Bifoldable instance for Map

[Zemyla]

Set is a Foldable, but not a Functor or Traversable. So the precedent is
there.

On Mon, Jun 1, 2020, 05:16 Andreas Abel <andreas.abel at ifi.lmu.de> wrote:

>  > We have Functor, Foldable, and Traversable instances
>
> Yes, and good so, since I use these type classes every day.  Haven't
> ever used Bifoldable, though.  This is why I question the need.
>
> In general, I am a proponent of fat libraries, thus, I am not strongly
> against such an addition, especially if it is only a light-weight
> overlay over foldrWithKey.
>
> I guess since Bifoldable is in base already (wasn't aware of this), the
> ship has already sailed on discussing its wide-spread usability.  I
> remain unconvinced though.
>
> What weights in for me is that Map does not have sensible Bifunctor or
> Bitraversable instances (since manipulating the keys does not preserve
> the tree structure in general).  Thus, we cannot make the chord
> (Bi)functor/(Bi)foldable/(Bi)traversable complete.
>
> My take would be to add Bifoldable only on public demand.
> But let's hear other opinions as well.
>
> --Andreas
>
>
> On 2020-05-31 18:49, Joseph C. Sible wrote:
> > 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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