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   SPDX-License-Identifier: Apache-2.0

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.. rst-class:: align-right

**control construct**

.. index:: pair: (@)/1; Control construct
.. _call_in_this_1:

``(@)/1``
=========

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

::

   @Predicate

Calls a predicate definition in :term:`this`. The argument must be a callable
term at compile-time. The predicate must be declared (by a scope directive).
This control construct provides access to predicate definitions in *this* from
categories. For example, it allows overriding a predicate definition from a
complementing category with a new definition that calls goals before and
after calling the overridden definition (the overriding definition is sometimes
described in other programming languages as an *around method*). When used
within an object, it's the same as calling its argument.

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

::

   @ +callable - zero_or_more

Errors
------

| ``Predicate``, with predicate indicator ``Name/Arity``, is not declared:
|     ``existence_error(predicate_declaration, Name/Arity)``

Examples
--------

Assuming an object declaring a ``make_sound/0`` predicate, define an
*around method* in a complementing category:

::

   make_sound :-
       write('Started making sound...'), nl,
       @make_sound,
       write('... finished making sound.'), nl.

.. seealso::

   :ref:`control_send_to_object_2`,
   :ref:`control_send_to_self_1`,
   :ref:`control_delegate_message_1`
