Sleepsort implementation in Haskell

SleepSort.hs

module Main where

import Data.Foldable

import Control.Concurrent
import Control.Monad

import System.IO.Unsafe

sleepSort :: [Int] -> [Int]
sleepSort xs = unsafePerformIO $ do
  res <- newMVar []
  ws  <- traverse (sleepSort' res) xs
  traverse_ takeMVar ws
  takeMVar res
  where
    sleepSort' res x = do
      w <- newEmptyMVar
      void $ forkIO $ do
        threadDelay (x * 1000000)
        print x -- just so you can see the magic happen
        modifyMVar_ res (return . (++ [x]))
        putMVar w ()
      return w

main :: IO ()
main = print (sleepSort [7,8,4,5,6,9,1,2,3,0])