[xmonad] Beginner problem with XMonad and appending to a list rather than prefixing

7 May 2015

      XMonad Community,

After all this time, I'm still just a beginner with XMonad.  I only know Haskell
through XMonad, and I wouldn't say I know it well.  I learn just enough to get
my XMonad configuration to work, then don't return to it for a while.  

I'm trying to make a minor change to my XMonad file, where a new
workspace is appended to the list rather than prefixed like I have now,
but I can't seem to figure out the code to do it.  Here's the relevant
details of what I have:

import XMonad.Actions.DynamicWorkspaces as DW                                                                                                                                                                                                
import qualified XMonad.Util.ExtensibleState as XS                                                                                                                                                                                           
import Control.Applicative                                                                                                                                                                                                                   
filterWorkspaces :: [WorkspaceId] -> [WindowSpace] -> [WindowSpace]                                                                                                                                                                          
filterWorkspaces ws = filter (\(W.Workspace tag _ _) -> tag `elem` ws)                                                                                                                                                                       
  -- xScreen are the type classes which hold the workspace name lists                                                                                                                                                                        
newtype LeftScreen = LeftScreen {getLeftScreen :: [WorkspaceId]} deriving (Typeable,Read,Show)                                                                                                                                               
instance ExtensionClass LeftScreen where                                                                                                                                                                                                     
  initialValue = LeftScreen []                                                                                                                                                                                                               
  extensionType = PersistentExtension                                                                                                                                                                                                        

newtype RightScreen = RightScreen {getRightScreen :: [WorkspaceId]} deriving (Typeable,Read,Show)                                                                                                                                            
instance ExtensionClass RightScreen where                                                                                                                                                                                                    
  initialValue = RightScreen []                                                                                                                                                                                                              
  extensionType = PersistentExtension             

-- snip --

main = do                                                                                                                                                                                                                                    
    dbus <- D.connectSession                                                                                                                                                                                                                 
    getWellKnownName dbus;                                                                                                                                                                                                                   
    xmonad $ xfceConfig {                                                                                                                                                                                                                    
    workspaces = ["sh","sb","of","wc","ws","wb","cht"]                                                                                                                                                                                       
  , terminal = "urxvtc"                                                                                                                                                                                                                      
  -- Goodbye, my sweet, sweet, sloppy focus                                                                                                                                                                                                  
  , focusFollowsMouse = False                                                                                                                                                                                                                
  , manageHook = manageDocks <+> myManageHook -- <+> manageHook defaultConfig                                                                                                                                                                
  , layoutHook = avoidStruts $ onWorkspace "gimp" gimp $ layoutH                                                                                                                                                                             
  , logHook = dynamicLogWithPP (ppL dbus)                                                                                                                                                                                                    
        >> dynamicLogWithPP (ppR dbus)                                                                                                                                                                                                       
        >> fadeHook                                                                                                                                                                                                                          
  , borderWidth = 1                                                                                                                                                                                                                          
  , normalBorderColor = "#333333"                                                                                                                                                                                                            
  , focusedBorderColor = "#CCCC00"                                                                                                                                                                                                           
    , modMask = winKey                                                                                                                                                                                                                       
    , startupHook = setWMName "LG3D"                                                                                                                                                                                                         
    } `additionalKeys` myKeys                       

myKeys =  [
-- snip 
-- Here's what I want to change
  , ((winKey , xK_m) ,DW.withWorkspace myXPConfigNew $ \wk -> do                                                                                                                                 
      sc <- screenBy 0                                                                                                                                                                                                                       
      if sc == 0                                                                                                                                                                                                                             
      then XS.modifyf $ LeftScreen . (wk :) . getLeftScreen -- prefix left list with new workspace
      --then XS.modify $  LeftScreen . getLeftScreen ++[wk] -- My first attempt at appending the workspace
      else XS.modify $ RightScreen . (wk :) . getRightScreen -- prefix right list with new workspace
      --else XS.modify $ RightScreen . (wk ++) . getRightScreen  --  another attempt
      windows $ W.view wk)

Normally I'd ask on #xmonad@irc.freenode.net, but I haven't had time to get on
there.  I'm sure this demonstrates my lack of knowledge of XMonad and Haskell in
general, hopefully you can help.

Thanks in advance,

Trey Blancher
trey@blancher.net