[Haskell-cafe] MonoidHashMap?

Konstantine Rybnikov k-bx at k-bx.com
Tue Jan 20 12:33:22 UTC 2015


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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