Do magik

magick.hs

import Data.Default
import Debug.Trace

data Proxy a = Proxy

class Magick a where
  magickStorage :: Proxy [a] -> String

instance Magick Int where
  magickStorage = const "Int"

instance Magick Char where
  magickStorage = const "String"

instance Default Char where
  def = '\0'

doMagick :: (Default a, Magick a) => [a]
doMagick = inner Proxy
  where inner :: (Default a, Magick a) => Proxy [a] -> [a]
        inner s | trace (magickStorage s) True  = [def]


{-
% runhaskell magick.hs
Int
[0]
String
"\NUL"
-}  

main = do 
  let x = doMagick :: [Int]
      y = doMagick :: String
  print x
  print y