Data.Vector.Algorithms.Intro には sortBy がある

競プロ典型 90 問 14 日目 - We Used to Sing a Song Together（★3)

import Control.Monad
import qualified Data.ByteString.Char8 as C
import Data.List
import qualified Data.Vector.Algorithms.Intro as A
import qualified Data.Vector.Unboxed as U

main = sol <$> readLn <*> get <*> get >>= print

get = unfoldr (C.readInt . C.dropWhile (==' ')) <$> C.getLine

sol n as bs = U.sum $ U.zipWith ((abs .) . (-)) (f as) (f bs)
  where
  f = U.modify (A.sortBy compare) . U.fromListN n