DEFMESSAGE HANDLER

Objects are manipulated by sending them messages via the function send. The

result of a message is a useful return-value or side-effect. A

defmessage-handler is a construct for specifying the behavior of a class of

objects in response to a particular message. The implementation of a message

is made up of pieces of procedural code called message-handlers (or handlers

for short). Each class in the class precedence list of an object's class can

have handlers for a message. In this way, the object's class and all its

superclasses share the labor of handling the message. Each class's handlers

handle the part of the message which is appropriate to that class. Within a

class, the handlers for a particular message can be further subdivided into

four types or categories: primary, before, after and around.

A defmessage-handler is comprised of seven elements: 1) a class name to which

to attach the handler (the class must have been previously defined), 2) a

message name to which the handler will respond, 3) an optional type (the

default is primary), 4) an optional comment, 5) a list of parameters that will

be passed to the handler during execution, 6) an optional wildcard parameter

and 7) a series of expressions which are executed in order when the handler

is called. The return-value of a message-handler is the evaluation of the last

expression in the body.

The syntax of the defmessage-handler construct is:

(defmessage-handler <class-name> <message-name>
   [<handlertype>] [<comment>]
   (<parameter>* [<wildcard-parameter>])
   <action>*)

<handler-type>       ::= around | before | primary | after

<parameter>          ::= <single-field-variable>

<wildcard-parameter> ::= <multifield-variable>