Re: Proposal: Remove Semigroup and Monoid instances for Data.Map, Data.IntMap, Data.HashMap

Hi Oleg, On 14 February 2018 at 09:48, Oleg Grenrus wrote:

I agree with Oliver, was just writing the very same.

`Map.map First = Map.map coerce = coerce`

i.e. mapping First is _zero-cost_, thanks to the fact `v` in `Map k v` has represenational role.

Yes, I agree that mapping First is zero-cost. I'm saying that `unionWith (<>)` over the First monoid costs more than `union`, due to allocation of extra thunks. - Akio