Bifoldable instance for Map

[David Feuer]

Yes, folding generally loses structure. This just seems especially
egregious. Can you think of even a single function polymorphic over
`Bifoldable` containers that you'd find it useful to pass a `HashMap`
to?

On Mon, Jun 1, 2020 at 5:34 PM Joseph C. Sible <josephcsible at gmail.com> wrote:
>
> Aren't you basically just saying that you lose some of the structure
> (namely, the knowledge that the "key" and its "value" go together)?
> But doesn't every Foldable instance on a type that's more complex than
> a list also do that? For example, if you fold a rose tree, you lose
> the knowledge of which elements came from which branches.
>
> (Completely unrelated: "Loost" and the names of its data constructors
> sound like something straight from Dr. Seuss.)
>
> Joseph C. Sible
>
> On Mon, Jun 1, 2020 at 8:28 AM David Feuer <david.feuer at gmail.com> wrote:
> >
> > Let me be more specific. Whereas we can get intuition for Foldable from
> >
> >     toList :: t a -> [a]
> >
> > we get intuition for Bifoldable from the hypothetical
> >
> >     toEitherList :: t a b -> [Either a b]
> >
> > This seems quite reasonable for some types.
> >
> >     data Loost a b
> >       = Nool
> >       | Corns b (Loost a b)
> >       | Colns a (Loost a b)
> >
> > But for something like
> >
> >     newtype Plist a b
> >       = PNil
> >       | PCons a b (PList a b)
> >
> > it feels awfully strange. Independent parts of the structure just get lumped together.