..
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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.
   You may obtain a copy of the License at

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

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.


.. index:: pair: implements_protocol/2-3; Built-in predicate
.. _predicates_implements_protocol_2_3:

implements_protocol/2-3
=======================

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

::

   implements_protocol(Object, Protocol)
   implements_protocol(Category, Protocol)

   implements_protocol(Object, Protocol, Scope)
   implements_protocol(Category, Protocol, Scope)

Enumerates, by backtracking, all pairs of entities such that an object
or a category implements a protocol. The relation scope is represented
by the atoms ``public``, ``protected``, and ``private``. This predicate
only returns direct implementation relations; it does not implement a
transitive closure.

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

::

   implements_protocol(?object_identifier, ?protocol_identifier) - zero_or_more
   implements_protocol(?category_identifier, ?protocol_identifier) - zero_or_more

   implements_protocol(?object_identifier, ?protocol_identifier, ?scope) - zero_or_more
   implements_protocol(?category_identifier, ?protocol_identifier, ?scope) - zero_or_more

Errors
------

| ``Object`` is neither a variable nor a valid object identifier:
|     ``type_error(object_identifier, Object)``
| ``Category`` is neither a variable nor a valid category identifier:
|     ``type_error(category_identifier, Category)``
| ``Protocol`` is neither a variable nor a valid protocol identifier:
|     ``type_error(protocol_identifier, Protocol)``
| ``Scope`` is neither a variable nor an atom:
|     ``type_error(atom, Scope)``
| ``Scope`` is an atom but an invalid entity scope:
|     ``domain_error(scope, Scope)``

Examples
--------

::

   % check that the list object implements the listp protocol:
   | ?- implements_protocol(list, listp).

   % check that the list object publicly implements the listp protocol:
   | ?- implements_protocol(list, listp, public).

   % enumerate only objects that implement the listp protocol:
   | ?- current_object(Object), implements_protocol(Object, listp).

   % enumerate only categories that implement the serialization protocol:
   | ?- current_category(Category), implements_protocol(Category, serialization).

.. seealso::

   :ref:`predicates_current_object_1`,
   :ref:`predicates_current_protocol_1`,
   :ref:`predicates_current_category_1`,
   :ref:`predicates_conforms_to_protocol_2_3`
