Bifoldable instance for Map

[Andreas Abel]

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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