Re: A more useful Monoid instance for Data.Map

On Sat, 28 Apr 2012, Evan Laforge wrote:

On Fri, Apr 27, 2012 at 7:00 PM, Daniel Peebles wrote:

...

Why not be explicit about the replacement strategy by injecting your values into First/Last? My point is that in terms of functionality, using mappend on the values is strictly more general than the current instance. It seems unfortunate to be stuck with the current instance for historical reasons, but I guess that's how a lot of this stuff works :/

Yeah, I suppose it would be a bit more regular that way. I'm always reluctant to map newtypes over things other than lists because I don't trust there to be a RULES that will eliminate it, but I suppose for Map there must be. I guess I wouldn't mind updating my code if the definition changed. It's hard to change a general purpose method though, simply because searching for it in your code will turn up so many false positives.

The version containers-0.6 may not have a Monoid Map instance at all, such that all people have to note the change and then containers-0.7 should contain the new instance. :-)