On 5/1/07, DavidA
1) Using State GameState r and then call execState for each game event (i.e. user input) so I can do IO 2) Using StateT GameState IO () and have the entire game live in one big execStateT call. (I note XMonad does something similar.)
I'm also interested in the answer to this question. One concern I would have about option 2 is that it looks like it "breaks encapsulation", to use a phrase from OOP.
What I mean is, it seems like good design would mean that you could write and test the game logic totally independently of any IO. Game functions such as "makeMove" ought to have type signatures that don't involve any IO. Can this be achieved in option 2?
You could, of course, do both. I do think it's a good idea to write your own "game monad" which encapsulates all of the unsafe functions that you might want to do in an application specific approprate way, rather than just having your "unsafe skin" be the IO monad. So for example, if you want to spawn an actor in the game world and attach an AI thread to govern its behaviour you'd need a function which interanlly uses forkIO, sets up a bunch of TVars for messaging etc., but from the Game monad's pespective that can be encapsulated. There *are* things which are just inherently imperative in nature, and are easier to do like that (threads with message passing, for example, is not necessarily bad, sometimes they are the Right Abstraction -- actor AI is probably a good example), but you can make it a lot nicer and less unsafe by writing your own "unsafe skin" which supplies slightly safer encapsulations tailored for your specific application. But of course, you could have another monad which ONLY deals with pure changes to the game state that you then evaluate from the underlying monad. So the GameEngine monad is the "unsafe skin", and then you have the Game monad which just does a pure computation on the game state. Then you'd have a function like: game :: Game a -> GameEngine a game g = do state <- get gstate <- gameState state let ( res, gstate' ) = runState g put ( state{ gameState = gstate' } ) return res Which simply runs a game action on the game part of the GameEngine state. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862