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diamonds

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

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Start by loading the example:

logtalk_load(diamonds(loader)).

First variant of the "diamond problem", defined in the diamond1.lgt source file:

d1::m.

<!-- Redefinition of method m/0 in object b1

true. -->

Second variant of the "diamond problem", defined in the diamond2.lgt source file:

d2::m.

<!-- Redefinition of method m/0 in object c2

true. -->

Third variant of the "diamond problem", defined in the diamond3.lgt source file:

d3::b3_m.

<!-- Redefinition of method m/0 in object b3

true. -->

d3::c3_m.

<!-- Redefinition of method m/0 in object c3

true. -->

d3::m.

<!-- Redefinition of method m/0 in object b3

true. -->

d4::m.

<!-- Redefinition of method m/0 in object b3 Redefinition of method m/0 in object c3

true. -->