..
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   SPDX-FileCopyrightText: 1998-2025 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.
   You may obtain a copy of the License at

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

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   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: define_events/5; Built-in predicate
.. _predicates_define_events_5:

``define_events/5``
===================

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

::

   define_events(Event, Object, Message, Sender, Monitor)

Defines a new set of events. The two types of events are represented by
the atoms ``before`` and ``after``. When the predicate is called with
the first argument unbound, both types of events are defined. The object
``Monitor`` must define the event handler methods required by the
``Event`` argument.

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

::

   define_events(@term, @term, @term, @term, +object_identifier) - one

Errors
------

| ``Event`` is neither a variable nor a valid event identifier:
|     ``type_error(event, Event)``
| ``Object`` is neither a variable nor a valid object identifier:
|     ``type_error(object_identifier, Object)``
| ``Message`` is neither a variable nor a callable term:
|     ``type_error(callable, Message)``
| ``Sender`` is neither a variable nor a valid object identifier:
|     ``type_error(object_identifier, Sender)``
| ``Monitor`` is a variable:
|     ``instantiation_error``
| ``Monitor`` is neither a variable nor a valid object identifier:
|     ``type_error(object_identifier, Monitor)``
| ``Monitor`` does not define the required ``before/3`` method:
|     ``existence_error(procedure, before/3)``
| ``Monitor`` does not define the required ``after/3`` method:
|     ``existence_error(procedure, after/3)``

Examples
--------

::

   % define "debugger" as a monitor for member/2 messages
   % sent to the "list" object:
   | ?- define_events(_, list, member(_, _), _ , debugger).

.. seealso::

   :ref:`predicates_abolish_events_5`,
   :ref:`predicates_current_event_5`,
   :ref:`methods_before_3`,
   :ref:`methods_after_3`
