How to use "when"

when.hs

-- from:  https://mail.haskell.org/pipermail/xmonad/2009-May/007979.html

  logHook = ...other stuff... >> checkKBLayout

  ...

  checkKBLayout :: X ()
  checkKBLayout = do
    curTag <- gets (W.currentTag . windowset)
    when (curTag == "foo") $ spawn "set keyboard layout"
 
-- 
--result:
--
xmonad.hs:68:5: error:
    * Variable not in scope: when :: Bool -> m0 () -> X ()
    * Perhaps you meant `whenX' (imported from XMonad)
   |
68 |     when ( curTag == "3:browser") $ spawn "xinput --disable 11"