On Feb 2, 2007, at 21:10 , Sergey Zaharchenko wrote:
Hello list,
Suppose I want show Nothing to return "", and show (Just foo) return show foo. I don't seem to be able to. Looks like I either have to use some other function name, like `mShow', or have to import Prelude hiding Maybe, reimplement Maybe, write all the other useful instances (Functor, Monad) for it, etc. Not particularly hard, but looks ugly. Isn't there a better solution? I recall some discussion about this, but can't find it in the archives...
With GHC you can at least avoid rewriting all the instances: make a newtype, use newtype deriving to get all the instances except Show, and write your own Show instance.
{-# OPTIONS_GHC -fglasgow-exts #-}
newtype MyMaybe a = MyMaybe (Maybe a) deriving (Eq,Ord,Monad,Functor)
instance Show a => Show (MyMaybe a) where showsPrec _ (MyMaybe Nothing) = id showsPrec n (MyMaybe (Just x)) = showsPrec n x
/Björn