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

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.. rst-class:: align-right

**built-in method**

.. index:: pair: this/1; Built-in method
.. _methods_this_1:

``this/1``
==========

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

::

   this(This)

Unifies its argument with the identifier of the object calling this method.
When this method is called from a category, the argument is unified with the
object importing the category on whose behalf the clause containing the call
is being used to prove the current goal. 

This private method is implemented as a unification between its argument and
the corresponding implicit execution-context argument in the predicate clause
making the call. This unification occurs at the clause head when the argument
is not bound at compile-time (the most common case).

This method is useful for avoiding hard-coding references to an object
identifier or for retrieving all object parameters with a single call
when using parametric objects.

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

::

   this(?object_identifier) - zero_or_one

Errors
------

(none)

Examples
--------

::

   % after compilation, the write/1 call will
   % be the first goal in the clause body
   test :-
       this(This),
       write('Using a predicate clause contained in '),
       writeq(This), nl.

.. seealso::

   :ref:`methods_context_1`,
   :ref:`methods_parameter_2`,
   :ref:`methods_self_1`,
   :ref:`methods_sender_1`
