[containers] Proposal: Change to the Data.Map Monoid

[Alec]

On Wed, May 21, 2014 at 12:11 PM,  David Thomas wrote:
> For what it's worth, I'd find Semigroup v => Monoid (Map k v) both
> more useful and less surprising

There is something somewhat surprising about the (Monoid v) => Monoid
(Map k v) instance never using v's mempty.

> ...than the current unqualified monoid instance for all maps
Also (and I may have missed this argument being made already,
apologies if so): It would be weird if Data.Map had the new monoid
behavior but Data.IntMap didn't.  And then of course, it would be
weird if Data.Map had it but Data.HashMap didn't.  Data.Set should
probably remain the same, though it might feel a bit weird that Map k
() wasn't isomorphic to Set k.  Anyway, the real upshot is that
unordered-containers would probably need to move along with
containers, and that in addition to the social problem of moving the
community in general, there would be additional gnashing of teeth from
implementors of other map types who have followed the convention laid
down in the "signature" for Data.Map.

-A