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.. This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2024 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. You may obtain a copy of the License at
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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(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.
::
threaded_engine_yield(@term)
- zero_or_one
(none)
::
% 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