Am Donnerstag, 19. Mai 2005 10:39 schrieb Stefan Holdermans:

Wukaichen,

...

data People = A | B | C | D | E | F deriving (Show, Eq, Ord, Enum) data Pair a = Pair a a deriving Eq

instance Show (Pair People) where show (Pair x y) = "<" ++ show x ++ ", " ++ show y ++ ">"

GHC is right, actually. Haskell 98 requires instance declarations to have the form

instance (...) => C (T a1 ... an)

where a1 are type variables, not type constructors as in your example.

However, you can use the Glasgow extensions to get rid of this restrictions. For example by adding

{-# OPTIONS -fglasgow-exts#-}

to the top of your file.

HTH,

Stefan

Alternatively, one might define instance Show a => Show (Pair a) where ... By the way, the original code also needs the '-98' option for hugs, so the behaviour isn't really different. Cheers, Daniel