Bifoldable instance for Map

[David Feuer]

No idea. Traversing in a Biapplicative can definitely be interesting. I
don't know what Bifoldable or Bitraversable can do.

On Tue, Jun 2, 2020, 12:12 AM Joseph C. Sible <josephcsible at gmail.com>
wrote:

> 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.
>
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