Haskell instanceof analogue?

Question

I am new to Haskell, so my question is probably stupid.

I want a function

show2 :: (Show a) => a -> String

which would return show a for any a, but a if a is itself String. How can I implement it?

P.S. it is great if this function is already implemented somewhere, but I still want to see an example of the implementation.

Answer 1

You can use cast from Data.Typeable

show2 :: (Typeable a, Show a) => a -> String
show2 s = maybe (show s) id ms
  where ms = cast s :: Maybe String

Answer 2

You can do this with this piece of dirty and dangerous code:

class Showable a where
  show2 :: a -> String

instance Showable String where
  show2 = id

instance (Show a) => Showable a where
  show2 = show

You need -XOverlappingInstances -XFlexibleInstances -XUndecidableInstances to compile and use it.

*Main> show2 "abc"
"abc"
*Main> show2 3
"3"