unionWith package:dhall

unionWith :: Ord k => (v -> v -> v) -> Map k v -> Map k v -> Map k v
dhall Dhall.Map
Combine two Maps using a combining function for colliding keys 
>>> unionWith (+) (fromList [("D",1),("C",2)]) (fromList [("B",3),("A",4)])
fromList [("D",1),("C",2),("B",3),("A",4)]

>>> unionWith (+) (fromList [("D",1),("C",2)]) (fromList [("C",3),("A",4)])
fromList [("D",1),("C",5),("A",4)]