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From: scaletti@uxc.cso.uiuc.edu
Newsgroups: comp.lang.smalltalk
Subject: Re: addition
Message-ID: <178400003@uxc.cso.uiuc.edu>
Date: Thu, 27-Aug-87 19:49:00 EDT
Article-I.D.: uxc.178400003
Posted: Thu Aug 27 19:49:00 1987
Date-Received: Sun, 30-Aug-87 00:48:17 EDT
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Nf-From: uxc.cso.uiuc.edu!scaletti    Aug 27 18:49:00 1987


I needed double dispatching for implementing matrix
arithmetic.  While it is easy to use the coercion facility for
standard numeric values, matrix arithmetic has two kinds of
multiplication and an unusual addition.  That is, you can have
either scalar or matrix multiplication, which could possibly be
taken care of with coercion (coerce a scalar to a properly scaled
identity matrix of the right size, assuming you know which side of
the matrix the scalar is going to multiply). Also, addition is only allowed 
between matrices of the same size.  There would be no error 
checking, unless additional checks are placed in the 
retry:coercing: methods.

In addition, double dispatching is much faster than the retry:coercing:
mechanism, the resulting code is much cleaner (no class checks),
and the code can also be verified to be type safe.

The only drawback is the order n^2 number of methods.

			Kurt Hebel