Using ST to modify arrays in Haskell

Array.hs

import Control.Monad (replicateM, forM_)
import Data.Ord (comparing)

import qualified Data.Array as Array
import qualified Data.Array.ST as Array.ST
import Control.Monad.ST
import Data.STRef 

bsort :: (Ord a, Enum a, Bounded a) => Int -> (e -> a) -> [e] -> [e] 
bsort numBuckets toKey items = List.concatMap (List.sortBy (comparing toKey)) $ Array.elems array
  where array = 
    -- convert back into immutable array
    Array.ST.runSTArray $ do
        -- use ST to contain impure computations (modifying a mutable array)
        -- see Data.Array.ST for docs
        buckets <- Array.ST.newArray (0, numBuckets - 1) []

        forM_ items $ \item -> do 
            let index = bucket numBuckets (toKey item)
            -- there is no Array.ST.modifyArray, sadly
            elements <- Array.ST.readArray buckets index
            Array.ST.writeArray buckets index (item : elements)

        return buckets