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— 123 — CLIPS> (describe-class INTEGER) ================================================================ **************************************************************** Abstract: direct instances of this class cannot be created. Direct Superclasses: NUMBER Inheritance Precedence: INTEGER NUMBER PRIMITIVE OBJECT Direct Subclasses: **************************************************************** ================================================================ CLIPS> The problem is that the INTEGER class has no handler for "+". In fact, it has no handlers at all since none are listed. As you recall, the USER class and its subclasses always have print, delete, and other handlers automatically defined by CLIPS. If we want to send print messages to an object like 1 of the INTEGER class, we'll have to make our own print handler. Before writing this handler, let's answer a question that you may have concerning how INTEGER can have instances. Since INTEGER is an abstract class, you may wonder how it can have instances such as 1, 2, 3, etc. Although you cannot make direct instances of an abstract class, you can make use of existing instances. For the case of the predefined system class INTEGER, all the integers up to the maximum allowed are available as objects. Likewise, strings and symbols are available for the system defined abstract classes STRING and SYMBOL, and so forth for the other predefined classes. Shown following is the definition of a handler for the NUMBER class that will handle addition by messages. The handler is defined for NUMBER rather than INTEGER because we would also like to handle FLOAT objects too. Instead of defining the same handler for FLOAT and for INTEGER, it's easier to just define a handler for the superclass NUMBER. If a handler for a message is not defined in the class of the object, CLIPS tries all the handlers in the inheritance precedence list. Since "+" is not defined for INTEGER, CLIPS tries NUMBER next, finds the applicable handler, and returns the result of 3. CLIPS> (defmessage-handler NUMBER + (?arg) ; Argument of handler (+ ?self ?arg)) ; Function addition of handler CLIPS> (send 1 + 2) 3 CLIPS> (send 2.5 + 3)
