On Wed, Feb 14, 2018 at 8:49 AM, Akio Takano
I can think of 3 reasonable definitions for (<>) for lazy maps:
1. (<>) = union 2. (<>) = unionWith (<>) 3. (<>) = Strict.unionWith (<>)
Of these, (1) is by far the most useful operation in my experience. (2) has the advantage that it seems like the most obvious choice, but it's not very useful in practice. (3) is slightly more useful than (2), but feels less canonical. Also (3) seems inconsistent with how fmap is defined.
I think you are very much in the minority here. (1) which we have now is almost never what I want. (2) is much more likely to be what I need. And of course, you can recover (1) with (2) by simply mapping First into every element. So there doesn't seem to be a clear winner. Perhaps there shouldn't
have been a Monoid instance for maps in the first place. Given that we already have one, however, it seems that the marginal gain of removing it doesn't justify the cost of breaking code.
There should be a Monoid instance, and http://conal.net/papers/type-class-morphisms/ argues that it should be (2).