sugoi haskell chapter14 problem

either_bird.hs

import Control.Monad()

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

landLeft :: Birds -> Pole -> Either String Pole 
landLeft n (left, right)
    | abs ((left + n) - right) < 4 = Right (left + n, right)
    | otherwise = Left ("Last state: " ++ show (left + n, right))

landRight :: Birds -> Pole -> Either String Pole
landRight n (left, right)
    | abs (left - (right + n)) < 4 = Right (left, right + n)
    | otherwise = Left ("Last state: " ++ show (left, right + n))