{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleInstances #-}
 
data Circle = Circle Int deriving Show
data Square = Square Int deriving Show
 
class Drawable a where
  draw :: a -> IO ()
 
instance Drawable Circle where
  draw (Circle x) = print $ ("I'm circle " ++ show x)
 
instance Drawable Square where
  draw (Square x) = print $ ("I'm square " ++ show x)
 
data Proxy = forall a. Drawable a => Proxy a
instance Drawable Proxy where
  draw (Proxy a) = draw a
 
instance Drawable [Proxy] where
  draw = mapM_ draw
 
main = do
  draw [Proxy $ Circle 42, Proxy $ Square 100500]
 

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