X.U.Stack: Add zipperFocusedAtFirstOf

XMonad/Util/Stack.hs

@@ -27,6 +27,7 @@ module XMonad.Util.Stack ( -- * Usage
                          , toIndex
                          , fromTags
                          , toTags
+                         , zipperFocusedAtFirstOf
 
                            -- * 'Zipper' manipulation functions
                            -- ** Insertion, movement
@@ -123,6 +124,18 @@ toTags Nothing = []
 toTags (Just s) = map Left (reverse . W.up $ s) ++ [Right . W.focus $ s]
                   ++ map Left (W.down s)
 
+-- | @differentiate zs xs@ takes the first @z@ from @z2 that also belongs to
+-- @xs@ and turns @xs@ into a stack with @z@ being the current element. Acts
+-- as 'XMonad.StackSet.differentiate' if @zs@ and @xs@ don't intersect.
+zipperFocusedAtFirstOf :: Eq q => [q] -> [q] -> Zipper q
+zipperFocusedAtFirstOf []       xs = W.differentiate xs
+zipperFocusedAtFirstOf (z : zs) xs
+  | z `elem` xs = Just $
+        W.Stack { W.focus = z
+                , W.up    = reverse $ takeWhile (/= z) xs
+                , W.down  = drop 1  $ dropWhile (/= z) xs
+                }
+  | otherwise = zipperFocusedAtFirstOf zs xs
 
 -- * Zipper functions