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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

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

This example illustrates some variants of the "diamond problem" (multi-inheritance conflicts and ambiguities) and its respective solutions on Logtalk.

This classical problem can be simply described by constructing a "diamond" of objects and inheritance links as follows:

A -- contains default definition for a predicate m/0 / \ B C -- contains redefinitions of the predicate m/0 \ / D -- inherits both redefinitions of the predicate m/0

As such, the object D inherits two conflicting definitions for the predicate m/0, one from object B and one from object C. If we send the message m/0 to object D, is ambiguous which inherited definition should be used to answer it. Depending on the nature of the objects A, B, C, and D, the correct answer can be the redefinition of m/0 in object B, the redefinition m/0 in object C, or both redefinitions. A programming language supporting multi-inheritance should provide programming mechanisms allowing easy implementation of each possible solution.

Note that, in the context of Logtalk, the diamond problem may occur with prototype hierarchies, class hierarchies, protocol hierarchies, or when using category composition.

This example deals with three variants of the diamond problem, illustrated using prototype hierarchies:

[100, 105, 97, 109, 111, 110, 100, 49]

illustrates the inherited definition which is visible due to the Logtalk predicate lookup algorithm

[100, 105, 97, 109, 111, 110, 100, 50]

presents a solution for making the overridden inherited definition the visible one

[100, 105, 97, 109, 111, 110, 100, 51]

presents a solution which allows both inherited definitions to be used in D