On Thu, 27 Mar 2008, Sittampalam, Ganesh wrote:
Well, Ord Foo doesn't hold, does it? So Ord (a, b) isn't equivalent to
(Ord a, Ord b).
It seems you (can) throw logic out of the window with flexible instances.
So there's no point in talking about equivalences any more. We could still
capture the operational aspect of it, but we'd need the type/data family
counterpart of flexible instances.
Considering overlapping instances, there is still a logic, but it's
implicit in the notation. You'd get for your
example:
(a /= Foo \/ b \= Foo) ==> (Ord a /\ Ord b <=> Ord (a,b))
Again, an overlapping type/data family would be needed for encoding this
in dictionaries.
Cheers,
Tom
-----Original Message-----
From: Simon Peyton-Jones [mailto:[EMAIL PROTECTED]
Sent: 27 March 2008 09:05
To: Sittampalam, Ganesh; 'Tom Schrijvers'; Ganesh Sittampalam
Cc: glasgow-haskell-users@haskell.org; Martin Sulzmann
Subject: RE: flexible contexts and context reduction
Why "unfortunately"? Looks fine to me.
Simon
|
| Unfortunately, GHC accepts the following:
|
| {-# LANGUAGE FlexibleInstances #-}
| module Foo2 where
|
| data Foo = Foo
| deriving Eq
|
| instance Ord (Foo, Foo) where
| (Foo, Foo) < (Foo, Foo) = False
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Tom Schrijvers
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