Bifoldable instance for Map

[Joseph C. Sible]

Let me turn this around: do you believe the Bifoldable class is a
useful abstraction at all? Are there any real types (i.e., that exist
in `base` or some other popular library or program, rather than made
up for the sake of example) that you think have a useful Bifoldable
instance? In other words, is your issue actually with this instance,
or is it really with the entire class?

Joseph C. Sible

On Mon, Jun 1, 2020 at 5:51 PM David Feuer <david.feuer at gmail.com> wrote:
>
> 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.