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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
http://www.apache.org/licenses/LICENSE-2.0
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.. rst-class:: align-right
built-in predicate
.. index:: pair: threaded_peek/1-2; Built-in predicate .. _predicates_threaded_peek_1_2:
threaded_peek/1-2
::
threaded_peek(Goal)
threaded_peek(Goal, Tag)
Checks if the result of proving Goal
in a new thread is already
available. This call succeeds or fails without blocking execution
waiting for a reply to be available. When there is no thread proving
the goal, the predicate generates an exception.
The argument of this predicate should be a variant of the argument of
the corresponding :ref:`threaded_call/1 <predicates_threaded_call_1_2>`
or :ref:`threaded_once/1 <predicates_threaded_once_1_2> call.
When the predicate argument is subsumed by the
threaded_call/1 or
threaded_once/1` call argument, the threaded_peek/1 call will
succeed iff its argument unifies with an already available solution of
the (more general) goal.
The threaded_peek/2 variant accepts a threaded call identifier tag generated by the calls to the :ref:`threaded_call/2 <predicates_threaded_call_1_2>` and :ref:`threaded_once/2 <predicates_threaded_once_1_2>` predicates. Tags shall be regarded as an opaque term; users shall not rely on its 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.
::
threaded_peek(+callable)
- zero_or_one
threaded_peek(+callable, +nonvar)
- zero_or_one
| Goal
is a variable:
| instantiation_error
| Goal
is neither a variable nor a callable term:
| type_error(callable, Goal)
| No thread is running for proving Goal
in the Object
calling context:
| existence_error(thread, Object)
| Tag
is a variable:
| instantiation_error
| To check for an asynchronous goal proof result:
| threaded_peek(Goal)
| To check for an asynchronous message to self result:
| threaded_peek(::Goal)
| To check for an asynchronous message result:
| threaded_peek(Object::Goal)
.. seealso::
:ref:predicates_threaded_call_1_2
,
:ref:predicates_threaded_exit_1_2
,
:ref:predicates_threaded_ignore_1
,
:ref:predicates_threaded_once_1_2
,
:ref:predicates_threaded_cancel_1
,
:ref:predicates_threaded_1