Selection algorithm in Haskell

SelectBy.hs

module SelectBy (selectBy, medianBy, select, median)
where

import Data.List

splitEvery _ [] = []
splitEvery n xs = xn : splitEvery n r
  where (xn, r) = splitAt n xs

-- | O(n) kth smallest selection
selectBy :: (a -> a -> Ordering) -> Int -> [a] -> a
selectBy c k xs
    | k >= length xs = maximumBy c xs
    | k == 0 = minimumBy c xs
    | null . drop 10 $ xs = sortBy c xs !! k
    | k <= ll = selectBy c k l
    | k > le = selectBy c (k - le) g
    | otherwise = m 
  where ms = map (selectBy c 3) . splitEvery 5 $ xs
        m = medianBy c ms
        l = filter ((GT ==) . c m) xs
        ll = length l
        e = filter ((EQ ==) . c m) xs
        le = length e + ll
        g = filter ((LT ==) . c m) xs

medianBy :: (a -> a -> Ordering) -> [a] -> a
medianBy c xs = selectBy c (length xs `div` 2) xs

select :: Ord a => Int -> [a] -> a
select = selectBy compare

median :: Ord a => [a] -> a
median = medianBy compare