This way we don't have to zipper together each "frame" of the output tree trying to figure out which bit goes to which other bit, we get an explicit delta saying "the 5th thing got deleted". You do probably need a language with HKT for this.

I mentioned this to Phil Freeman years ago and he did a prototype in Purescript, but I haven't seen it anywhere else: https://github.com/paf31/purescript-purview

Another thing I'd like to see is a _truly_ immediate mode framework where each widget is a pure function to render it given its state, and it's up to the user to manage the UI state (scroll position, text cursor location, etc), you can mix it in with/ your data model, or whatever. Even egui/etc aren't truly immediate as they need to keep some sort of stable identity for events, UI state and focus. You could of course build a React like thing on top of that, but it would be helpful to have that for experimentation with wildly different state models without having to reinvent the text box and everything yet again. I've started some experimentation w/ this in Haskell and it's going well so far.