From "Learn you a Haskell for Great Good!" Simulation of Pole's tightrope walking by Either monad.

PoleEither.hs

module PoleEither where

import Control.Monad.Error ()
import Data.List (intersperse)

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

landLeft :: Birds -> Pole -> Either String Pole
landLeft n (left, right) =
  if balance < 4
    then Right (left', right)
    else Left $ shoutBirdsNum "Too many birds! I can't stand!" right left'
  where
    left' = left + n
    balance = abs $ left' - right

landRight :: Birds -> Pole -> Either String Pole
landRight n (left, right) =
  if balance < 4
    then Right (left, right')
    else Left $ shoutBirdsNum "Too many birds! I can't stand!" right' left
  where
    right' = right + n
    balance = abs $ left - right'

shoutBirdsNum :: String -> Birds -> Birds -> String
shoutBirdsNum cause l r = concat $ intersperse "\n" msg
  where
    msg =
      [cause
      ,"  Left:  " ++ show l
      ,"  right:  " ++ show r]

banana :: Pole -> Either String Pole
banana (l, r) = Left $ shoutBirdsNum "Oh, banana! slip down!" l r

routine :: Either String Pole
routine = do
  start  <- return (0,0)
  first  <- landLeft 2 start
  second <- landRight 2 first
  banana second