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List:       haskell-beginners
Subject:    Re: [Haskell-beginners] Type classes and synonyms
From:       sterl <s.clover () gmail ! com>
Date:       2009-11-22 18:08:24
Message-ID: 4B097E18.2030002 () gmail ! com
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Brent Yorgey wrote:
> "free theorem" will be of the form
>
>   "Any function f of type T, *no matter how f is implemented*, will
>   always satisfy the following property:
>
>     blah blah f blah = blah f blah
>   "
>
> This has nothing to do with whether or not there is only one possible
> implementation of f that does not involve undefined, which is a
> different phenomenon.
>   
If it hasn't been mentioned, djinn turns type signatures into code, as 
has been discussed, although if f has multiple implementations, it will 
simply produce one of them.
http://hackage.haskell.org/package/djinn

The discussion on ltu helps flesh out the concept: 
http://lambda-the-ultimate.org/node/1178

This is, of course, as Brent pointed out, very different from free theorems.

--S
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