Why the instance declaration , passing on Hug but failing on GHC 6.2.1?
I am a novice from China. All the coeds are very simple as below: 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 ++ ">" And the error in GHCi: Compiling Main ( Main.hs, interpreted ) Main.hs:5: Illegal instance declaration for `Show (Pair People)' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Show (Pair People)' Failed, modules loaded: none. But it is puzzling that these code can pass on Hug! I thought that the syntaxn of instance declaration is right! Then, Why did it fail? Best regards, Wukaichen
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
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Stefan Holdermans -
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