
|     ``type_error(callable, Goal)``
| ``Tag`` is not a variable:
|     ``type_error(variable, Goal)``

Examples
--------

| Prove ``Goal`` asynchronously in a new thread:
|     ``threaded_call(Goal)``
| Prove ``::Message`` asynchronously in a new thread:
|     ``threaded_call(::Message)``
| Prove ``Object::Message`` asynchronously in a new thread:
|     ``threaded_call(Object::Message)``

.. seealso::

   :ref:`predicates_threaded_exit_1_2`,
   :ref:`predicates_threaded_ignore_1`,
   :ref:`predicates_threaded_once_1_2`,
   :ref:`predicates_threaded_peek_1_2`,
   :ref:`predicates_threaded_cancel_1`,
   :ref:`predicates_threaded_1`,
   :ref:`directives_synchronized_1`
