Create NonEmptySet type

Data/Map/Internal.hs

@@ -1923,7 +1923,7 @@ difference t1 (Bin (NonEmptyMap _ k _ l2 r2)) = case split k t1 of
 withoutKeys :: Ord k => Map k a -> Set k -> Map k a
 withoutKeys Tip _ = Tip
 withoutKeys m Set.Tip = m
-withoutKeys m (Set.Bin _ k ls rs) = case splitMember k m of
+withoutKeys m (Set.Bin (Set.NonEmptySet _ k ls rs)) = case splitMember k m of
   (lm, b, rm)
      | not b && lm' `ptrEq` lm && rm' `ptrEq` rm -> m
      | otherwise -> link2 lm' rm'
@@ -3316,7 +3316,8 @@ assocs m
 
 keysSet :: Map k a -> Set.Set k
 keysSet Tip = Set.Tip
-keysSet (Bin (NonEmptyMap sz kx _ l r)) = Set.Bin sz kx (keysSet l) (keysSet r)
+keysSet (Bin (NonEmptyMap sz kx _ l r)) = Set.Bin $
+  Set.NonEmptySet sz kx (keysSet l) (keysSet r)
 
 -- | /O(n)/. Build a map from a set of keys and a function which for each key
 -- computes its value.
@@ -3326,7 +3327,7 @@ keysSet (Bin (NonEmptyMap sz kx _ l r)) = Set.Bin sz kx (keysSet l) (keysSet r)
 
 fromSet :: (k -> a) -> Set.Set k -> Map k a
 fromSet _ Set.Tip = Tip
-fromSet f (Set.Bin sz x l r) = Bin $ NonEmptyMap sz x (f x) (fromSet f l) (fromSet f r)
+fromSet f (Set.Bin (Set.NonEmptySet sz x l r)) = Bin $ NonEmptyMap sz x (f x) (fromSet f l) (fromSet f r)
 
 {--------------------------------------------------------------------
   Lists

Data/Map/Strict/Internal.hs

@@ -1468,7 +1468,8 @@ mapKeysWith c f = fromListWith c . foldrWithKey (\k x xs -> (f k, x) : xs) []
 
 fromSet :: (k -> a) -> Set.Set k -> Map k a
 fromSet _ Set.Tip = Tip
-fromSet f (Set.Bin sz x l r) = case f x of v -> v `seq` Bin (NonEmptyMap sz x v (fromSet f l) (fromSet f r))
+fromSet f (Set.Bin (Set.NonEmptySet sz x l r)) = case f x of
+  v -> v `seq` Bin (NonEmptyMap sz x v (fromSet f l) (fromSet f r))
 
 {--------------------------------------------------------------------
   Lists

Data/Set.hs

@@ -70,6 +70,7 @@ module Data.Set (
               Set          -- instance Eq,Ord,Show,Read,Data,Typeable
 #else
               Set(..)
+            , NonEmptySet(..)
 #endif
 
             -- * Construction