add 'withNthWorkspace' to DynamicWorkspaceOrder.

Note this is very similar to the function of the same name exported by
DynamicWorkspaces.  Ultimately it would probably be cleaner to
generalize the one in DynamicWorkspaces to accept an arbitrary
workspace sort as a parameter; this is left as an exercise for future
hackers.

XMonad/Actions/DynamicWorkspaceOrder.hs

@@ -28,6 +28,8 @@ module XMonad.Actions.DynamicWorkspaceOrder
     , moveToGreedy
     , shiftTo
 
+    , withNthWorkspace
+
     ) where
 
 import XMonad
@@ -162,4 +164,15 @@ moveToGreedy dir t = doTo dir t getSortByOrder (windows . W.greedyView)
 -- | Shift the currently focused window to the next workspace of the
 -- given type in the given direction, using the dynamic workspace order.
 shiftTo :: Direction1D -> WSType -> X ()
-shiftTo dir t = doTo dir t getSortByOrder (windows . W.shift)
+shiftTo dir t = doTo dir t getSortByOrder (windows . W.shift)
+
+-- | Do something with the nth workspace in the dynamic order.  The
+--   callback is given the workspace's tag as well as the @WindowSet@
+--   of the workspace itself.
+withNthWorkspace :: (String -> WindowSet -> WindowSet) -> Int -> X ()
+withNthWorkspace job wnum = do
+  sort <- getSortByOrder
+  ws <- gets (map W.tag . sort . W.workspaces . windowset)
+  case drop wnum ws of
+    (w:_) -> windows $ job w
+    []    -> return ()