GHC (5.03.20020410) wrongly accepts the following:
:::::::::::::: C.lhs ::::::::::::::
module C where
class A a where a :: a -> Int :::::::::::::: X.lhs :::::::::::::: module X where import qualified C
instance C.A Int where C.a = id
This has been fixed, and GHC 5.04 will behave correctly.
Incidentally, I also noticed that GHC and Hugs behave differently for the following variant of X.lhs.
:::::::::::::: X.lhs ::::::::::::::
module X where import qualified C
instance C.A Int where a = const X.a
a = 4
GHC happily accepts the file while Hugs complains that
ERROR "X.lhs":6 - Type error in instance member binding *** Term : a *** Type : a -> Integer *** Does not match : Int -> Int
There seems to be a difference in defaulting. [Don't ask me which is the correct behaviour.] The problem disappears if I supply an explicit type signature for `a'.
Hugs's defaulting behaviour differs from Haskell 98 (and always has done). See for example this discussion on the Haskell mailing list: http://www.haskell.org/pipermail/haskell/2002-May/009616.html (plus other messages with the same subject). Could someone from the Hugs camp please document this? Cheers, Simon