..
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   SPDX-FileCopyrightText: 1998-2025 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 succeed 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 their 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.

Meta-predicate template
-----------------------

::

   threaded_call(0)
   threaded_call(0, *)

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:
|     ``uninstantiation_error(Tag)``

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`
