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
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.