Quicksort in Haskell in imperative style

This is the Haskell implementation of quicksort but in imperative style; yes, it's terrible and very naive implementation :-)

Difference between all C/C++/Java implementations from the Web is that all indexes are checking on out of bounds.

-- exit from array bounds is checking !
import Data.Array
import qualified Data.Array.ST as DAS
import qualified Control.Monad.ST as CMS
import qualified Data.Array.MArray as DAM


findLeft :: (Ix i, Num i, Ord e) => DAS.STArray s i e -> i -> e -> CMS.ST s i
findLeft arr l p = do
  let n = l + 1
  e <- DAM.readArray arr l
  b <- DAM.getBounds arr
  if e < p && (inRange b n) then findLeft arr n p else return l


findRight :: (Ix i, Num i, Ord e) => DAS.STArray s i e -> i -> e -> CMS.ST s i
findRight arr r p = do
  let n = r - 1
  e <- DAM.readArray arr r
  b <- DAM.getBounds arr
  if e > p && (inRange b n) then findRight arr n p else return r


swap :: Ix i => DAS.STArray s i e -> i -> i -> CMS.ST s ()
swap arr i j = do
  vi <- DAM.readArray arr i
  vj <- DAM.readArray arr j
  DAM.writeArray arr i vj
  DAM.writeArray arr j vi


swapAll :: (Ix i, Num i, Ord e) => DAS.STArray s i e -> i -> i -> e -> CMS.ST s (i, i)
swapAll arr l r p = do
  l' <- findLeft arr l p
  r' <- findRight arr r p
  b <- DAM.getBounds arr
  let nl = l' + 1
      nr = r' - 1
  if l' <= r' && (inRange b nl) && (inRange b nr) then do
    swap arr l' r'
    swapAll arr (l' + 1) (r' - 1) p
  else return (l', r')


qsortArray :: (Integral i, Ix i, Ord e) => DAS.STArray s i e -> i -> i -> CMS.ST s ()
qsortArray arr i0 i1 =
  let l = i0
      r = i1
      mid = (i0 + i1) `div` 2 -- pivot index
  in do
    p <- DAM.readArray arr mid -- pivot
    (l', r') <- swapAll arr l r p
    if i0 < r' then do
      qsortArray arr i0 r'
      else return ()
    if l' < i1 then do
      qsortArray arr l' i1
      else return ()


qsort  :: (Num e, Ord e) => [e] -> [e]
qsort xs =
  let len = length xs
      i1 = len - 1
  in elems $ DAS.runSTArray $
    do
      arr <- DAM.newListArray (0, i1) xs
      qsortArray arr 0 i1
      return arr


main :: IO()
main = print $ qsort [0, 0, 9, 10, 4, 3, 98, 100, 99, 2, 6, 1]