将函数映射到haskell中的列表/数组？

Question

I am trying to initiate a random array to begin the game. To do that I create a function randomBoard which returns '*' or ' ' to represent a space on the game board.

I want to be able to create the game board array with this function. I havent been able to succesfully instantiate an array yet. I am hoping there is a way i can declare an array of say size 100 and use my random function to set each element.

This obviously doesnt work or even compile. I am sure there are a couple things wrong with it, as I am not sure really how to even work with IO in haskell and produce this outcome. Any guidance is much appreciated...

Answer 1

This should work for you:

import Control.Monad
import System.Random
import Data.Array
import Data.List

randomBoard :: IO Char
randomBoard =
   do
   f1 <- randomIO :: IO Int
   if(f1 `mod` 2) == 0
     then return  '*'
     else return  ' '

boardArray :: IO (Array Int Char)
boardArray = listArray (0, 99) <$> replicateM 100 randomBoard

Here's what I changed:

1. I added the type signature randomBoard :: IO Char for clarity. (The code would still work without it, as Haskell correctly infers this type if you don't supply it.)
2. I changed the type of boardArray to use IO . Anything that uses IO , no matter how indirectly, need to be in IO itself.
3. I changed listArray (0, 100) to listArray (0, 99) , as the former would actually be 101 elements.
4. map randomBoard $ 100 (0,100) isn't right at all. To get a list of several of the same thing, you'd usually use replicate , but since the thing you care about here is in the IO monad, you use replicateM instead. replicateM 100 randomBoard gives an IO [Char] with 100 random elements of either '*' or ' ' .
5. I added an import of Control.Monad , which is needed to use replicateM .
6. I use <$> in boardArray . Since you want to call listArray with a [Char] and get an Array Int Char , but replicateM 100 randomBoard is an IO [Char] , you can't just apply the argument directly. Using <$> (which is also called fmap ) applies it "inside" of IO , giving you back an IO (Array Int Char) .