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