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From: stevev@tekchips.TEK.COM (Steve Vegdahl)
Newsgroups: comp.lang.smalltalk
Subject: Re: A question...
Message-ID: <1549@tekchips.TEK.COM>
Date: Wed, 5-Aug-87 12:27:25 EDT
Article-I.D.: tekchips.1549
Posted: Wed Aug  5 12:27:25 1987
Date-Received: Sat, 8-Aug-87 09:17:25 EDT
References: <4357@felix.UUCP>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 30

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:
> 
>   + delta
>     (delta class == Point)
>       ifTrue: [^ Point new x: (self x + (delta x)) y: (self y + (delta y)) ]
>       ifFalse: [^ Point new x: (self x + delta) y: (self y + delta) ]

I believe that most Smalltalk images have a coercion selector "asPoint",
where
	3@7 asPoint --> 3@7
but
	6 asPoint --> 6@6

Then your '+' method for Point looks something like:
	+ delta
	    | temp |
	  temp <- delta asPoint
	  ^(self x + temp x) @ (self y + temp y)

		Steve Vegdahl
		Computer Research Lab
		Tektronix Labs
		Beaverton, Oregon