Scroll is a roguelike, with a twist, which I won't reveal until I've finished building it. I'll just say: A playable roguelike pun, set in a filesystem near you.

I'm creating Scroll as part of the 7DRL Challange. If all goes well, I'll have a usable roguelike game finished in 7 days.

This is my first time developing a roguelike, and my first time writing a game in Haskell, and my first time writing a game to a time limit. Wow!

First, some groundwork. I'm writing Scroll in Haskell, so let's get the core data types and monads and IO squared away. Then I can spend days 2-7 writing entirely pure functional code, in the Haskell happy place.

To represent the current level, I'm using a Vector of Vectors of Chars. Actually, MVectors, which can be mutated safely by pure code running inside the ST monad, so it's fast and easy to read or write any particular location on the level.

-- Writes a Char to a position in the world.
writeWorld :: Pos -> Char -> M ()
writeWorld (x, y) c = modWorld $ \yv -> do
    xv <- V.read yv y
    V.write xv x c

showPlayer :: M ()
showPlayer = writeWorld (5,8) '@'

(I wish these Vectors had their size as part of their types. There are vector libraries on hackage that do, but not the standard vector library, which has mutable vectors. As it is, if I try to access outside the bounds of the world, it'll crash at runtime.)

Since the game will need some other state, I'm using the state monad. The overall monad stack is type M = StateT S (ST RealWorld). (It could be forall s. StateT S (ST s), but I had some trouble getting that to type check, so I fixed s to RealWorld, which is ok since it'll be run using stToIO.

Next, a concept of time, and the main event loop. I decided to use a continutation passing style, so the main loop takes the current continuation, and runs it to get a snapshot of the state to display, and a new continutation. The advantage of using continuations this way is that all the game logic can be handled in the pure code.

I should probably be using the Cont monad in my monad stack, but I've not learned it and lack time. For now I'm handling the continuations by hand, which seems ok.

updateWorld :: Step
updateWorld (Just 'Q') = do
        addMessage "Are you sure you want to quit? [yn]"
        next $ \i -> case i of
                Just 'y' -> quit
                _ -> continue
updateWorld input = do
        addMessage ("pressed " ++ show input)
        continue

Finally, I wrote some ncurses display code, which is almost working.

Start time: After midnight last night. Will end by midnight next Friday.

Lines of code written today: 368

Craziest type signature today: writeS :: forall a. ((Vec2 a -> ST RealWorld ()) -> M ()) -> Pos -> a -> M ()

By the way, there's a whole LambdaHack library for Haskell, targeted at just this kind of roguelike construction. It looks excellent. I'm not using it for two reasons:

1. Scroll is going to be unusual in a lot of ways, and LambdaHack probably makes some assumptions that don't fit.
2. mainSer :: (MonadAtomic m, MonadServerReadRequest m) => [String] -> COps -> (m () -> IO ()) -> (COps -> DebugModeCli -> ((FactionId -> ChanServer ResponseUI RequestUI -> IO ()) -> (FactionId -> ChanServer ResponseAI RequestAI -> IO ()) -> IO ()) -> IO ()) -> IO ()
   That's a lot of stuff to figure out! I only have a week, so it's probably easier to build my own framework, and this gives me an opportunity to learn more generally useful stuff, like how to use mutable Vectors.

Much as I want to get my @ displayed and moving around screen, last night and today have focused on level generation instead.

Scroll has kinda strange level generation method, compared to how I suppose most roguelikes do it. There are only 3 calls to rand in all of Scroll. Just a 3 random parameters, but that's enough to ensure a different level each time.

-- Random level generation function.
level :: Bool -> StdGen -> [String]
level randomize r = concat
        [ final (length tutorial + extra) $ concat $ rand mariner1body
        , concat $ rand mariner1end
        , concatMap rand kubla
        ]
  where
    -- here be spoilers

You could say there are two influences in Scroll's level generation method: Nick Montfort and Samuel Taylor Coleridge.

I have thought some about Scroll before starting the 7drl week, but my idea for the game was missing some key concepts. There was nothing to keep the player engaged in moving forward, an unclear victory condition, no clue how to generate appropriate random levels, a large potential for getting stuck, and no way to lose the game. This is all a problem for a roguelike.

But, I had an idea finally last night, about a single unified thing that all of that stuff falls out from. And it's right there in the name of the game!

Now that I understand Scroll better, I wrote the tutorial level. It's a very meta tutorial level that sets the scene well and serves more purposes than are at first apparent. I count a total of 6 things that this "tutorial level" will let the user do.

And interestingly, while the tutorial level is static, it interacts with the rest of the game in a way that will make it be experienced differently every time through.

The strangest line of code I wrote today is:

import GPL

Somehow, I have never before, in all my time programming, written a line like that one.

Finally, after 7 hours of nonstop coding, I got ncurses to display the generated game world, scrolling around in the display viewport. No @ yet; that will need to wait for tonight or tomorrow!

Got the player moving in the map! And, got the map to be deadly in its own special way.

        HeadCrush -> do
                showMessage "You die."
                endThread

Even winning the game is implemented. The game has a beginning, a middle, and an end.

I left the player movement mostly unconstrained, today, while I was working on things to do with the end of the game, since that makes it easier to play through and test them. Tomorrow, I will turn on fully constrained movement (an easy change), implement inventory (which is very connected to movement constraints in Scroll), and hope to start on the spell system too.

At this point, Scroll is 622 lines of code, including content. Of which, I notice, fully 119 are types and type classes.

Only 4 days left! Eep! I'm very glad that scroll's central antagonist is already written. I don't plan to add other creatures, which will save some time.

Last night as I was drifting off to sleep, it came to me a way to implement my own threading system for my roguelike. Since time in a roguelike happens in discrete ticks, as the player takes each action, normal OS threads are not suitable. And in my case, I'm doing everything in pure code anyway and certianly cannot fork off a thread for some background job.

But, since I'm using continuation passing style, I can just write my own fork, that takes two continuations and combines them, causing both to be run on each tick, and recursing to handle combining the resulting continuations.

It was really quite simple to implement. Typechecked on the first try even!

fork :: M NextStep -> M NextStep -> M NextStep
fork job rest = do
        jn <- job
        rn <- rest
        runthread jn rn
  where
        runthread (NextStep _ (Just contjob)) (NextStep v (Just contr)) =
                return $ NextStep v $ Just $ \i -> do
                        jn <- contjob i
                        rn <- contr i
                        runthread jn rn
        runthread (NextStep _ Nothing) (NextStep v (Just contr)) =
                return $ NextStep v (Just contr)
        runthread _ (NextStep v Nothing) =
                return $ NextStep v Nothing

endThread :: M NextStep
endThread = nextStep Nothing

background :: M NextStep -> M NextStep
background job = fork job continue

demo :: M NextStep
demo = do
    showMessage "foo"
    background $ next $ const $
        clearMessage >> endThread

That has some warts, but it's good enough for my purposes, and pretty awesome for a threading system in 66 LOC.

Slow start today; I was pretty exhausted after yesterday and last night's work. Somehow though, I got past the burn and made major progress today.

All the complex movement of both the player and the scroll is finished now, and all that remains is to write interesting spells, and a system for learning spells, and to balance out the game difficulty.

I haven't quite said what Scroll is about yet, let's fix that:

In Scroll, you're a bookworm that's stuck on a scroll. You have to dodge between words and use spells to make your way down the page as the scroll is read. Go too slow and you'll get wound up in the scroll and crushed.

The character is multiple chars in size (size is the worm's only stat), and the worm interacts with the scroll in lots of ways, like swallowing letters, or diving through a hole to the other side of the scroll. While it can swallow some letters, if it gets too full, it can't move forward anymore, so letters are mostly consumed to be used as spell components.

I think that I will manage to get away without adding any kind of monsters to the game; the scroll (and whoever is reading it) is the antagonist.

As I'm writing this very post, I'm imagining the worm wending its way through my paragraphs. This dual experience of text, where you're both reading its content and hyper-aware of its form, is probably the main thing I wanted to explore in writing Scroll.

As to the text that fills the scroll, it's broadly procedurally generated, in what I hope are unusual and repeatedly surprising (and amusing) ways. I'm not showing any screenshots of the real text, because I don't want to give that surprise away.

But, the other thing about Scroll is that it's scroll, a completely usable (if rather difficult..) Unix pager!

I want my 7drl game Scroll to have lots of interesting spells. So, as I'm designing its spell system, I've been looking at the types, and considering the whole universe of possible spells that fit within the constraints of the types.

My first throught was that a spell would be a function from World -> World. That allows any kind of spell that manipulates the game map. Like, for instance a "whiteout" that projects a stream of whitespace from the player's mouth.

Since Scroll has a state monad, I quickly generalized that; making spell actions a state monad M (), which lets spells reuse other monadic actions, and affect the whole game state, including the player. Now I could write a spell like "teleport", or "grow".

But it quickly became apparent this was too limiting: While spells could change the World map, the player, and even change the list of supported spells, they had no way to prompting for input.

I tried a few types of the Event -> M () variety, but they were all too limiting. Finally, I settled on this type for spell actions: M NextStep -> M NextStep.

And then I spent 3 hours exploring the universe of spells that type allows! To understand them, it helps to see what a NextStep is:

type Step = Event -> M NextStep
data NextStep = NextStep View (Maybe Step)

Since NextStep is a continuation, spells take the original continuation, and can not only modify the game state, but can return an altered continuation. Such as one that prompts for input before performing the spell, and then calls the original continuation to get on with the game.

That let me write "new", a most interesting spell, that lets the player add a new way to cast an existing spell. Spells are cast using ingredients, and so this prompts for a new ingredient to cast a spell. (I hope that "new farming" will be one mode of play to possibly win Scroll.)

And, it lets me write spells that fail in game-ending ways. (Ie, "genocide @"). A spell can cause the game to end by returning a continuation that has Nothing as its next step.

Even better, I could write spells that return a continuation that contains a forked background task, using the 66 line contiuation based threading system I built in day 3. This allows writing lots of fun spells that have an effect that lasts for a while. Things like letting the player quickly digest letters they eat, or slow down the speed of events.

And then I thought of "dream". This spell stores the input continuation and game state, and returns a modified continuation that lets the game continue until it ends, and then restores from the point it saved. So, the player dreams they're playing, and wakes back up where they cast the spell. A wild spell, which can have different variants, like precognitive dreams where the same random numbers are used as will be used upon awaking, or dreams where knowledge carries back over to the real world in different ways. (Supports Inception too..)

Look how easy it was to implement dreaming, in this game that didn't have any notion of "save" or "restore"!

runDream :: M NextStep -> M NextStep -> (S -> S) -> M NextStep
runDream sleepcont wakecont wakeupstate = go =<< sleepcont
   where
         go (NextStep v ms) = return $ NextStep v $ Just $
                 maybe wake (go <=<) ms
         wake _evt = do
                 modify wakeupstate
                 wakecont

I imagine that, if I were not using Haskell, I'd have just made the spell be an action, that can do IO in arbitrary ways. Such a spell system can of course do everything I described above and more. But, I think that using a general IO action is so broad that it hides the interesting possibilities like "dream".

By starting with a limited type for spells, and exploring toward more featureful types, I was able to think about the range of possibilities of spells that each type allowed, be inspired with interesting ideas, and implement them quickly.

Just what I need when writing a roguelike in just 7 days!

Fun comment thread on reddit

Last night I put up a telnet server and web interface to play a demo of scroll and send me playtester feedback, and I've gotten that almost solid today. Try it!

Today was a scramble to add more features to Scroll and fix bugs. The game still needs some balancing, and generally seems a little too hard, so added a couple more spells, and a powerup feature to make it easier.

Added a way to learn new spells. Added a display of spell inventory on 'i'. For that, I had to write a quick windowing system (20 lines of code).

Added a system for ill effects from eating particular letters. Interestingly, since such a letter is immediately digested, it doesn't prevent the worm from moving forwards. So, the ill effects can be worth it in some situations. Up to the player to decide.

I'm spending a lot of time now looking at letter frequency historgrams to decide which letter to use for a new feature. Since I've several times accidentially used the same letter for two different things (most amusingly, I assigned 'k' to a spell, forgetting it was movement), I refactored all the code to have a single charSet which defines every letter and what it's used for, be that movement, control, spell casting, or ill effects. I'd like to use that to further randomize which letters are used for spell components, out of a set that have around the same frequency. However, I doubt that I'll have time to do that.

In the final push tonight/tomorrow, I hope to add an additional kind of level or two, make the curses viewport scroll when necessary instead of crashing, and hopefully work on game balance/playtester feedback.

I've written ~2800 lines of code so far this week!

A frantic last day of work on Scroll.

Until 3 am last night, I was working on adding a new procedurally generated level.

This morning, fixed two major bugs reported by playesters overnight. Also fixed crashes on small screens and got the viewport to scroll. Added a victory animation in time for lunch.

After lunch, more level generation work. Wasted an entire hour tracking down a bug in level gen I introduced last night, when I was bad and didn't create a data type to express an idea. Added a third type of generated level, with its own feel.

Finished up with a level selection screen, which needed just 47 lines of code and features a playable character.

I have six hours until my 7drl is officially over, but I'm done! Success! You can download the code, play, etc, at Scroll's homepage