{-# 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

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