At 7:17 PM +0200 5/2/09, Nicolas Martyanoff wrote:

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Hi,

I don't think I already presented myself; I'm Nicolas, a 23y french student, trying to learn and use haskell.

I've been using C for years, for all sort of tasks, and am quite comfortable with it. I'm also using it 40h a week in my internship for network systems, so I kind of know how to use it.

I discovered Haskell some monthes ago, bought `Real World Haskell', quickly read, and enjoyed it.

So now I'd want to use it for a small project of mine, a simple multiplayer roguelike based on telnet. I wrote a minimal server in C, and it took me a few hours. Now I'm thinking about doing the same in Haskell, and I'm in trouble.

I don't really know how to map my ideas in haskell code. For example, a character can cast spells, so I'd have something like this in C:

struct hashtable spells;

struct character { int n_spells; struct spell **spells; };

I thought I could do something like this in haskell:

spells = Data.Map.Map Int Spell

data Character = Character { charSpells :: [Int] }

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.

I have the same problem for a bout every problem. I love writing pure functions in haskell, but as soon as I try to write some code involving states or side effects, I can't write a line.

I just wanted a 2d array to store a zone, for example, dead simple in C, but this kind of link http://greenokapi.net/blog/2009/03/10/rough-grids-in-haskell make me shiver.

Point is, I'd like to use haskell, but I don't know how, it seems totally alien.

How did you manage to change the way you map ideas to code, from imperative to pure functional ?

Thank you.

Regards,

-- Nicolas Martyanoff http://codemore.org khaelin@gmail.com

Nicolas, First, bienvenue à Haskell ! Learning it will stretch your mind; it may be rocky at times, but it will be rewarding. You've quickly come upon a key difference between imperative and functional programming. State management in Haskell is more explicit, which is a double-edged sword. It requires greater discipline and mechanism, but provides greater control and security. In your example program you could manage your state with a State monad. Assuming you'll want to be able to do I/O, you'll probably want to combine State with IO. For starters, something like:

import Control.Monad.State import qualified Data.Map as M

data Spell = ... data Character = Character { charName :: String, charSpells :: [Spell], ... } data MyState = MyState { characters :: M.Map String Character, ... } initialState = MyState { characters = M.empty, ... }

type MyMonad = StateT MyState IO

addSpellForCharacter :: String -> Spell -> MyMonad () addSpellForCharacter name spell = do state <- get let chars = characters state case M.lookup name chars of Just char -> let char' = char { charSpells = spell : charSpells char } state' = state { characters = M.insert name char' chars } in put state' Nothing -> ... -- leave these issues for another time

main = do ... finalState <- execStateT game initialState ...

game :: MyMonad () game = do ... addSpellForCharacter ... liftIO $ putStrLn "Added spell ..." ...

Dean

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