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hpr3582 :: Rolling a new character

Tuula continues writing an example Haskell game, this time rolling a new character

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Hosted by Tuula on 2022-04-26 is flagged as Clean and is released under a CC-BY-SA license.
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Duration: 00:29:53


A series looking into the Haskell (programming language)

Quick peek at some places in code

Main.hs has our Main module definition. It was generated by Stack when we started. In the end of the main function, it calls run function, which is defined in Run.hs file. This is the place where we can see overall flow of the program in one glance.

run :: RIO App ()
run = do
  choice <- showMainMenu
  case choice of
    StarNewGame -> do
      logDebug "New game starting..."
      logDebug "Rolling new character..."
      player <- liftIO $ evalRandIO rollNewCharacter
      displayNewCharacter player
      logDebug "Selecting starting gear..."
      gear <- selectStartingGear $ playerGear player
      logDebug "Preparing game..."
      game <- liftIO $ evalRandIO $ startGame player gear
      logDebug "Dealing first card..."
      finishedGame <- playGame game
      logDebug "Displaying game over..."
      displayGameOver finishedGame

    ExitGame ->
      return ()

Another interesting module is Types. Here you can find how player, items, monsters and such are represented.

Third and biggest module is UserInterface, which contains functions to display game status to player and ask their input.

So, what does our run function do? Lets have a look:

  • choice <- showMainMenu
    • show main menu and ask for player input
  • case choice of
    • depending on the choice, continue with game logic or exit the function
  • player <- liftIO $ evalRandIO rollNewCharacter
    • roll a new character
    • evalRandIO indicates we're dealing with random numbers
  • displayNewCharacter player
    • display the new character on screen
  • gear <- selectStartingGear $ playerGear player
    • select starting gear
  • game <- liftIO $ evalRandIO $ startGame player gear
    • shuffle the deck and set up the game
    • again using random numbers here
  • finishedGame <- playGame game
    • play game until we're done
  • displayGameOver finishedGame
    • display game over screen

Word about input and output

One of the features of Haskell I like is the ability to show which functions are pure (always returning same output with given set of inputs and not having any side effects). In our program, every function that returns RIO a b has access to input and output. In addition to that, it also has access to system wide configuration (which we don't use much here) and logging functions.

To write on the screen, we use putStrLn and reading a user input readLine. Since they're designed to work with IO instead of RIO a b, we have to use liftIO. But all that is technical details that we won't worry now.

App is our configuration. We aren't directly using it, so it's safe to ignore for now.

Showing main menu

showMainMenu function will print out the menu and then call mainMenuInput. mainMenuInput will read input, validate that it's either 1 or 2 and return respectively StarNewGame or ExitGame. In case user enters something else, mainMenuInput will recurse until user enters valid input.

-- | Display main menu
showMainMenu :: RIO App MainMenuChoice
showMainMenu = do
  logDebug "Displaying main menu"
  liftIO $ putStrLn "\n\n"
  liftIO $ putStrLn "Treasure Dungeon"
  liftIO $ putStrLn "****************"
  liftIO $ putStrLn ""
  liftIO $ putStrLn "1. Start a new game"
  liftIO $ putStrLn "2. Quit"

mainMenuInput :: RIO App MainMenuChoice
mainMenuInput = do
  i <- liftIO getLine
  case i of
    "1" -> return StarNewGame
    "2" -> return ExitGame
      _ -> do
            logDebug $ displayShow $ "Incorrect menu choice: " <> i
            liftIO $ putStrLn "Please select 1 or 2"

You might wonder, why mainMenuInput can keep calling itself without filling the stack? That's because Haskell doesn't use stack in the same sense as many other programming languages. Haskell compiler is also smart enough to notice that call to mainMenuInput is last operation of the mainMenuInput, there is no work to be done after the call, and thus can optimize things even more. I don't know all the dirty details how this has been implemented and how things work behind the curtains.

Rolling new character

player <- liftIO $ evalRandIO rollNewCharacter rolls a new character, but what exactly is going on here? rollNewChacter has following signature: rollNewCharacter :: (RandomGen g) => Rand g Player. It doesn't take any parameters and returns Rand g Player, where g implements RandomGen. So, it's Rand monad that returns Player. In order to get the result of the computation, we call evalRandIO that uses global random number generator to compute. And since it's an IO operation, we need liftIO. It's bit confusing at first, so don't worry if you don't get all the details. The main point is that we're doing computation with random numbers.

Implementation is not too complex:

rollNewCharacter :: (RandomGen g) => Rand g Player
rollNewCharacter = do
  str <- dice 3
  dex <- dice 3
  mind <- dice 3
  maxHp <- dice 4
  return $ Player
    { playerStrength  = MkStrength str
    , playerDexterity = MkDexterity dex
    , playerMind      = MkMind mind
    , playerHP        = MkHP maxHp
    , playerMaxHP     = MkHP maxHp
    , playerGear      = [ ]

This rolls three six sided dice for each attribute and 4 for hit points. The values are then used to create Player record that is returned.

dice is implemented as following:

dice :: (RandomGen g) => Natural -> Rand g Natural
dice n = do
  rolls <- getRandomRs (1, 6)
  let roll = sum $ take (fromIntegral n) rolls
  return $ fromInteger roll

Again, we're using Rand monad for random number generation. getRandomRs supplies us an infinite list of numbers between 1 and 6. Then we use take to some of them and sum to add them together. fromIntegral n is needed, because take doesn't operate on Natural type, but Int. I wanted to use Natural though, because that ensures that the parameter n will always be 0 or more.

In closing

Now we have a basic layout for our program and know how to roll a character with random stats. Next time we'll finally look into getting some gear on them. The code for the game is available at my codeberg repository.

ad astra!


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