data State a = State (Int -> (a, Int))

instance Monad State where
  return x = State $ s -> (x, s)

  (State f) >>= k = State $ s ->
    let
      (x, s') = f s
      State f' = k x
    in f' s'

get :: State Int
get = State $ s -> (s, s)

put :: Int -> State ()
put s = State $ _ -> ((), s)

increment :: State ()
increment = do
  a <- get
  put(a+1)