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   Copyright 1998-2023 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

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   you may not use this file except in compliance with the License.
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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: threaded_engine_yield/1; Built-in predicate
.. _predicates_threaded_engine_yield_1:

``threaded_engine_yield/1``
===========================

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

::

   threaded_engine_yield(Answer)

Returns an answer independent of the solutions of the engine goal. Fails
if not called from within an engine. This predicate is usually used when
the engine goal is a call to a recursive predicate processing terms from
the engine term queue.

This predicate blocks until the returned answer is consumed.

Note that this predicate should not be called as the last element of a
conjunction resulting in an engine goal solution as, in this case, an
answer will always be returned. For example, instead of
``(threaded_engine_yield(ready); member(X,[1,2,3]))`` use
``(X=ready; member(X,[1,2,3]))``.

.. note::

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

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

::

   threaded_engine_yield(@term) - zero_or_one

Errors
------

(none)

Examples
--------

::

   % returns the atom "ready" as an engine answer:
   ..., threaded_engine_yield(ready), ...

.. seealso::

   :ref:`predicates_threaded_engine_create_3`,
   :ref:`predicates_threaded_engine_next_2`,
   :ref:`predicates_threaded_engine_next_reified_2`
