Here's a version, just to prove you can convert code from Wikipedia almost exactly into Haskell ;)
import Control.Monad.ST
import Data.Array.ST
import Data.Foldable
import Control.Monad
-- wiki-copied code starts here
partition arr left right pivotIndex = do
pivotValue <- readArray arr pivotIndex
swap arr pivotIndex right
storeIndex <- foreachWith [left..right-1] left (\i storeIndex -> do
val <- readArray arr i
if (val <= pivotValue)
then do
swap arr i storeIndex
return (storeIndex + 1)
else do
return storeIndex )
swap arr storeIndex right
return storeIndex
qsort arr left right = when (right > left) $ do
let pivotIndex = left + ((right-left) `div` 2)
newPivot <- partition arr left right pivotIndex
qsort arr left (newPivot - 1)
qsort arr (newPivot + 1) right
-- wrapper to sort a list as an array
sortList xs = runST $ do
let lastIndex = length xs - 1
arr <- newListArray (0,lastIndex) xs :: ST s (STUArray s Int Int)
qsort arr 0 lastIndex
newXs <- getElems arr
return newXs
-- test example
main = print $ sortList [212498,127,5981,2749812,74879,126,4,51,2412]
-- helpers
swap arr left right = do
leftVal <- readArray arr left
rightVal <- readArray arr right
writeArray arr left rightVal
writeArray arr right leftVal
-- foreachWith takes a list, and a value that can be modified by the function, and
-- it returns the modified value after mapping the function over the list.
foreachWith xs v f = foldlM (flip f) v xs
