In-place bubble sort

bubblesort.hs

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

sortPair arr (i, j) = do x <- readArray arr i
                         y <- readArray arr j
                         if (x > y) then do writeArray arr i y
                                            writeArray arr j x
                                    else return ()

-- e.g. (pairs 3) produces (0,1), (1,2), (2,3), (0,1), (1,2), (0,1)
pairs n = concat [zip [0..n] $ tail [0..n] | n <- [n, n-1 .. 1]]

sortArray :: Ord a => (STArray s Int a) -> ST s ()
sortArray arr = do len <- getNumElements arr
                   foldr (\p b -> b >> sortPair arr p) (return ()) (pairs $ len-1)

bubbleSort :: Ord a => [a] -> [a]
bubbleSort xs = runST $ do arr <- newListArray (0, length xs - 1) xs -- convert list to mutable array
                           sortArray arr -- sort array in place
                           getElems arr -- convert sorted array to list