Replace 'findIndex' with 'findTag', which more accurately describes what the function does.

I realize this is a big change, but the name 'findIndex' was confusing for me, since I expected it to return some sort of integer.  What it actually does, of course, is return a workspace tag, which might be more general than an index.
Of course, this change breaks several contrib modules; I'll submit a patch to make the change there as well.

Operations.hs

@@ -564,7 +564,7 @@ float :: Window -> X ()
 float w = do
     (sc, rr) <- floatLocation w
     windows $ \ws -> W.float w rr . fromMaybe ws $ do
-        i <- W.findIndex w ws
+        i <- W.findTag w ws
         guard $ i `elem` map (W.tag . W.workspace) (W.screens ws)
         f <- W.peek ws
         sw <- W.lookupWorkspace sc ws

StackSet.hs

@@ -26,7 +26,7 @@ module StackSet (
         -- $stackOperations
         peek, index, integrate, integrate', differentiate,
         focusUp, focusDown, focusMaster, focusWindow,
-        tagMember, renameTag, ensureTags, member, findIndex, mapWorkspace, mapLayout,
+        tagMember, renameTag, ensureTags, member, findTag, mapWorkspace, mapLayout,
         -- * Modifying the stackset
         -- $modifyStackset
         insertUp, delete, delete', filter,
@@ -389,7 +389,7 @@ reverseStack (Stack t ls rs) = Stack t rs ls
 focusWindow :: (Eq s, Eq a, Eq i) => a -> StackSet i l a s sd -> StackSet i l a s sd
 focusWindow w s | Just w == peek s = s
                 | otherwise        = maybe s id $ do
-                    n <- findIndex w s
+                    n <- findTag w s
                     return $ until ((Just w ==) . peek) focusUp (view n s)
 
 -- | Get a list of all screens in the StackSet.
@@ -439,13 +439,13 @@ mapLayout f (StackSet v vs hs m) = StackSet (fScreen v) (map fScreen vs) (map fW
 
 -- | /O(n)/. Is a window in the StackSet.
 member :: Eq a => a -> StackSet i l a s sd -> Bool
-member a s = maybe False (const True) (findIndex a s)
+member a s = maybe False (const True) (findTag a s)
 
 -- | /O(1) on current window, O(n) in general/.
--- Return Just the workspace index of the given window, or Nothing
+-- Return Just the workspace tag of the given window, or Nothing
 -- if the window is not in the StackSet.
-findIndex :: Eq a => a -> StackSet i l a s sd -> Maybe i
-findIndex a s = listToMaybe
+findTag :: Eq a => a -> StackSet i l a s sd -> Maybe i
+findTag a s = listToMaybe
     [ tag w | w <- workspaces s, has a (stack w) ]
     where has _ Nothing         = False
           has x (Just (Stack t l r)) = x `elem` (t : l ++ r)
@@ -559,7 +559,7 @@ shiftWin :: (Ord a, Eq a, Eq s, Eq i) => i -> a -> StackSet i l a s sd -> StackS
 shiftWin n w s | from == Nothing                     = s -- not found
                | n `tagMember` s && (Just n) /= from = go
                | otherwise                           = s
-    where from   = findIndex w s
+    where from   = findTag w s
 
           go     = on n (insertUp w) . on (fromJust from) (delete' w) $ s
           curtag = tag (workspace (current s))

tests/Properties.hs

@@ -351,14 +351,14 @@ prop_focusWindow_local (n :: NonNegative Int) (x::T ) =
                    in hidden_spaces (focusWindow (s !! i) x) == hidden_spaces x
 
 -- ---------------------------------------------------------------------
--- member/findIndex
+-- member/findTag
 
 --
--- For all windows in the stackSet, findIndex should identify the
+-- For all windows in the stackSet, findTag should identify the
 -- correct workspace
 --
 prop_findIndex (x :: T) =
-    and [ tag w == fromJust (findIndex i x)
+    and [ tag w == fromJust (findTag i x)
         | w <- workspace (current x) : map workspace (visible x)  ++ hidden x
         , t <- maybeToList (stack w)
         , i <- focus t : up t ++ down t
@@ -529,7 +529,7 @@ prop_shift_win_indentity i w (x :: T) =
 -- shiftWin leaves the current screen as it is, if neither i is the tag
 -- of the current workspace nor w on the current workspace
 prop_shift_win_fix_current i w (x :: T) =
-    i `tagMember` x && w `member` x && i /= n && findIndex w x /= Just n
+    i `tagMember` x && w `member` x && i /= n && findTag w x /= Just n
         ==> (current $ x) == (current $ shiftWin i w x)
     where
         n = tag (workspace $ current x)
@@ -707,7 +707,7 @@ main = do
         ,("focusWindow is local", mytest prop_focusWindow_local)
         ,("focusWindow works"   , mytest prop_focusWindow_works)
 
-        ,("findIndex"           , mytest prop_findIndex)
+        ,("findTag"           , mytest prop_findIndex)
         ,("allWindows/member"   , mytest prop_allWindowsMember)
 
         ,("insert: invariant"   , mytest prop_insertUp_I)