Immediate super-objects are declared by defining the method
super/2 within the object. (Any definition
super(Super) is transformed to
super(Super,[])). The objects declared by super/2
are the immediate objects from which a method is inherited if not
defined within the object. This implies that the inheritance mechanism
is an overriding one. One could possibly have a union inheritance,
whereby all clauses defining a method are collected from the super
hierarchy and executed in a Prolog fashion. This can easily be
programmed in SICStus Objects, using delegation to super objects.
The following example shows some objects used for animal classification.
animal :: {}.
bird :: {
super(animal) &
skin(feather) &
habitat(tree) &
motions(fly)
}.
penguin :: {
super(bird) &
habitat(land) &
motions(walk) &
motions(swim) &
size(medium)
}.
| ?- penguin :: motions(M).
M = walk ;
M = swim ;
no
| ?- penguin :: skin(S).
S = feather ;
no
The following is an example of multiple inheritance: an object john is
both a sportsman and a professor:
john :: {
super(sportsman) &
super(professor) &
:
}.
Inheritance will give priority to the super-objects by the order defined in
the super/2 method. Therefore in the above example John's
characteristics of being a sportsman will dominate those of being professor.
Other kinds of hierarchy traversal can be programmed explicitly using the
delegation mechanism.