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This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2023 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.

To load this example and for sample queries, please see the SCRIPT.txt file.

A parametric object may be used to represent objects whose "state" is static and set when the object is defined. However, there can be only one parametric object with a given functor and arity. For example, if we define the following parametric object:

:- object(circle(_Radius, _Color)).
        ...
:- end_object.

then the following terms may be interpreted as references to the object above:

circle(1, blue)
circle(2, yellow)

In the context of parametric objects, the above terms are known as "parametric object proxies". Proxies represent different instantiations of a parametric object parameters. Proxy terms may be stored on the database as Prolog facts or as Prolog rules (parameter instantiation can be deduced instead of being fixed). This results in a very compact representation, which can be an advantage when dealing with a large number of objects with immutable state. In addition, all the predicates managing these compact representation are encapsulated in a parametric object. This can be, however, a fragile solution as changes on the parametric object ancestors may imply changes to the number and meaning of the parametric object parameters which, in turn, may imply changes to all the Prolog facts used to represent the individual objects.

Note that parametric objects can co-exist with "normal" objects. For example, when using a class-based design, we may use "normal" instances together with a parametric instance of the same class.