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From: halvers@altair.crd.ge.com (Pete Halverson)
Newsgroups: comp.object
Subject: Re: Should Shapes Display Themselves?
Message-ID: <10885@crdgw1.crd.ge.com>
Date: 9 Aug 90 14:42:10 GMT
References: <1681@dinl.mmc.UUCP> <25966@bellcore.bellcore.com> <CLINE.90Aug8130349@cheetah.ece.clarkson.edu>
Sender: news@crdgw1.crd.ge.com
Reply-To: halverson@crd.ge.com (Pete Halverson)
Organization: General Electric Corporate R&D Center
Lines: 46

In article <CLINE.90Aug8130349@cheetah.ece.clarkson.edu> cline@sun.soe.clarkson.edu (Marshall Cline) writes:

>QUESTION: Should a `Shape' be able to `draw' itself?
>
>The usual answer is ABSOLUTELY....However: what if I add a new terminal
>type?  Or what if I want a textual `view' (using MVC terminology) rather
>than a graphical `view'?

CLOS explicitly deals with this kind of "orthogonal polymorphism" in the
form of multimethods, in which behavior can be dependent on any or all of
the arguments, not just the first.   Thus, one can define a method for
each combination of shapes and view types (at whatever level of abstraction
is necessary). For instance, given a class structure like 

    (defclass shape ()
        ()
      (:documentation "base shape class"))

    (defclass square (shape))
    (defclass circle (shape))
    (defclass hexagon (shape))

    (defclass view ()
        ()
      (:documentation "base view class"))

    (defclass text-view (view))
    (defclass graph-view (view))

you could define multimethods like

    (defmethod draw ((shape square) (view text-view))
      ;; print "I'm a square"
    )

    (defmethod draw ((shape square) (view graph-view))
      ;; render square
    )

   ...etc.
          
--
===============================================================================
Pete Halverson                        		    INET: halverson@crd.ge.com 
GE Corporate R&D Center	                      UUCP: uunet!crd.ge.com!halverson
Schenectady, NY                     "Money for nuthin' and your MIPS for free"