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From: susser@apple.com (Joshua Susser)
Newsgroups: comp.object
Subject: Re: the need for classes as objects
Message-ID: <9959@goofy.Apple.COM>
Date: 28 Aug 90 20:14:59 GMT
References: <2259@esquire.UUCP>
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Organization: Apple Computer, Inc. (ATG)
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When I was at Caltech in '84, Tzu-Mu Lin (one of Carver Mead's grad 
students) created a VLSI analysis tool (I think it used predicted signal 
delay times) in Smalltalk-80 as part of his thesis work.  As I recall, he 
had a nice graphical tool for describing circuits.  Once the circuits were 
specified, those descriptions were translated into Smalltalk code in the 
form of classes and methods.  Since a VLSI circuit often has common 
circuit elements duplicated many times, using classes as the 
representation of behavior gave a lot of leverage.  Since Smalltalk-80 
supports creating classes dynamically, the user never had to leave and 
restart the system to include new changes.

Another better known example is Alan Borning's ThingLab, also written in 
Smalltalk-80.  ThingLab is a constraint-based graphics environment.  It 
looks a bit like a MacDraw-type editor, but you can connect the various 
shapes through constraints.  A typical example would be constraining a 
point to be the midpoint of a line segment P stretch the line, and the 
point moves as well.  The types of objects were represented as Smalltalk 
classes, with methods for the various constraints, all added dynamically 
and incrementally.

In both of these cases, using classes really paid off.  Both of these 
systems allowed sharing of behavior, and classes provided a perfect 
mechanism for that.  If Smalltalk-80 hadn't had the ability to create and 
use classes at runtime, both these systems would have had to implement a 
similar, special-purpose mechanism for managing that sharing.  In such a 
case, the code would end up much more obtuse, be harder to understand and 
maintain, and would probably run slower.

Joshua Susser, Object Percussionist
Apple Computer, Advanced Technology Group
inet: susser@apple.com | link: susser.j | phone: 408/974-6997