Ticket #4908: STUArray.hs

import Data.Int
import Data.Array.Base
import Data.Array.ST
import Control.Monad.ST
import System.Environment

update :: Int -> Int -> Int
update orig round = orig + round

incrArray :: STUArray s Int Int -> Int -> ST s ()
incrArray arr round = do k <- getNumElements arr
                         spin 0 (k - 1)
  where spin i n | i > n = return ()
        spin i n = do x <- unsafeRead arr i
                      unsafeWrite arr i $ update x round
                      spin (i + 1) n

main :: IO ()
main = do
  [_nr, _len] <- getArgs
  let nRounds = read _nr :: Int
      len = read _len :: Int
      loop :: STUArray s Int Int -> Int -> ST s ()
      loop arr r | r > nRounds = return ()
      loop arr r = incrArray arr r >> loop arr (r + 1)
  stToIO $ do
    arr <- newArray (1, len) 0 :: ST s (STUArray s Int Int)
    loop arr 1