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From: johnson@uiucdcsp.cs.uiuc.edu
Newsgroups: comp.lang.smalltalk
Subject: Re: A question...
Message-ID: <80500012@uiucdcsp>
Date: Thu, 6-Aug-87 16:40:00 EDT
Article-I.D.: uiucdcsp.80500012
Posted: Thu Aug  6 16:40:00 1987
Date-Received: Sat, 8-Aug-87 16:54:02 EDT
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Nf-From: uiucdcsp.cs.uiuc.edu!johnson    Aug  6 15:40:00 1987


The + method for Point in Smalltalk-80 converts the argument to a point
and then adds it.  It looks something like

+ anArg
    | aPoint |
    aPoint <- anArg asPoint.
    ^(x + aPoint x) @ (y + aPoint y)

The asPoint message to a Point returns the receiver, while the asPoint
message to a number returns (self @ self).

This seems to me a kludge.  A better solution is to use "double dispatching"
to pick the method based on the classes of both the receiver and the
argument.  Dan Ingals had a paper describing this in OOPSLA'86.  The basic
idea is that the + method for Point would look something like

+ anArg
    ^anArg addToPoint: self

The class of anything that might ever be added to a point would then need
an addToPoint: method.  You can do this with all the number classes and
the arithmetic operators.  It results in a nicer system than the one in
Smalltalk-80 based on "coercion", especially if you want to have matrices,
polynomials, functions, etc. as part of your arithmetic.

I teach my students that good Smalltalk code should NEVER explicitly test
the class of an object.  Double dispatching is usually a better alternative.