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From: budd@mist.cs.orst.edu (Tim Budd)
Newsgroups: comp.lang.smalltalk
Subject: Re: Message dispatch on argument class
Message-ID: <7220@orstcs.CS.ORST.EDU>
Date: 7 Nov 88 17:14:32 GMT
References: <AXRDuKt78k-04Ce11X@andrew.cmu.edu>
Sender: usenet@orstcs.CS.ORST.EDU
Reply-To: budd@mist.UUCP (Tim Budd)
Organization: Oregon State University - CS - Corvallis, Oregon
Lines: 21


There is a technique called double polymorphism that does exactly what
you want.  It is described in more detail in a paper by Dave Unger in the
first OOPSLA conference.  Basically, you turn the argument around, and
encode the type of the receiver for the first message in the message
selector for a second message.  Let me use mixed type arithmetic for an 
example.  Class Integer would have something like the following:
	+ x
		^ x addToInteger: self
	addToFloat: x
		^ x + self asFloat

A class like Float would, when it receives the message addToInteger, 
know that it must convert the argument into float before adding.
Here are a couple methods in class Float.
	+ x
		^ x addToFloat: self
	addToInteger: x
		^ x asFloat + x
The primitive methods are now addToInteger in class Integer and addToFloat
in class Float.