{-# LANGUAGE ExistentialQuantification #-}

data Square    = Square Int
data Circle    = Circle Int
data Rectangle = Rectangle Int Int

class HasArea a where
    area :: a -> Double

instance HasArea Square where
    area (Square n) = fromIntegral n * fromIntegral n

instance HasArea Circle where
    area (Circle r) = pi * fromIntegral r

instance HasArea Rectangle where
    area (Rectangle n m) = fromIntegral n * fromIntegral m

data Shape = forall s. HasArea s => Shape s

shapes :: [Shape]
shapes = [Shape (Square 5), Shape (Circle 2), Shape (Rectangle 10 5)]

shapeArea :: Shape -> Double
shapeArea (Shape s) = area s

main = print $ map shapeArea shapes