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   Copyright 1998-2020 Paulo Moura <pmoura@logtalk.org>

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   you may not use this file except in compliance with the License.
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.. index:: pair: threaded_peek/1-2; Built-in predicate
.. _predicates_threaded_peek_1_2:

threaded_peek/1-2
=================

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

::

   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.

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.

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

::

   threaded_peek(+callable) - zero_or_one
   threaded_peek(+callable, +nonvar) - zero_or_one

Errors
------

| ``Goal`` is a variable:
|     ``instantiation_error``
| ``Goal`` is neither a variable nor a callable term:
|     ``type_error(callable, Goal)``
| ``Tag`` is a variable:
|     ``instantiation_error``

Examples
--------

| 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`
