{-# LANGUAGE ExistentialQuantification #-}

import Control.Monad

data Cube = Cube { side :: Float }
data Sphere = Sphere { radius :: Float }

class Volume a where
    volume :: a -> Float

instance Volume Cube where
    volume = (^ 3) . side

instance Volume Sphere where
    volume = (4 * pi / 3 *) . (^3) . radius

data AnyPolygon = forall v. Volume v => AP v

volumeObjects :: [AnyPolygon]
volumeObjects = [AP (Cube 5), AP (Sphere 10)]

printVolume :: Float -> IO ()
printVolume = putStrLn . (++) "The volume is: " . show

getVolume :: AnyPolygon -> Float
getVolume (AP x) = volume x

main = forM_ volumeObjects (printVolume . getVolume)