gistfile1.hs

{-# LANGUAGE ExistentialQuantification #-}
import System

class Hi_ a where
  hi :: a -> String
data Hi = forall a. Hi_ a => Hi a
instance Hi_ Hi where
  hi (Hi x) = hi x

data Ujihisa = Ujihisa
instance Hi_ Ujihisa where
  hi = const "my name is ujihisa"
ujihisa :: Hi
ujihisa = Hi Ujihisa

data Ujm = Ujm
instance Hi_ Ujm where
  hi = const "this is a pen"
ujm :: Hi
ujm = Hi Ujm

main = do x <- getArgs
          let the_klass = if length x == 1 then ujihisa else ujm
          print $ hi the_klass