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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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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: threaded_engine_next_reified/2; Built-in predicate
.. _predicates_threaded_engine_next_reified_2:

``threaded_engine_next_reified/2``
==================================

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

::

   threaded_engine_next_reified(Engine, Answer)

Retrieves an answer from an engine and signals it to start computing the
next answer. This predicate always succeeds and blocks until an answer
becomes available. Answers are returned using the terms ``the(Answer)``,
``no``, and ``exception(Error)``.

.. 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_next_reified(@nonvar, ?nonvar) - one

Errors
------

| ``Engine`` is a variable:
|     ``instantiation_error``
| ``Engine`` is neither a variable nor the name of an existing engine:
|     ``existence_error(engine, Engine)``

Examples
--------

::

   % get the next reified answer from the worker_1 engine:
   | ?- threaded_engine_next_reified(worker_1, Answer).

.. seealso::

   :ref:`predicates_threaded_engine_create_3`,
   :ref:`predicates_threaded_engine_next_2`,
   :ref:`predicates_threaded_engine_yield_1`
