unionWith package:rio

unionWith :: (Eq k, Hashable k) => (v -> v -> v) -> HashMap k v -> HashMap k v -> HashMap k v
rio RIO.HashMap
The union of two maps. If a key occurs in both maps, the provided function (first argument) will be used to compute the result.
unionWith :: Ord k => (a -> a -> a) -> Map k a -> Map k a -> Map k a
rio RIO.Map
O(m*log(n/m + 1)), m <= n. Union with a combining function. 
unionWith (++) (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == fromList [(3, "b"), (5, "aA"), (7, "C")]
unionWithKey :: (Eq k, Hashable k) => (k -> v -> v -> v) -> HashMap k v -> HashMap k v -> HashMap k v
rio RIO.HashMap
The union of two maps. If a key occurs in both maps, the provided function (first argument) will be used to compute the result.
unionWithKey :: Ord k => (k -> a -> a -> a) -> Map k a -> Map k a -> Map k a
rio RIO.Map
O(m*log(n/m + 1)), m <= n. Union with a combining function. 
let f key left_value right_value = (show key) ++ ":" ++ left_value ++ "|" ++ right_value
unionWithKey f (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == fromList [(3, "b"), (5, "5:a|A"), (7, "C")]