yap-6.3/Logtalk/examples/points/NOTES

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Logtalk - Object oriented extension to Prolog
Release 2.23.1

Copyright (c) 1998-2005 Paulo Moura.  All Rights Reserved.
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To load this example and for sample queries, please see the SCRIPT file.

You can find the original description of this example (and a solution using 
SICStus Objects) at the URL:

	http://www.sics.se/ps/sicstus/sicstus_32.html#SEC254

Suppose you wish to represent points in a two-dimensional space. The
protocol you will have to define consists on the operation move/2, which
allows you to move one point to a new position, and on the operation
print/0, which prints the point position. From the base class point,
which contains the indicated operations, we wish to build three
variants. One class bounded_point in which one point can only move in a
restricted area in space. One class history_point, characterized from
the particularity that each point recalls its previous positions.
Finally, a class bounded_history_point combining the functionality of
classes bounded_point and history_point.

At first sight, this looks like the kind of ideal problem to illustrate
the advantages of the multiple inheritance mechanisms. However, this type 
of solution holds several problems. If the methods move/2 and
print/0 are inherited by bounded_history_point of classes
history_point and bounded_point simultaneously, then one point will be
moved and shown twice. If the inheritance is carried out, for each
method, only from one of the superclasses (assuming that it is possible
to do so, only by breaking the apparent problem symmetry), then the
interfaces of classes history_point and bounded_point will have to
contain separately the necessary operations to verify the limits (in the
case of bounded_point), or to recall the previous positions (in the
case of history_point). This way, the class bounded_history_point could
build its own versions of methods move/2 and print/0, adding to the
inherited definitions of one of the superclasses the calling of the
operation missing in the other superclass. This is the solution adopted
in the SICStus Objects. However, this solution also implies a few
problems. Let's suppose that method move/2 is inherited from class
history_point. Then, any changing operated in the definition of the same
method in class bounded_point is ignored by bounded_history_point. The
problem can be unnoticed, once the symmetry suggested by the use of
multiple inheritance does not reflect on the present implementation.

The solution just suggested is, in short, a generalization of the
problem previously described. Instead of using multiple inheritance,
let's use composition mechanisms. In order to do so, let's separate the
operations on one point, while an object state, of the classes
representing each one of the point types. This can be achieved through
the definition of two new categories, bounded_coordinate and
point_history, that will define the operations associated both to the
memorization of previous values, and to the verification of feasible
limits for a coordinate value. Each one of the point, bounded_point,
history_point and bounded_history_point classes will import this
category, using his operations so as to define the methods affecting the
solutions that use multiple inheritance.