Monad transformers

I’ve been spending some time working through Haskell Programming From First Principles recently which is a very comprehensive introduction to functional programming and Haskell.

One of the exercises to implement a simplified command line version of Morra, which involves keeping track of scores and reading user input.

My initial method to play a round looked something like this:

playRound :: Config -> Scores -> IO (Bool, Scores)
  playRound config scores = do
  putStr "P1: "
  p1HandMaybe <- getHumanHand
  case p1HandMaybe of
    Nothing -> return (True, scores)               -- error case 1
    Just x -> do
      putStr "P2: "
      p2HandMaybe <- getHumanHand
      case p2HandMaybe of
        Nothing -> return (True, scores)           -- error case 2
        Just y -> do
          let evenWins = (x + y) `mod` 2 == 0
          return (False, updateScores config evenWins scores)

playRound takes the configuration for the game and the current score, and returns a side effecting computation that will return a tuple with the new scores and a boolean indicating if the game is finished.

The method getHumanHand used above returns a IO (Maybe Int), which can be interpreted as a side effecting action that might return an integer (in this case, the side effect is reading from the console and we can’t trust the user to enter an integer, hence the Maybe).

The problem then is that we’re then manually unpacking these Maybe Int values, which leads to the ugly nesting and case statements. However, we can see on the lines marked ‘error case’ above that the handling for both cases is the same - we assume that if the user has entered something other than an Int that they want to end the game.

I recently learned about Monad transformers, which allow you to compose monads. In this case, we want to compose the Maybe monad with the IO monad, so we will use MaybeT.

Rewriting getHumanHand to return a MaybeT IO Int and rewriting playRound results in the following:

playRound' :: Config -> Scores -> IO (Bool, Scores)
playRound' config scores = do

  newScores <- runMaybeT $ do
    liftIO $ putStr "P1: "
    p1Hand <- getHumanHand'
    liftIO $ putStr "P2: "
    p2Hand <- getHumanHand'
    let evenWins = (p1Hand + p2Hand) `mod` 2 == 0
    return $ updateScores config evenWins scores

  return $ case newScores of
    Nothing -> (True, scores)
    Just x -> (False, x)

The nice thing about this implementation is that we’ve avoided the need for pattern matching, as the do block above where we’re dealing with the potentially failing computations will immediately short circuit and return a Nothing if either user fails to provide a legitimate value.

This is the first time I’ve actually used a Monad transformer, and it was good to see how it cleans up the implementation of playRound.