[Haskell] ghc question

[Simon Peyton-Jones]

GHC does context reduction as late as possible, so that when overlapping
instances are involved the commitment is made where maximum information
is available.

Simon

| -----Original Message-----
| From: haskell-bounces at haskell.org [mailto:haskell-bounces at haskell.org]
On Behalf Of Jurriaan Hage
| Sent: 17 March 2004 15:35
| To: haskell at haskell.org
| Subject: [Haskell] ghc question
| 
| Hello,
| 
| Given the fact that Haskell 98 demands that class constraints in an
| explicit type are
| in a normal form (either a variable, or a type variable applied to a
| list of types), it struck me
| that in the following (not very useful) program ghci yields a type
| which is not of that form.
| 
| class X a where
|    (><) :: a -> a -> Bool
| 
| class Y a where
|    (<>) :: a -> a -> Bool
|    _ <> _ = True
| 
| instance Y a => X [a] where
|    x >< y = not(head x  <> head y)
| 
| --f :: Y a => a -> a -> Bool
| f g h = [g] >< [h]
| 
| Now, in ghci
| :t f yields f :: forall t. (X [t]) => t -> t -> Bool
| 
| Hugs does reduce the type of f to the explicit type in comments.
| 
| My question is: is there any special reason for this behaviour?
| 
| Jur
| --
| http://www.cs.uu.nl/people/jur/progrock.html -- jur at cs.uu.nl
| If e-mail does not work try jurriaanhage at hotmail.com
| 
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