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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Preventing type extensions
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Date: Tue, 21 Sep 2010 20:11:08 +0200
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Date: 2010-09-21T20:11:06+02:00
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On Tue, 21 Sep 2010 19:11:33 +0200, Georg Bauhaus wrote:

> On 21.09.10 18:25, Dmitry A. Kazakov wrote:
> 
>> horse /= animal
> 
> It will help immensely, I think, at least those of us who do
> not do type theory on an every day basis, if we had a glossary
> of terms for interpreting expression like "horse /= animal".

Gosh, it was no more than just this: if horse is an animal that does not
imply they are interchangeable. Compare: "all horses have hoofs" and "all
animals have hoofs."

is-a /= is
member /= set

Moreover:

is-a /= substitutable (greeting to the LSP!)

Compare: "some animals have tentacles" and "some horses have tentacles."
Absence of unconditional substitutability is IMO a killer argument against
re-dispatch, because then see what J-P. Rosen wrote about testing.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de