Caterpillar

Infrastructure for rendering arbitrary types is just the start. We also need the same kind of infrastructure for editing arbitrary types.

Yes, you can always type a new number in the editor. But for some values, it might be better to have a slider. Imagine tweaking a value that way, and experiencing the resulting change in your program right away. How much more immediate of an experience would that be? How much more efficiently could you tweak your code's behavior?

Other data types would require other kinds of special editors. And IDEs already do that, sometimes. It's not uncommon to see a color picker when editing CSS, for example. But as far as I've seen, it's always special-purpose. And I think it needs to be a standard thing supported by the language and its tooling. Something that every library author can use without hurdles.