Hello Nicolas, Saturday, May 2, 2009, 9:17:55 PM, you wrote:
But now I don't know how to dynamically add new spells (new spells can be created in my gameplay). Since I can't assign a new value to the `spells' variable (Data.Map.insert returns a new map), I just don't know where to go.
well, i've written mid-size program that combines pure haskell, imperative haskell and c++ and that's my vision: in C, you don't have the barrier between pure and imperative worlds and as result you don't got habit to distinguish them. haskell forces you to separate pure and imperative code: pure code is much simpler to write but it cannot call imperative one (i.e. function can't call procedures) so you should understand the difference and carefully make your choice you are *forced* to use imperative code when it responds to some external events - user input, network input, so on. so when your spell list mutates as a result of external events, you should do something imperatively. although it may be as easy as tail-recursively calling itself with purely modified value: mainCycle spells = do x <- userInput case x of AddSpell -> mainCycle (newSpell:spells) .... further improving this idea, you may develop data structire contating whole state info: data GameState = GameState {spells: [Spell], .....} and update appropriate parts of GameState on tail-recursive calls: mainCycle gameState = do x <- userInput case x of AddSpell -> mainCycle (gameState {spells = newSpell:spells gameState}) .... as you see, this approach allows you to live in imperative world but use only pure computations to update state but once your program will become larger, you may find that this approach needs too much typing. in that case, you can program using IORefs, MVars and other types of C-like global vars. or, you may use them semi-globally - i.e. create this vars in outside procedure and then pass to all interested routines: main = do vspells <- newIORef [] let stateVars = StateVars {spells=vspells, ...} mainCycle stateVars you may even use implicit parameters to hide passing details from program code but i don't have experience of using them -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com