Reduce head usage

XMonad/Actions/OnScreen.hs

@@ -27,7 +27,7 @@ module XMonad.Actions.OnScreen (
     ) where
 
 import XMonad
-import XMonad.Prelude (fromMaybe, guard)
+import XMonad.Prelude (fromMaybe, guard, empty)
 import XMonad.StackSet hiding (new)
 
 
@@ -140,10 +140,9 @@ toggleOrView' f i st = fromMaybe (f i st) $ do
     let st' = hidden st
     -- make sure we actually have to do something
     guard $ i == (tag . workspace $ current st)
-    guard $ not (null st')
-    -- finally, toggle!
-    return $ f (tag . head $ st') st
-
+    case st' of
+      []      -> empty
+      (h : _) -> return $ f (tag h) st  -- finally, toggle!
 
 -- $usage
 --