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.. rst-class:: align-right

built-in predicate

.. 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 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_once(0) threaded_once(0, *)

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: | uninstantiation_error(Tag)

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