On Friday, June 11, 2010, Felipe Lessa
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.
OverlappingInstances (of course), and IncoherrentInstances, since neither instance is more specific than the other.
2) After compiled, if it works as intended or not.
It's hard for me to concieve of a situation where something requiring IncoherrentInstances is work as intended, but maybe that's a failure of imagtination. -Antoine