in general though i have mixed feeling about making structured editing more text-like; the above is just about trying to patch a hole in existing structured editors, which doesn't in-itself improve on text. i think we can do better than that; we're exploring more radical directions in a separate project. but i do think it is interesting to see how close to regular text entry we can stay while always maintaining a well-formed (though incomplete) parse state which we can use to constantly run type-checking and evaluation.
the current model partially succeeds in the above, but at some significant usability cost, including the fact the the backpack obscures what the actual underlying parse state is. the ghost model i'm describing can basically be thought of a generalization of the vscode parentheses insertion you describe; it just works for every multi-delimiter form (eg when you insert 'let' you get ghosts '=' and 'in', with appropriate holes inserted). the utility is (A) the same as an incremental parser in a language server (you get semantic feedback in every state), but because of the ghost/hole insertions it's crystal clear what the parse state is that you're getting feedback from. but yeah the current version doesn't live up to that standard