[erlang-questions] Merging maps with a fun like dict:merge/3

[Richard A. O'Keefe]

Merging whatevers has three cases:
  - this is in the left but not the right
  - this is in the right but not the left
  - this is in both with these values

That is, given
   data Map k v
I'd expect
   merge :: (k -> v -> Maybe r) ->         -- left handler
            (k -> w -> Maybe r) ->         -- right handler
            (k -> v -> w -> Maybe r) ->    -- union handler
            Map k v ->                     -- left map
            Map k w ->                     -- right map
            Map k r                        -- result map

The left handler is given a key and value from the left map
and the assurance that the key is not in the right map.
It either returns Nothing, discarding this element,
or Just r meaning that k :-> r should be in the result.
The other handlers work in the same way.

Of course there can be specialisations, but this is what I
would expect as the foundation.

The frames proposal didn't include such a function because
a good record substitute and a good dictionary are not the
same kind of animal and should not be mixed up.