Inheritance

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.