Implement Data.Map.foldrWithKeyM

containers/src/Data/Map/Internal.hs

@@ -264,6 +264,7 @@ module Data.Map.Internal (
 
     -- ** Monadic folds
     , foldlWithKeyM
+    , foldrWithKeyM
 
     -- * Conversion
     , elems
@@ -3366,16 +3367,33 @@ foldlWithKey' f z = go z
 {-# INLINE foldlWithKey' #-}
 
 -- | /O(n)/. Monadic variant of 'foldlWithKey'.
+--
+-- > 'foldlWithKeyM\'' f z0 (fromList [(0,'a'),(1,'b'),(2,'c')]) =
+-- >   f z0 0 'a' >>= (\z1 -> f z1 1 'b' >>= (\z2 -> f z2 2 'c'))
 foldlWithKeyM :: Monad m => (a -> k -> b -> m a) -> a -> Map k b -> m a
 foldlWithKeyM f z = go z
   where
-    go z' Tip             = return z'
+    go z' Tip              = return z'
     go z' (Bin _ kx x l r) = do
       z'' <- go z' l
       z''' <- f z'' kx x
       go z''' r
 {-# INLINE foldlWithKeyM #-}
 
+-- | /O(n)/. Monadic variant of 'foldrWithKey'.
+--
+-- > 'foldrWithKeyM\'' f z0 (fromList [(0,'a'),(1,'b'),(2,'c')]) =
+-- >   (f 2 'c' >=> f 1 'b' >=> f 0 'a') z0
+foldrWithKeyM :: Monad m => (k -> a -> b -> m b) -> b -> Map k a -> m b
+foldrWithKeyM f z = go z
+  where
+    go z' Tip              = return z'
+    go z' (Bin _ kx x l r) = do
+      z'' <- go z' r
+      z''' <- f kx x z''
+      go z''' l
+{-# INLINE foldrWithKeyM #-}
+
 -- | /O(n)/. Fold the keys and values in the map using the given monoid, such that
 --
 -- @'foldMapWithKey' f = 'Prelude.fold' . 'mapWithKey' f@