Thanks all, it works fine (see below). I lamentably try to make the same for show:
showTypeable :: (Typeable a) => a -> String showTypeable x = case cast x of Just x' -> show x' Nothing -> ""
Because it really upsets me to add this show constraints to the Equ constructor ;) what if i want to make an Obs instance with non showable elements, with no intention to show it of course? Corentin
instance Typeable1 Obs where typeOf1 _ = mkTyConApp (mkTyCon "Obs") []
(===) :: (Typeable a, Typeable b, Eq b) => a -> b -> Bool (===) x y = cast x == Just y
data Obs a where Player :: Obs Player Official :: Obs Bool Equ :: (Eq a, Show a, Typeable a) => Obs a -> Obs a -> Obs Bool Plus :: (Num a) => Obs a -> Obs a -> Obs a Time :: (Num a) => Obs a -> Obs a -> Obs a Minus :: (Num a) => Obs a -> Obs a -> Obs a And :: Obs Bool -> Obs Bool -> Obs Bool Or :: Obs Bool -> Obs Bool -> Obs Bool Not :: Obs Bool -> Obs Bool Konst :: (Show a, Eq a) => a -> Obs a
instance Show t => Show (Obs t) where show Player = "Player" show Official = "Official" show (Equ a b) = (show a) ++ " Eq " ++ (show b) show (Plus a b) = (show a) ++ " Plus " ++ (show b) show (Minus a b) = (show a) ++ " Minus " ++ (show b) show (Time a b) = (show a) ++ " Time " ++ (show b) show (Konst a) = " (Konst " ++ (show a) ++ ")" show (And a b) = (show a) ++ " And " ++ (show b) show (Or a b) = (show a) ++ " Or " ++ (show b) show (Not a) = " (Not " ++ (show a) ++ ")"
instance Eq t => Eq (Obs t) where Player == Player = True Official == Official = True Equ a b == Equ c d = (a,b) === (c,d) Plus a b == Plus c d = (a == c) && (b == d) Minus a b == Minus c d = (a == c) && (b == d) Time a b == Time c d = (a == c) && (b == d) And a b == And c d = (a == c) && (b == d) Or a b == Or c d = (a == c) && (b == d) Not a == Not b = (a == b) Konst a == Konst b = a == b _ == _ = False