Balancing routine example from http://learnyouahaskell.com/ book.

balancing.hs

import           Control.Monad
import           Control.Monad.State

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

type Balance = Either String Pole

birdLand :: Pole -> Balance
birdLand pole@(left, right) = if balanced
  then return pole
  else Left "Lost balance!"
  where balanced = abs (left - right) < 4

landLeft :: Birds -> State Pole Balance
landLeft n = state (\(left,right) -> let newpole = (left + n, right) in (birdLand newpole, newpole))

landRight :: Birds -> State Pole Balance
landRight n = state (\(left,right) -> let newpole = (left, right + n) in (birdLand newpole, newpole))

routine :: State Pole Balance
routine = do
  landLeft 3
  landRight 2
  landLeft 6

main = do
  print $ runState routine (0, 0)