Feature: `:exit` listener equivalents?

[JoshuaKGoldberg]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

This library is a great clean way to run a visitor on either preorder (NLR) or reverse preorder (NRL) per API > visit. Thanks for making it! ❤️

I'm using this to build visitors in a linter's language support for YAML nodes and other languages. Visiting in NLR works fine, but we'd also like to have "exit" visitors akin to ESLint's :exit listeners. As in, essentially have both NLR and NRL in the same call, with intermixed enter & exit calls: not running all enter calls and then all exit calls.

Current solutions

I couldn't figure out how to do it from this package or ones linked from its README.md such as unist-util-visit. I'm not exhaustively familiar with the unist ecosystem, though. Is there a package that could be used for this? Say, a lower-level-ish one that gives more fine-grained control over walking?

Proposed solutions

As a vague first suggestion, how about allowing visit to either be a function (current behavior) or an object with enter and exit keys?

visit(root, {
  enter: (node) => console.log("Entering:", node),
  exit: (node) => console.log("Exiting:", node),
});

As for an implementation suggestion, I think the :exit addition could run at the bottom of the visit() function after visiting children (subresult) and before returning.

[remcohaszing]

Personally I think this would be nice to have.

On the other hand, you don’t need to transform the AST while visiting, right? So it would be fairly trivial to write your own visit function. For example, I wrote one here.

[wooorm]

I have also thought about it, it’s a noticeable gap in the API!
I’d wonder a bit about the interface, typing it, indeed as Remco mentions there’s no SKIP and it’s mostly trivial to implement a walker yourself.
Generally I think I am up to it! Is this something you’d want to work on? It’s somewhat of a fun exercise at the start, but maybe gets more difficult to get thoroughly working?