[Haskell-cafe] GATD and pattern matching
aslatter at gmail.com
Fri Jun 11 21:12:43 EDT 2010
On Friday, June 11, 2010, Felipe Lessa <felipe.lessa at gmail.com> wrote:
> 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
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