QuickSort using mutable STArray

qsort.hs

import qualified Data.ByteString.Char8 as BS

import Control.Monad
import Control.Monad.ST
import Data.Array.ST
import Data.Array

qsort :: (STArray s Int Int) -> Int -> Int -> ST s ()
qsort arr min mx =
	if mx - min < 1 then
		return ()

	else do
		p <- readArray arr min
		final_i <- foldM (partitioner p) (min+1) [(min+1)..mx]
		swap min $ final_i - 1
		qsort arr min     (final_i-2)
		qsort arr final_i mx     

	where
		swap i j = do
			arr_i <- readArray arr i
			arr_j <- readArray arr j
			writeArray arr i arr_j
			writeArray arr j arr_i

		partitioner p i idx = do
			arr_idx <- readArray arr idx
			if arr_idx > p then
				return i
			else do
				swap i idx
				return $ i+1

main = do
	lines <- BS.lines `fmap` BS.readFile "10.txt"
	let
		inputData = Prelude.map (maybe (error "can't read Int") fst . BS.readInt) lines
		--inputData = [1,10,4,3,2]
	print $ elems $ runSTArray $ do
		--newListArray :: (MArray a e m, Ix i) => (i, i) -> [e] -> m (a i e)
		state <- newListArray (1, length inputData) inputData
		qsort state 1 (length inputData)
		return state