insertWith package:dhall

Insert a key-value pair into a Map, using the supplied function to combine the new value with any old value underneath the same key, if present

>>> insertWith (+) "C" 1 (fromList [("B",2),("A",3)])  -- No collision
fromList [("C",1),("B",2),("A",3)]

>>> insertWith (+) "C" 1 (fromList [("C",2),("A",3)])  -- Collision
fromList [("C",3),("A",3)]