X.A.DynamicWorkspaceOrder: add transformation-aware withNthWorkspace

The user may modify the list of workspace tags that results form
applying the dynamic order. This way one may filter workspaces they
don't want in the order (e.g. "NSP") or apply any transformation he
wishes to the list of tags.

CHANGES.md

@@ -49,6 +49,11 @@
   * `XMonad.Actions.MessageHandling`
     Refresh-performing functions updated to better reflect the new `sendMessage`.
 
+  * `XMonad.Actions.DynamicProjects`
+    Add a version of `withNthWorkspace` that takes a `[WorkspaceId] ->
+	[WorkspaceId]` transformation to apply over the list of workspace tags
+	resulting from the dynamic order.
+
 ## 0.14
 
 ### Breaking Changes

XMonad/Actions/DynamicWorkspaceOrder.hs

@@ -30,6 +30,7 @@ module XMonad.Actions.DynamicWorkspaceOrder
     , moveToGreedy
     , shiftTo
 
+    , withNthWorkspace'
     , withNthWorkspace
 
     ) where
@@ -183,13 +184,19 @@ moveToGreedy dir t = doTo dir t getSortByOrder (windows . W.greedyView)
 shiftTo :: Direction1D -> WSType -> X ()
 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
+-- | Do something with the nth workspace in the dynamic order after
+--   transforming it.  The callback is given the workspace's tag as well
+--   as the 'WindowSet' of the workspace itself.
+withNthWorkspace' :: ([WorkspaceId] -> [WorkspaceId]) -> (String -> WindowSet -> WindowSet) -> Int -> X ()
+withNthWorkspace' tr job wnum = do
   sort <- getSortByOrder
-  ws <- gets (map W.tag . sort . W.workspaces . windowset)
+  ws <- gets (tr . map W.tag . sort . W.workspaces . windowset)
   case drop wnum ws of
     (w:_) -> windows $ job w
     []    -> return ()
+
+-- | 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 = withNthWorkspace' id