+1 On Tue, Jan 20, 2015 at 1:45 PM, Michael Snoyman wrote:

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 mono-traversable[1].

[1] http://www.stackage.org/haddock/nightly-2015-01-20/mono-traversable-0.7.0/Da...

On Tue Jan 20 2015 at 4:34:00 AM 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 :)

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I do have many newtypes for different datas, and defined correct Monoid instances for them. The problem arises when you need to convert between them in different ways, or when you write generic HashMap-related code. On Tue, Jan 20, 2015 at 3:44 PM, Tobias Dammers wrote:

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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-- Tobias Dammers - tobias@twokings.nl - 070-3457628 - www.twokings.nl Maandag t/m donderdag van 9.00 tot 17.30 Voor dringende vragen, mail naar support@twokings.nl _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe