Bifoldable instance for Map

[Joseph C. Sible]

I don't find this argument convincing. We have Functor, Foldable, and
Traversable instances even though mapWithKey, foldrWithKey, and
traverseWithKey give more control than they do. We have a Semigroup
instance even though unionWith and unionWithKey give more control than
it does.

As for the implementation, I'd be fine with changing it to be in terms
of foldrWithKey. I just care that we have this instance somehow.

Joseph C. Sible

On Sun, May 31, 2020 at 6:23 AM Andreas Abel <andreas.abel at ifi.lmu.de> wrote:
>
> Do we have any need for Bifoldable?  We have
>
>    foldrWithKey :: (k -> a -> b -> b) -> b -> Map k a -> b
>    foldlWithKey :: (a -> k -> b -> a) -> a -> Map k b -> a
>
> These functions seem to give more control of how the fold happens.
>
> And if we'd want Bifoldable, shouldn't the Bifoldable instance be
> implemented in terms of just foldrWithKey?  Why have a recursive
> implementation?
>
> Personally, I consider Bifoldable a fringe class, as I would hope that
> in general with fusion, I can just go via bimap and ordinary Foldable
> without much penalty.
>
> On 2020-05-31 07:20, George Wilson wrote:
> > If you have a non-commutative monoid, the result could easily be
> > different than you expect. eg. if you think it would do all keys and
> > then all values, rather than interleaving (which is I assume the most
> > efficient to implement).
> > Despite that I'm still +1 on this.
> >
> > On Sun, 31 May 2020 at 14:59, Joseph C. Sible <josephcsible at gmail.com> wrote:
> >>
> >> I'll admit I don't have a strong use case for this. I was just looking
> >> at instances and noticed that these seemed to be a weird omission,
> >> since there was an obvious definition. But I also don't think it's
> >> confusing at all. Since (,), Set, and HashSet are Foldable, and (,)
> >> and (,,) x are Bifoldable, it seems only logical to have Map and
> >> HashMap be Bifoldable too. What exactly would be confusing about any
> >> of these?
> >>
> >> Joseph C. Sible
> >>
> >> On Sat, May 30, 2020 at 8:33 PM David Feuer <david.feuer at gmail.com> wrote:
> >>>
> >>> Let me take that back. I forgot how weird Bifoldable and Bitraversable are for product types and product-like types. Is this instance actually useful for anything, or is it mostly confusing?
> >>>
> >>> On Mon, Apr 13, 2020, 9:46 PM David Feuer <david.feuer at gmail.com> wrote:
> >>>>
> >>>> I would go as far as to say we don't need to continue the proposal process here. We're doing it.
> >>>>
> >>>> On Mon, Apr 13, 2020, 9:44 PM David Feuer <david.feuer at gmail.com> wrote:
> >>>>>
> >>>>> This seems eminently reasonable to me. We must also be sure to add one to Data.HashMap if that's missing too.
> >>>>>
> >>>>> On Mon, Apr 13, 2020, 9:36 PM Joseph C. Sible <josephcsible at gmail.com> wrote:
> >>>>>>
> >>>>>> I'd like to propose a change to the containers package: adding a
> >>>>>> Bifoldable instance to Map. I briefly mentioned this on Reddit [1] and
> >>>>>> no obvious problems were brought up. I submitted a PR implementing it
> >>>>>> [2]. This seems like an obvious and straightforward instance to me.
> >>>>>> Thoughts?
> >>>>>>
> >>>>>> Joseph C. Sible
> >>>>>>
> >>>>>> [1]: https://old.reddit.com/r/haskell/comments/fsgqd6/monthly_hask_anything_april_2020/fn90d6k/
> >>>>>> [2]: https://github.com/haskell/containers/pull/714
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