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From: lady0065@sable.ox.ac.uk (David J Hopwood)
Subject: Re: Cows, Hogs & Farms
Message-ID: <1995Jan12.010822.19169@inca.comlab.ox.ac.uk>
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References: <1995Jan3.223031@hobbit> <1995Jan4.191910.13968@schbbs.mot.com> <1995Jan9.021043@hobbit>
Date: Thu, 12 Jan 95 01:08:22 GMT
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In article <1995Jan9.021043@hobbit>,
Mark S. Hathaway <hathawa2@marshall.edu> wrote:
...
> David Shang wrote:
...
>> In reality, we always have exceptions. We can use covariance to rule out a  
>> general principle and use run-time type check for a few exceptions. As to
>> the animal example, it is clearly worthwhile to divide animals into
>> herbivires, carnivores and (?)(animals eating both food) according to the
>> type of their food. We can use covaraince. But it might be very hard to
>> rule out a general principle for the futher classification for a certain
>> number of animals.
...
>> We are not developing a commercial software for zoologists, are we? These
>> are only pedagogical examples. Be wise to make things as simple as possible
>> -- just for the key points you want to say. 
>
>I have created a few examples in the past which thoroughly confused people
>because they weren't realistic enough.  I'm trying to avoid the sins of
>my past.

Same here.

...
>I'd certainly be interested to read further discussion on the differences
>between these classes and those of Sather & Eiffel.  There seem to be some
>rather major differences.

It might help to read about the typing systems of Cecil, Trellis/Owl, or ML
(and various other functional languages), which use a similar approach to
Cluster.

A paper on the Cecil typing system is at
http://www.cs.washington.edu/research/projects/cecil/www/cecil-spec.html
At least, I think that's the right paper - I can't read postscript on this
terminal to check. Don't get too confused by the stuff on multi-methods - it's
fascinating, but not really relevant. Genericity is described near the end of
the paper.

A good account of dynamic genericity (also called Milner polymorphism) in
functional languages is given in

An Introduction to Functional Programming
Richard Bird and Phillip Wadler
Prentice Hall series in Computing Science
(sorry, don't have the ISBN on me).

Hope this helps,

David Hopwood
david.hopwood@lmh.oxford.ac.uk