[Haskell] MPTCs and type inference

[Andreas Rossberg]

This may well be stupidity on my side, but some experiments with multi 
parameter type classes got me slightly confused. Can somebody explain 
the following behaviour to me?

   class D a b where fd :: a -> b -> ()

   d1 x = let p = fd x in ()
   d2 x = let p y = fd x y in ()

GHC derives the following types:

   d1 :: D a b => a -> ()
   d2 :: a -> ()

Hugs rejects d1 on the grounds that the type is ambiguous, but agrees on 
the type of d2. I do not understand where the context disappears to in 
this example - in particular, when I compare with the single parameter case:

   class C a where fc :: a -> a -> ()

   c1 x = let p = fc x in ()
   c2 x = let p y = fc x y in ()

where

   c1 :: C a => a -> ()
   c2 :: C a => a -> ()

is inferred, as I would expect.

-- 
Andreas Rossberg, rossberg at ps.uni-sb.de

Let's get rid of those possible thingies!  -- TB