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

   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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       http://www.apache.org/licenses/LICENSE-2.0

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.. index:: pair: once/1; Built-in method
.. _methods_once_1:

once/1
======

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

::

   once(Goal)

This predicate behaves as ``call(Goal)`` but it is not re-executable.
This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

This meta-predicate is opaque to cuts in its argument.

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

::

   once(+callable) - zero_or_one

Errors
------

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

Examples
--------

| Call a goal deterministically in the context of the object or category containing the call:
|     ``once(Goal)``
| To send a goal as a non-backtracable message to :term:`self`:
|     ``once(::Goal)``
| To send a goal as a non-backtracable message to an explicit object:
|     ``once(Object::Goal)``

.. seealso::

   :ref:`methods_call_N`,
   :ref:`methods_ignore_1`,
   :ref:`methods_not_1`
