Make 'index' return Nothing, rather than error

Operations.hs

@@ -48,7 +48,7 @@ refresh = do
     flip mapM_ (M.assocs (W.screen2ws ws)) $ \(scn, n) -> do
         let sc        = genericIndex xinesc scn -- temporary coercion!
             (Just l)  = fmap fst $ M.lookup n fls
-        mapM_ (\(w, rect) -> io $ moveWindowInside d w rect) $ (doLayout l) sc $ W.index n ws
+        whenJust (W.index n ws) $ mapM_ (\(w, rect) -> io $ moveWindowInside d w rect) . doLayout l sc
         whenJust (W.peekStack n ws) (io . raiseWindow d)
     whenJust (W.peek ws) setFocus
     clearEnterEvents
@@ -235,7 +235,7 @@ setFocus w = do
 
     -- clear mouse button grab and border on other windows
     flip mapM_ (W.visibleWorkspaces ws) $ \n -> do
-        flip mapM_ (W.index n ws) $ \otherw -> do
+        flip mapM_ (fromMaybe [] $ W.index n ws) $ \otherw -> do
             setButtonGrab True otherw
             io $ setWindowBorder dpy otherw (color_pixel nbc)
 
@@ -297,7 +297,7 @@ view n = do
     ws' <- gets workspace
     -- If the old workspace isn't visible anymore, we have to hide the windows
     -- in case we're switching to an empty workspace.
-    when (m `notElem` W.visibleWorkspaces ws') (mapM_ hide (W.index m ws))
+    when (m `notElem` W.visibleWorkspaces ws') $ maybe (return ()) (mapM_ hide) $ W.index m ws
     clearEnterEvents
     setTopFocus
 

StackSet.hs

@@ -98,9 +98,9 @@ peekStack :: Integral i => i -> StackSet i j a -> Maybe a
 peekStack i w = M.lookup i (focus w)
 
 -- | /O(log s)/. Index. Extract the stack at workspace 'n'.
--- If the index is invalid, an exception is thrown.
-index :: Integral i => i -> StackSet i j a -> [a]
-index k w = uncurry (++) $ fromJust $ M.lookup k (stacks w)
+-- If the index is invalid, returns Nothing.
+index :: Integral i => i -> StackSet i j a -> Maybe [a]
+index k w = fmap (uncurry (++)) $ M.lookup k (stacks w)
 
 -- | view. Set the stack specified by the argument as being visible and the
 -- current StackSet. If the stack wasn't previously visible, it will become
@@ -173,7 +173,7 @@ delete k w = maybe w del (M.lookup k (cache w))
   where
     del i = w { cache  = M.delete k (cache w)
               , stacks = M.adjust (\(xs, ys) -> (L.delete k xs, L.delete k ys)) i (stacks w)
-              , focus  = M.update (\k' -> if k == k' then elemAfter k (index i w)
+              , focus  = M.update (\k' -> if k == k' then elemAfter k =<< index i w
                                                      else Just k') i (focus w) }
 
 -- | /O(log n)/. If the given window is contained in a workspace, make it the