This is a good example of Haskell trying to be smart and causing confusion. Remember that the compiler needs a type for everything and if you don't provide one it will try to guess. The best way to avoid this issue is to start writing a function by first figuring out its type signature and then writing the function. You need some way to track game state. There's two (at least) ways to do that. One way is using a monad which we'll ignore for now. The second way is by defining some sort of state type and explicitly passing it in and out of your functions. This could look like: data State = State { turn :: Int, piles :: [Int] } deriving (Show, Eq) runOn :: (a -> a) -> Int -> [a] -> [a] runOn g i xs = runOn' g (splitAt i xs) where runOn' g (h, t) = h ++ ((g (head t)) : (drop 1 t)) removeStone :: State -> Int -> Int -> State removeStone s i num = State ((turn s)+1) (runOn (\x -> x - num) i (piles s)) Note that there's a potential failure condition here if you provide an index off the end of the pile array (because of the use of head without a guard). removeStone would be used like so: Main> removeStone (State 4 [5,6,7]) 0 2 State { turn = 5, piles = [3,6,7]} Main> removeStone (State 5 [3,6,7]) 1 3 State { turn = 6, piles = [3,3,7]} Using a Monad to do this is very similar, but instead of manual passing the State around, you store the State inside of the enclosing Monad and the various functions fetch and store the state from the Monad. Also bear in mind that once again you're not modifying a List in any of these functions, that's impossible, rather you are making a *new* list using pieces of the old list. On Sunday, October 25, 2009, Peter Verswyvelen wrote:

Btw, it's often a good idea to introduce type signatures:

nim :: IO [Int]

then you don't need to provide type signatures either, and don't have to disable the monomo restriction

On Sun, Oct 25, 2009 at 8:07 PM, Peter Verswyvelen wrote:

...

Hi John,

regarding the first problem, just provide the type of your range explicitly:

nim = do  let range = (1,10) :: (Int,Int)  x <- getStdRandom $ randomR range  y <- getStdRandom $ randomR range  z <- getStdRandom $ randomR range  return [x,y,z]

This is because the annoying monomorphism restriction, which will most likely be reduced in the next version of Haskell.

You can also disable this restriction:

{-# LANGUAGE NoMonomorphismRestriction #-}

import System.Random

nim = do  let range = (1,10) -- no explicit type annotation needed anymore  x <- getStdRandom $ randomR range  y <- getStdRandom $ randomR range  z <- getStdRandom $ randomR range  return [x,y,z]

Cheers, Peter

On Sun, Oct 25, 2009 at 7:57 PM, John Moore wrote:

...

Hi All,         I'm attempting to write a program for the game nim.(The game of Nim is played with two players and several piles of stones. On each move a player removes as many stones as they would like but form only one pile. The player who takes the last stone wins) It not as simple as I first thought. Here is my basic starting points. Any comments would be greatly appreciated. I not well versed in Haskell yet so simple(basic) Haskell rather than well written haskell if you understand what I mean. Complicated monads are way out of my league.

1) I first get the program to give me three random piles by doing     nim = do   x <- getStdRandom $ randomR (1,10)   y <- getStdRandom $ randomR (1,10)   z <- getStdRandom $ randomR (1,10)   return [x,y,z]  Cant get this to work! 2) Now I need to get the program to ask for a number and which pile to remove the number  from. This is tricky I thought about asking to find the elementAt elementAt :: [a] -> Int -> a elementAt list i = list !! (i-1) put this in a variable then asking the palyer how many to take away.  and then subtracting the number from and then putting it back into the list but this seem impossible. Then the second player would do the same. 3) Finally we would end up with a case statement like f x = in case of x [0,0,1]-> You win [0,1,0]-> You win [0,0,1]-> You win [_,_,_]-> keep playing.

Lets know what you think please, getting confused.

John _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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