birds2.hs

type Birds = Int
type Pole = (Birds, Birds)

-- 左に鳥が止まる
landLeft :: Birds -> Pole -> Maybe Pole
landLeft n (left, right)
    | abs ((left + n) - right) < 4 = Just (left + n, right)
    | otherwise                    = Nothing

-- 右に鳥が止まる
landRight :: Birds -> Pole -> Maybe Pole
landRight n (left, right)
    | abs (left - (right + n)) < 4 = Just (left, right + n)
    | otherwise                    = Nothing


(-:) :: a -> (a -> b) -> b
x -: f = f x


main :: IO ()
main = do
    print $
        (0, 0) -: landLeft 1 -: landRight 1 -: landLeft 2

    print $
        (0, 0) -: landLeft 1 -: landRight 4 -: landLeft (-1) -: landRight (-2)




banana :: Pole -> Maybe Pole
banana _ = Nothing