[Haskell-cafe] MonoidHashMap?

Tobias Dammers tdammers at gmail.com
Tue Jan 20 13:44:21 UTC 2015

Pragmatic workaround: newtype-wrap HashMap?

On Tue, Jan 20, 2015 at 02:33:22PM +0200, Konstantine Rybnikov 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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