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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

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. 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.

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) | No thread is running for proving Goal in the Object calling context: | existence_error(thread, Object) | 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