On Sat, Jun 12, 2010 at 12:13:14AM +0200, Dupont Corentin wrote:
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?
Ad hoc solution: class MaybeShow a where maybeShow :: a -> Maybe String instance Show a => MaybeShow a where maybeShow = Just . show instance MaybeShow a where maybeShow = Nothing data MyData where Something :: MaybeShow a => a -> MyData instance MaybeShow MyData where maybeShow (Something x) = fmap (\s -> "Something (" ++ s ++ ")") (maybeShow x) Hahahaha :). Try to guess without using GHC/GHCi: 1) Which extensions are required to make the code compile. 2) After compiled, if it works as intended or not. Cheers, -- Felipe.