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   Copyright 1998-2020 Paulo Moura <pmoura@logtalk.org>

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   you may not use this file except in compliance with the License.
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.. index:: pair: threaded_once/1-2; Built-in predicate
.. _predicates_threaded_once_1_2:

threaded_once/1-2
=================

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

::

   threaded_once(Goal)
   threaded_once(Goal, Tag)

Proves ``Goal`` asynchronously using a new thread. Only the first goal
solution is found. The argument can be a message sending goal. This call
always succeeds. The result (success, failure, or exception) is sent
back to the message queue of the object containing the call
(:term:`this`).

The ``threaded_once/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.

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

::

   threaded_once(@callable) - one
   threaded_once(@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_once(Goal)``
| Prove ``::Message`` asynchronously in a new thread:
|     ``threaded_once(::Message)``
| Prove ``Object::Message`` asynchronously in a new thread:
|     ``threaded_once(Object::Message)``

.. seealso::

   :ref:`predicates_threaded_call_1_2`,
   :ref:`predicates_threaded_exit_1_2`,
   :ref:`predicates_threaded_ignore_1`,
   :ref:`predicates_threaded_peek_1_2`,
   :ref:`predicates_threaded_cancel_1`,
   :ref:`predicates_threaded_1`,
   :ref:`directives_synchronized_1`
