2004-09-15 00:11:12 +01:00
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=================================================================
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Logtalk - Object oriented extension to Prolog
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2004-12-30 00:35:38 +00:00
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Release 2.22.3
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2004-09-15 00:11:12 +01:00
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Copyright (c) 1998-2004 Paulo Moura. All Rights Reserved.
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=================================================================
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2004-11-29 20:36:31 +00:00
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To load this example and for sample queries, please see the SCRIPT
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file.
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2004-09-15 00:11:12 +01:00
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This example illustrates some variants of the "diamond problem"
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(multi-inheritance conflicts and ambiguities) and its respective
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solutions on Logtalk.
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This classical problem can be simply described by constructing a
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"diamond" of objects and inheritance links as follows:
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A -- contains default definition for a predicate m/0
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/ \
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B C -- contains redefinitions of the predicate m/0
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\ /
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D -- inherits both redefinitions of the predicate m/0
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As such, the object D inherits two conflicting definitions for the
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predicate m/0, one from object B and one from object C. If we send
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the message m/0 to object D, is ambiguous which inherited definition
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should be used to answer it. Depending on the nature of the objects
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A, B, C, and D, the correct answer can be the redefinition of m/0 in
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object B, the redefinition m/0 in object C, or both redefinitions.
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A programming language supporting multi-inheritance should provide
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programming mechanisms allowing easy implementation of each possible
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solution.
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Note that, in the context of Logtalk, the diamond problem may occur with
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prototype hierarchies, class hierarchies, protocol hierarchies, or when
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using category composition.
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This example deals with three variants of the diamond problem, illustrated
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using prototype hierarchies:
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diamond1
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illustrates the inherited definition which is visible due to the
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Logtalk predicate lookup algorithm
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diamond2
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presents a solution for making the overridden inherited definition
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the visible one
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diamond3
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presents a solution which allows both inherited definitions to be
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used in D
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