change Dmenu functions to return IO/X (Maybe String)

dmenu exits with code 1 when you hit Escape, and I wanna create a contrib that
takes advantage of that.

This required changes in four contribs (Commands, DirectoryPrompt, ShellPrompt,
and WorkspaceDir), and might require changes in users' Configs. Also, I'm not
sure some of the changes I made to the client code are very Haskelly. Would
appreciate input there.

Commands.hs

@@ -102,7 +102,9 @@ runCommand :: [(String, X ())] -> X ()
 runCommand cl = do
   let m = commandMap cl
   choice <- dmenu (M.keys m)
-  fromMaybe (return ()) (M.lookup choice m)
+  case choice of
+    Just selection -> fromMaybe (return ()) (M.lookup selection m)
+    Nothing        -> return ()
 
 runCommand' :: String -> X ()
 runCommand' c = do

DirectoryPrompt.hs

@@ -18,6 +18,8 @@ module XMonadContrib.DirectoryPrompt (
                              directoryPrompt
                               ) where
 
+import Data.Maybe(fromMaybe)
+
 import XMonad
 import XMonadContrib.XPrompt
 import XMonadContrib.Dmenu ( runProcessWithInput )
@@ -34,7 +36,7 @@ directoryPrompt :: XPConfig -> String -> (String -> X ()) -> X ()
 directoryPrompt c prom job = mkXPrompt (Dir prom) c getDirCompl job
 
 getDirCompl :: String -> IO [String]
-getDirCompl s = (filter notboring . lines) `fmap`
+getDirCompl s = (filter notboring . lines . fromMaybe "") `fmap`
                 runProcessWithInput "/bin/bash" [] ("compgen -A directory " ++ s ++ "\n")
 
 notboring :: String -> Bool

Dmenu.hs

@@ -21,6 +21,7 @@ module XMonadContrib.Dmenu (
 
 import XMonad
 import qualified StackSet as W
+import System.Exit
 import System.Process
 import System.IO
 import Control.Monad.State
@@ -32,7 +33,9 @@ import Control.Monad.State
 
 -- %import XMonadContrib.Dmenu
 
-runProcessWithInput :: FilePath -> [String] -> String -> IO String
+-- | Returns Just output if the command succeeded, and Nothing if it didn't.
+-- This corresponds to dmenu's notion of exit code 1 for a cancelled invocation.
+runProcessWithInput :: FilePath -> [String] -> String -> IO (Maybe String)
 runProcessWithInput cmd args input = do
     (pin, pout, perr, ph) <- runInteractiveProcess cmd args Nothing Nothing
     hPutStr pin input
@@ -41,16 +44,17 @@ runProcessWithInput cmd args input = do
     when (output==output) $ return ()
     hClose pout
     hClose perr
-    waitForProcess ph
-    return output
-    
+    exitCode <- waitForProcess ph
+    case exitCode of
+      ExitSuccess   -> return (Just output)
+      ExitFailure _ -> return Nothing
+
 -- | Starts dmenu on the current screen. Requires this patch to dmenu:
 -- <http://www.jcreigh.com/dmenu/dmenu-3.2-xinerama.patch>
-dmenuXinerama :: [String] -> X String
+dmenuXinerama :: [String] -> X (Maybe String)
 dmenuXinerama opts = do
     curscreen <- (fromIntegral . W.screen . W.current) `liftM` gets windowset :: X Int
     io $ runProcessWithInput "dmenu" ["-xs", show (curscreen+1)] (unlines opts)
 
-dmenu :: [String] -> X String
+dmenu :: [String] -> X (Maybe String)
 dmenu opts = io $ runProcessWithInput "dmenu" [] (unlines opts)
-

ShellPrompt.hs

@@ -58,7 +58,7 @@ shellPrompt c = mkXPrompt Shell c getShellCompl spawn
 getShellCompl :: String -> IO [String]
 getShellCompl s 
     | s /= "" && last s /= ' ' = do
-  f <- fmap lines $ runProcessWithInput "/bin/bash" [] ("compgen -A file " ++ s ++ "\n")
+  f <- fmap (lines . fromMaybe "") $ runProcessWithInput "/bin/bash" [] ("compgen -A file " ++ s ++ "\n")
   c <- commandCompletionFunction s
   hPutStrLn stdout s
   return $ map escape . sort . nub $ f ++ c

WorkspaceDir.hs

@@ -68,8 +68,10 @@ workspaceDir :: LayoutClass l a => String -> l a
 workspaceDir s = ModifiedLayout (WorkspaceDir s)
 
 scd :: String -> X ()
-scd x = do x' <- io (runProcessWithInput "bash" [] ("echo -n " ++ x) `catch` \_ -> return x)
-           catchIO $ setCurrentDirectory x'
+scd x = do x' <- io (runProcessWithInput "bash" [] ("echo -n " ++ x) `catch` \_ -> return Nothing)
+           case x' of
+             Just newDir -> catchIO $ setCurrentDirectory newDir
+             Nothing     -> return ()
 
 changeDir :: XPConfig -> X ()
 changeDir c = directoryPrompt c "Set working directory: " (sendMessage . Chdir)