A simple State example in haskell

Main.hs

module Main where
import Control.Monad.Trans.State
import Data.Functor.Identity

type PScore = Int
type CScore = Int
type GameState = (PScore, CScore)

valFromGameState :: GameState -> (Int, Int)
valFromGameState = id

-- 1 for player's point, 2 for computer's point
nextGameState :: Int -> GameState -> GameState
nextGameState ind s = case ind of
                     1 -> (fst s + 1, snd s)
                     2 -> (fst s, snd s + 1)
                     _ -> s

type GameStateMonad = StateT GameState Identity

getNextGameState :: Int -> GameStateMonad (Int, Int)
getNextGameState ind = state $ \st -> let st' = nextGameState ind st in (valFromGameState st', st')
-- In this instance of the game, player scores two points and computer scores one point
oneGame :: GameStateMonad (Int, Int)
oneGame = do
  x <- getNextGameState 1
  y <- getNextGameState 1
  return =<< getNextGameState 2

main = do
  -- Final state should be (2, 1)
  print (evalState oneGame (0,0))