intersection package:dhall

intersection :: Ord k => Map k a -> Map k b -> Map k a
dhall Dhall.Map
Combine two Map on their shared keys, keeping the value from the first Map 
intersection = intersectionWith (\v _ -> v)

>>> intersection (fromList [("C",1),("B",2)]) (fromList [("B",3),("A",4)])
fromList [("B",2)]
intersectionWith :: Ord k => (a -> b -> c) -> Map k a -> Map k b -> Map k c
dhall Dhall.Map
Combine two Maps on their shared keys, using the supplied function to combine values from the first and second Map 
>>> intersectionWith (+) (fromList [("C",1),("B",2)]) (fromList [("B",3),("A",4)])
fromList [("B",5)]