Re: no non-typevariable in instance declarations

class C a where ty :: a -> String instance (Num a) => C a where ty _ = "NUM" instance C Integer where ty _ = "Integer"

Why GHC and NHC98 are more restrictive than Hugs?

The instances for (Num a=> a) and Integer overlap, and are therefore forbidden by Haskell'98. Hugs allows overlapping instances. So does GHC with a special flag (-foverlapping-instances I think). nhc98 does not provide type-system extensions to Haskell'98. Regards, Malcolm