unionWith package:unordered-containers

unionWith :: Eq k => (v -> v -> v) -> HashMap k v -> HashMap k v -> HashMap k v
unordered-containers Data.HashMap.Internal Data.HashMap.Internal.Strict Data.HashMap.Lazy Data.HashMap.Strict
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 :: Eq k => (k -> v -> v -> v) -> HashMap k v -> HashMap k v -> HashMap k v
unordered-containers Data.HashMap.Internal Data.HashMap.Internal.Strict Data.HashMap.Lazy Data.HashMap.Strict
The union of two maps. If a key occurs in both maps, the provided function (first argument) will be used to compute the result.