ShellPrompt: check for executables and better error handling

Code contributed by Spencer (basically I just removed FilePath
depenency).

ShellPrompt.hs

@@ -16,7 +16,6 @@ module XMonadContrib.ShellPrompt (
                              -- * Usage
                              -- $usage
                              shellPrompt
-                             , rmPath
                              , split
                               ) where
 
@@ -62,26 +61,29 @@ getShellCompl s
   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
+  return . map escape . sort . nub $ f ++ c
     | otherwise = return []
 
 commandCompletionFunction :: String -> IO [String]
 commandCompletionFunction str
     | '/' `elem` str = return []
     | otherwise      = do
-  p <- getEnv "PATH"
+  p  <- getEnv "PATH" `catch` const (return [])
-  cl p
+  let ds = split ':' p
-    where
+      fp d f = d ++ "/" ++ f  
-      cl     = liftM (nub . rmPath . concat) . mapM cmpl . split ':'  
+  es <- forM ds $ \d -> do
-      cmpl s = filter (isPrefixOf str) `fmap` getFileNames s
+          exists <- doesDirectoryExist d
+          if exists
+             then getDirectoryContents d >>= filterM (isExecutable . fp d)
+             else return []
+  return . filter (isPrefixOf str) . concat $ es
 
-getFileNames :: FilePath -> IO [FilePath]
+isExecutable :: FilePath ->IO Bool
-getFileNames fp = 
+isExecutable f = do
-    getDirectoryContents fp `catch` \_ -> return []
+    fe <- doesFileExist f
-
+    if fe
-rmPath :: [String] -> [String]
+        then fmap executable $ getPermissions f
-rmPath s = 
+        else return False
-    map (reverse . fst . break  (=='/') . reverse) s
 
 split :: Eq a => a -> [a] -> [[a]]
 split _ [] = []