..
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   Copyright 1998-2021 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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.. 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`
