如何在XMonad中获取Stackset中窗口的索引？

import XMonad
import XMonad.StackSet as W

-- 获取指定窗口的索引
getWindowIndex :: Window -> X (Maybe Int)
getWindowIndex win = withWindowSet $ \ws -> do
  let stack = W.stack $ W.workspace $ W.current ws
  return $ stack >>= W.findTag win >>= W.findIndex

main :: IO ()
main = xmonad def
  { ...
    -- 在某个键绑定中使用getWindowIndex函数
    , keys = \c -> myKeys c `M.union` keys def c
  }

myKeys :: XConfig Layout -> M.Map (KeyMask, KeySym) (X ())
myKeys conf@(XConfig {modMask = modm}) = M.fromList
  [ ...
    -- 使用Mod键加上某个键来获取当前窗口的索引
    , ((modm, xK_i), withFocused $ \win -> do
        index <- getWindowIndex win
        case index of
          Just i -> spawn $ "notify-send 'Window Index: " ++ show i ++ "'"
          Nothing -> return ()
      )
  ]