data P = Zero | Succ P deriving (Show, Read, Eq) (.+.) :: P -> P -> P Zero .+. y = y Succ x .+. y = Succ (x .+. y) one :: P one = Succ Zero two :: P two = Succ one toInt :: P -> Int toInt Zero = 0 toInt (Succ x) = 1 + toInt x fromInt :: Int -> P fromInt 0 = Zero fromInt i = Succ $ fromInt (i - 1) main = do print $ Zero .+. Zero print $ one .+. one print $ one .+. one == two print $ toInt $ two .+. one print $ fromInt 5