Re: OBJECT-ORIENTED CLX

["David A. Moon" <Moon@SCRC-STONY-BROOK.ARPA>]

    Date: Tue, 01 Nov 88 12:17:31 -0600
    From: "David C. Martin" <dcmartin@cs.wisc.edu>

    Lastly, I didn't quite understand the following:

    ----- from Kerry Kimbrough (10/31/88)

    This alternative would be much improved if there actually was a way for CLX to
    dispatch on programmer-defined subclasses *inside* process-event.  It's not
    clear how to implement this.  Another thing is that I've noticed it's
    frequently useful to define a new event subclass procedurally, either via a
    relation predicate over its slots (e.g.  :button-1-press, :shift-button-press,
    etc.) or by a function of the event stream (e.g.  :double-click).  But, CLOS
    doesn't handle these kind of (sub)classes.

    ------ end

I'm not sure I understood it either.  But if it means that you would like
to be able to have a predicate that controls whether a particular method
is applicable or not, like Common Lisp's SATISFIES type-specifiers or
guards in some other languages, CLOS doesn't have that, but it does have
something which gives about the same functionality:  You can define your
own method-combination type, in which each method is optionally associated
with a predicate function, and the effective method form is a COND form
that calls the predicates and then calls the method for the first satisfied
predicate.  Alternatively, you can use the builtin :OR method combination
type, and stick the predicates inside the bodies of the methods.

I hope this helps.