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From: patc@tekcrl.TEK.COM (Pat Caudill)
Newsgroups: comp.lang.smalltalk
Subject: Re: A question...
Message-ID: <1856@tekcrl.TEK.COM>
Date: Wed, 5-Aug-87 16:14:49 EDT
Article-I.D.: tekcrl.1856
Posted: Wed Aug  5 16:14:49 1987
Date-Received: Sat, 8-Aug-87 07:37:36 EDT
References: <4357@felix.UUCP> <1549@tekchips.TEK.COM>
Reply-To: patc@tekcrl.UUCP (Pat Caudill)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 37

>In article <4357@felix.UUCP>, preston@felix.UUCP (Preston Bannister) writes:
> 
> The class Point responds to '+'.  The examples show both:
> 	aPoint + aPoint		i.e.  (1 @ 2) + (3 @ 4) = (4 @ 6)
> and
> 	aPoint + aNumber	i.e.  (1 @ 2) + 3 = (4 @ 5)
> What I don't see is how to write the method for '+' without explicitly
> testing the class of the parameter.  Something like:

	Besides using the method that Steve Vegdahl describes, I
would like to suggest the method pointed out by Dan Ingalls in a
paper at the OOPSLA-86 conference. I will use the convention, as
he did, of showing the class of the receiver in <> before the message.

<Point> + aValue

    ^aValue pointPlus: self

Then define pointPlus in both number and point as

<Point> pointPlus: aPoint

    ^ (self x + aPoint x) @ (self x + aPoint y)

<Number> pointPlus: aPoint

    ^ (self + aPoint x) @ (self + aPoint y)

The full reference is:

Daniel H. H. Ingalls, "A Simple Technique for Handling Multiple
Polymorphism" OOPSLA'86 Conference Proceedings (Sigplan Notices
Vol 21 #1 (Nov 1986)) pp 347.

I hope this helps.
			Pat Caudill
			patc@tekcrl.TEK.COM