PROLOG OBJECT-ORIENTED PACKAGE
                       Author: Ben Staveley-Taylor
                     Received sometime in early 1987
                  Shelved on the 10th of December 1987
                  Updated on the 10th of September 1988
         with small changes to improve portability suggested by
         Nick Youd (Cambridge University Engineering Department)


The Prolog Object-oriented Embedded Manager, POEM, makes available some
of the features found in languages like Simula-67. Classes may be
defined, objects (instantiations of classes) created and operated on as
high- level entities.

An example is often the best way to introduce an idea. Suppose that
points are to be represented in 2-dimensional Cartesian co-ordinates,
and only the quadrant 0 <= x,y <= 10 is to be considered:

    class point(X, Y)
         checks
              ( X >= 0, X =< 10,
                Y >= 0, Y =< 10 )
         body
              identical( point(X1, Y1) ) =>
                   ( X1 = X, Y1 = Y )
          -&-
              distance2( point(X1, Y1), Dist ) =>
                   Dist is (X1-X)*(X1-X)+(Y1-Y)*(Y1-Y).

This declaration then sets up a class 'point'. The clauses following
'checks' are executed whenever a new point object is created, and the
'checks' goal must succeed for the object to be successfully
instantiated.

Two predicates are defined to manipulate the class: identical/1 succeeds
if the argument point structure is the same as the point that owns this
incarnation of identical/1. distance2/2 instantiates its second argument
to the square of the distance between the two points.

As in Simula, a hierarchy of classes may be established. This allows
subclasses to be defined with all the checks and predicates of their
superclasses, and further subclass specific ones.

For example, suppose a class 'rhombus' has been defined. A rhombus is
specified by any three of its vertices. Further to this, some operations
can be defined only for squares owing to their special symmetry. This
setup could be realised by:

    class rhombus(P1, P2, P3)
         checks
              /* ensure that P1, P2, P3 are valid points */
         body
             area(A) => /* find the rhombus's area */.

    square(P1, P2, P3) class rhombus
         checks
              /* extra checks for squares */
         body
              circle(C) => /* find the circumcircle */.

For objects of class 'square', both the predicates 'area' and 'circle'
are available. Note that all the initialisation checks defined for
rhombuses are automatically applied to squares too. Extra ones specific
to squares may be defined.

If class 'square' were to define another predicate 'area', then this new
version would be executed only on failure of the superclass 'rhombus'
version of 'area'.

Subclasses must have the same description list as their superclass,
otherwise the superclass predicates cannot meaningfully be applied.
[BS-T]


CHECKED ON EDINBURGH-COMPATIBLE (POPLOG) PROLOG : yes.


PORTABILITY :

    See notes at the top of POEM.PL and POEMSHAPES.PL

DOCUMENTATION :

    (1) A separate description of  POEM's class description syntax, main
    predicates, and how class descriptions are translated into Prolog.

    (2) Comments in the program for important predicates and methods.