import Control.Monad
import Data.Array.ST
import Data.Array.Unboxed
import Data.List

sumInPlace1 :: [Int] -> Int
sumInPlace1 xs = (! 0) . runSTUArray $ do
        a <- newArray (0, 0) 0
        forM_ xs $ \x -> do
            x' <- (readArray a 0)
            writeArray a 0 (x' + x)
        return a 

sumInPlace2 :: [Int] -> Int
sumInPlace2 xs = finArr ! 0
    where
        f a x = a // [(0, (a ! 0) + x)]
        arr = array (0,0) [(0,0)] :: UArray Int Int
        finArr = foldl' f arr xs

sumInPlace = sumInPlace1

main = do
    let s = sumInPlace [1..1000000]
    putStrLn $ show s