[Haskell-cafe] MonoidHashMap?

Michael Snoyman michael at snoyman.com
Tue Jan 20 12:45:08 UTC 2015

I've definitely wanted it in the past, and have even implemented limited
versions of it before. I'd be open to adding it to Data.Containers in


On Tue Jan 20 2015 at 4:34:00 AM Konstantine Rybnikov <k-bx at k-bx.com> wrote:

> Hi!
> I'm working with a lot of HashMap's and it's very frustrating how many
> times I've "lost" my data because of usage of either a Monoid instance of a
> HashMap (which is defined as H.union, which, upon collision, takes value
> from first hm and discards from second), or just using fromList in the
> wrong place. Whereas the data I'm working is is mostly defined as (Monoid v
> => HashMap k v), so what I need "by default" is actually something like
> `H.unionWith (<>)`.
> What I was wondering is this: is something like MonoidHashMap is desired
> to be in unordered-containers, or is this use-case only popular in my
> programs? I'm asking because I have a feeling that this thing might be
> useful quite a lot for others also. If not -- sorry for bothering :)
> Cheers!
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