Add strict foldlWithKeyM

Author: andrewthad

containers/src/Data/Map/Internal.hs

@@ -262,6 +262,9 @@ module Data.Map.Internal (
     , foldrWithKey'
     , foldlWithKey'
 
+    -- ** Strict monadic folds
+    , foldlWithKeyM'
+
     -- * Conversion
     , elems
     , keys
@@ -3362,6 +3365,17 @@ foldlWithKey' f z = go z
       in go (f z'' kx x) r
 {-# INLINE foldlWithKey' #-}
 
+-- | /O(n)/. Monadic variant of 'foldlWithKey\''.
+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' (Bin _ kx x l r) = do
+      !z'' <- go z' l
+      z''' <- f z'' kx x
+      go z''' r
+{-# INLINE foldlWithKeyM' #-}
+
 -- | /O(n)/. Fold the keys and values in the map using the given monoid, such that
 --
 -- @'foldMapWithKey' f = 'Prelude.fold' . 'mapWithKey' f@