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

landLeft :: Birds -> Pole -> Maybe Pole
landLeft n (left, right)
    | abs((left+n) - right) < 4 = Just (left + n, right)
    | otherwise                 = Nothing

landRight :: Birds -> Pole -> Maybe Pole
landRight n (left, right) 
    | abs((right+n) - left) < 4 = Just (left, right + n)
    | otherwise                 = Nothing

foo = do
    x <- Just 3
    y <- Just "!"
    Just (show x ++ y)

routine = do
    start <- return (0,0)
    first <- landLeft 2 start
    second <- landRight 2 first
    landLeft 1 second

wopwop = do
    (x:xs) <- Just ""
    return x