Specifying a Term Class Essence

As mentioned above, it is possible to specify a term class as the type of a slot of some other object. For example, we might declare

:- class colored_rectangle = [
        public origin:point,
        public size:size,
        public color:rgb_color].

This will store an rgb_color object (i.e., a color/3 term) in the color slot of each colored_rectangle object. Unfortunately, though, SICStus Objects cannot tell what is the best way to store a term object, and therefore it stores it the same way it stores a slot declared to be of term type: using the Prolog database. This has all the efficiency disadvantages of term slots. In this case, however, we know that all that really needs to be saved in order to save an rgb_color object is the three arguments. We also know that each of these arguments is a floating point number, and because precision isn’t terribly critical in representating colors, each of these numbers can be stored as a float, rather than a double. In effect, we know that the essence of a rgb_color object is these three numbers; if we have them, we can easily construct the color/3 term. If we provide this information in the declaration of the rgb_color class, SICStus Objects can store instances of the rgb_color class as 3 separate floats, rather than as a term, significantly improving the performance of creating or destroying a colored_rectangle object, as well as accessing or modifying its color slot.

The essence of a term class is specified with the following form of class declaration:

:- class ClassName = term(Term, Constraint, Essence).

where Essence is of the form

[Name1:Type1=i[Variable1], Name2:Type2=i[Variable2], …]

and each Name is a distinct atom naming a slot, each Type is a slot type as specified in obj-scl-slt, and each Variable is an unbound variable appering in Term. Providing a term essence not only makes storage of terms in ordinary object slots more efficient, it also gives a name to each “essential” slot of the term class. This allows you to use fetch_slot to fetch the slots of this class.

To extend our rgb_color example, we might introduce the rgb_color class with this declaration:

:- class rgb_color = 
              (float(Red),   Red >= 0.0,   Red =< 1.0,
               float(Green), Green >= 0.0, Green =< 1.0,
               float(Blue),  Blue >= 0.0,  Blue =< 1.0),
              [red:float=Red, green:float=Green, blue:float=Blue]).

This declaration defines the rgb_color class exactly as the example declaration of the previous section: every color/3 term whose arguments are all floating point numbers between 0.0 and 1.0 inclusive are instances of rgb_color. The difference is that with this declaration, ordinary classes that have slots of type rgb_color, such as the colored_rectangle example above, will be stored more efficiently, and their rgb_color slots will be accessed and modified much more efficiently. Also, it will be possible to use fetch_slot(red, Red) in the methods of the rgb_color class to fetch to red component of the message recipient, and similarly for green and blue.

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