The simplest sort of term class declaration has the following form:
:- class ClassName = term(Term).
This declares that any term that unifies with Term is an instance of class ClassName. For example, you might declare:
:- class rgb_color = term(color(_Red,_Green,_Blue)). color(R,_G,_B) >> red(R). color(_R,G,_B) >> green(G). color(_R,_G,B) >> blue(B). :- end_class rgb_color.
This would declare any term whose principal functor is
arity is three to be an object of class
rgb_color. Given this
declaration, entering the goal
color(0.5, 0.1, 0.6) >> blue(B)
B to 0.6.
Note that you cannot use
create/2 to create a term class
instance. Since they are just ordinary terms, you can create them the
same way you’d create any ordinary Prolog term. Similarly, you cannot
modify an existing term class instance.
You may specify a term class as the type of a slot of an ordinary
class. This is effectively the same as specifing the type to be
term. In particular, fetching and storing term class slots is not
very efficient. Also, the default value for slots of term class type
''; this is because not enough is known about a simple term
class to determine a better default.
For an explanation of how to avoid these pitfalls, see obj-tcl-tce.