Add a startupHook.

The only thing I am not sure about here is at what exact point the 
startupHook should get run.  I picked a place that seems to make sense: 
as late as possible, right before entering the main loop.  That way all
the layouts/workspaces/other state are set up and the startupHook can
manipulate them.

XMonad/Config.hs

@@ -25,11 +25,11 @@ module XMonad.Config (defaultConfig) where
 -- Useful imports
 --
 import XMonad.Core as XMonad hiding
-    (workspaces,manageHook,numlockMask,keys,logHook,borderWidth,mouseBindings
+    (workspaces,manageHook,numlockMask,keys,logHook,startupHook,borderWidth,mouseBindings
     ,defaultGaps,layoutHook,modMask,terminal,normalBorderColor,focusedBorderColor
     ,focusFollowsMouse)
 import qualified XMonad.Core as XMonad
-    (workspaces,manageHook,numlockMask,keys,logHook,borderWidth,mouseBindings
+    (workspaces,manageHook,numlockMask,keys,logHook,startupHook,borderWidth,mouseBindings
     ,defaultGaps,layoutHook,modMask,terminal,normalBorderColor,focusedBorderColor
     ,focusFollowsMouse)
 
@@ -134,6 +134,10 @@ manageHook = composeAll
 logHook :: X ()
 logHook = return ()
 
+-- | Perform an arbitrary action at xmonad startup.
+startupHook :: X ()
+startupHook = return ()
+
 ------------------------------------------------------------------------
 -- Extensible layouts
 --
@@ -256,6 +260,7 @@ defaultConfig = XConfig
     , XMonad.modMask            = defaultModMask
     , XMonad.keys               = keys
     , XMonad.logHook            = logHook
+    , XMonad.startupHook        = startupHook
     , XMonad.mouseBindings      = mouseBindings
     , XMonad.manageHook         = manageHook
     , XMonad.focusFollowsMouse  = focusFollowsMouse }

XMonad/Core.hs

@@ -88,6 +88,7 @@ data XConfig l = XConfig
                                                  -- ^ The mouse bindings
     , borderWidth        :: !Dimension           -- ^ The border width
     , logHook            :: !(X ())              -- ^ The action to perform when the windows set is changed
+    , startupHook        :: !(X ())              -- ^ The action to perform on startup
     , focusFollowsMouse  :: !Bool                -- ^ Whether window entry events can change focus
     }
 

XMonad/Main.hs

@@ -116,6 +116,8 @@ xmonad initxmc = do
             -- manage the as-yet-unmanaged windows
             mapM_ manage (ws \\ W.allWindows winset)
 
+            startupHook initxmc
+
             -- main loop, for all you HOF/recursion fans out there.
             forever_ $ handle =<< io (nextEvent dpy e >> getEvent e)
 

man/xmonad.hs

@@ -247,6 +247,16 @@ myFocusFollowsMouse = True
 --
 myLogHook = return ()
 
+------------------------------------------------------------------------
+-- Startup hook
+
+-- Perform an arbitrary action each time xmonad starts or is restarted
+-- with mod-q.  Used by, e.g., XMonad.Layout.PerWorkspace to initialize
+-- per-workspace layout choices.
+--
+-- By default, do nothing.
+myStartupHook = return ()
+
 ------------------------------------------------------------------------
 -- Now run xmonad with all the defaults we set up.
 
@@ -279,5 +289,6 @@ defaults = defaultConfig {
       -- hooks, layouts
         layoutHook         = myLayout,
         manageHook         = myManageHook,
-        logHook            = myLogHook
+        logHook            = myLogHook,
+        startupHook        = myStartupHook
     }