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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 method

.. 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.

Meta-predicate template

::

once(0)

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