{-# LANGUAGE FlexibleContexts #-}
import Data.Array.IO
import System.Random

randomArray :: Int -> Int -> Int -> Int -> IO (IOArray Int Int)
randomArray i1 i2 mn mx = do
    arr <- newArray (i1,i2) mn  -- 仮に mn で初期化する
    loop i1 i2 mn mx arr
    return arr
  where
    loop i i2 mn mx a
       | i > i2    = return ()
       | otherwise = do r <- randomRIO (mn, mx)
                        writeArray a i r
                        loop (i + 1) i2 mn mx a

printArray :: Show a => IOArray Int a -> IO ()
printArray arr = do
    (i1,i2) <- getBounds arr
    loop i1 i2 arr
  where
    loop i1 i2 arr | i1 > i2   = return ()
                   | otherwise = do
        a <- readArray arr i1
        putStr (show a)
        putStr ", "
        loop (i1 + 1) i2 arr

accumArray arr = do
    (i1, i2) <- getBounds arr
    loop i1 i2 0 arr 
  where   
    loop i1 i2 v arr | i1 > i2 = return ()
                     | otherwise = do
       a <- readArray arr i1
       let v' = a + v
       writeArray arr i1 v'
       loop (i1 + 1) i2 v' arr
       
main = do arr <- randomArray 1 5 0 100
          printArray arr
          putStrLn ""
          accumArray arr
          printArray arr
          putStrLn ""