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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.

This example illustrates the predicate declaration and predicate definition lookup algorithms used when sending a message to an object. For full details, see the Handbook section on inheritance.

The lookup algorithms differ for instances and for prototypes and also depend if the lookup is for a predicate declaration or for a predicate definition. Sending a message to an object, requires two predicate lookups (performed at compile time, when possible):

• Lookup the predicate declaration to check that the predicate in within the scope of the sender. In the most common cases, this means that the predicate is declared public.
• Assuming that the predicate exists and is within scope, lookup the predicate definition to answer the message. If none found, the message simply fails as per the Closed World Assumption (CWA).

  See the comments in the prototypes.lgt and classes.lgt source files and the sample queries in the SCRIPT.txt file for further details.