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.. This file is part of Logtalk https://logtalk.org/ 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. 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.
In this example, we will illustrate the use of:
.. _category:
We want to define a set of predicates to handle dynamic object
attributes. We need public predicates to set, get, and delete
attributes, and a private dynamic predicate to store the attributes
values. Let us name these predicates set_attribute/2 and
get_attribute/2, for getting and setting an attribute value,
del_attribute/2 and del_attributes/2, for deleting attributes,
and attribute_/2
, for storing the attributes values.
But we do not want to encapsulate these predicates in an object. Why? Because they are a set of useful, closely related, predicates that may be used by several, unrelated, objects. If defined at an object level, we would be constrained to use inheritance in order to have the predicates available to other objects. Furthermore, this could force us to use multi-inheritance or to have some kind of generic root object containing all kinds of possible useful predicates.
For this kind of situation, Logtalk enables the programmer to encapsulate the predicates in a category, so that they can be used in any object. A category is a Logtalk entity, at the same level as objects and protocols. It can contain predicates directives and/or definitions. Category predicates can be imported by any object, without code duplication and without resorting to inheritance.
When defining category predicates, we need to remember that a category
can be imported by more than one object. Thus, the calls to the built-in
methods that handle the private dynamic predicate (such as
:ref:methods_assertz_1
or :ref:methods_retract_1
) must be made
either in the context of self, using the message to self control
structure, :ref:control_send_to_self_1
, or in
the context of this (i.e. in the context of the object importing the
category). This way, we ensure that when we call one of the attribute
predicates on an object, the intended object own definition of
attribute_/2
will be used. The predicates definitions are
straightforward. For example, if opting for storing the attributes in
self:
::
:- category(attributes)
.
:- public(set_attribute/2). :- mode(set_attribute(+nonvar, +nonvar), one). :- public(get_attribute/2). :- mode(get_attribute(?nonvar, ?nonvar), zero_or_more). :- public(del_attribute/2). :- mode(del_attribute(?nonvar, ?nonvar), zero_or_more). :- public(del_attributes/2). :- mode(del_attributes(@term, @term), one). :- private(attribute_/2). :- mode(attribute_(?nonvar, ?nonvar), zero_or_more). :- dynamic(attribute_/2). set_attribute(Attribute, Value):- ::retractall(attribute_(Attribute, _)), ::assertz(attribute_(Attribute, Value)). get_attribute(Attribute, Value):- ::attribute_(Attribute, Value). del_attribute(Attribute, Value):- ::retract(attribute_(Attribute, Value)). del_attributes(Attribute, Value):- ::retractall(attribute_(Attribute, Value)).
:- end_category.
The alternative, opting for storing the attributes on this, is
similar: just delete the uses of the (::)/1
control structure from the
code above.
We have two new directives, :ref:directives_category_1_4
and
:ref:directives_end_category_0
, that
encapsulate the category code. If needed, we can put the predicates
directives inside a protocol that will be implemented by the category:
::
:- category(attributes,
implements(attributes_protocol))
.
...
:- end_category.
Any protocol can be implemented by either an object, a category, or both.
.. _importing:
We reuse a category's predicates by importing them into an object:
::
:- object(person,
imports(attributes))
.
...
:- end_object.
After compiling and loading this object and our category, we can now try queries like:
.. code-block:: text
| ?- person::set_attribute(name, paulo)
.
yes
| ?- person::set_attribute(gender, male)
.
yes
| ?- person::get_attribute(Attribute, Value)
.
Attribute = name, Value = paulo ; Attribute = gender, Value = male ; no