how to xmonad graceful exit

xmonad.hs

import Control.Concurrent
import Control.Monad.State
import qualified Data.Map as M
import System.Exit
import XMonad.Core
import XMonad.Operations
import qualified XMonad.StackSet as W

windowIDs :: W.StackSet i l a sid sd -> [a]
windowIDs ss
	=  goScreen (W.current ss)
	++ concatMap goScreen (W.visible ss)
	++ concatMap goWorkspace (W.hidden ss)
	++ M.keys (W.floating ss)
	where
	goScreen = goWorkspace . W.workspace
	goWorkspace = foldMap goStack . W.stack
	goStack s = W.up s ++ [W.focus s] ++ W.down s

gracefulExit :: X ()
gracefulExit = do
	ws <- gets windowset
	mapM_ killWindow (windowIDs ws)
	io (threadDelay 500000) -- adjust to taste, this is 0.5s
	io exitSuccess