treesit: how to get it to parse multiple languages

[Andrew De Angelis <bobodeangelis@gmail.com>]

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Hello everyone and thanks for all your work!

I'm trying to get a better understanding of treesit.el, and I've stumbled
on a couple of things that make me think the manual is either
outdated/faulty, or just not entirely clear and I'm missing something.

The latter is most likely, but I'd appreciate any help in figuring out what
exactly is wrong in my approach/setup. I would be happy to contribute to
the manual, if needed, to ensure it is clearer.

This is the relevant section of the manual:
https://www.gnu.org/software/emacs/manual/html_node/elisp/Multiple-Languages.html
I've started out with simply trying to recreate the setup described in the
manual, but I've run into some issues.
Here's what I've done so far:
 - I've defined a very simple `html-ts-mode`, using the elisp functions
from the manual:
https://github.com/andrewdea/poc-html-ts-mode/blob/main/html-ts-mode.el
 - I activate this mode when visiting the example.html file (which is also
copied from the manual):
https://github.com/andrewdea/poc-html-ts-mode/blob/main/example.html
 - the queries seem to be working as expected: when I'm in a buffer
visiting example.html, evaluating `(treesit-query-capture 'html css-query)`
and `(treesit-query-capture 'html js-query)` return the expected nodes
 - ISSUE: `treesit-update-ranges` doesn't seem to be working as expected:
even if I call it multiple times, the parser for the whole buffer seems to
still be 'html. `(treesit-language-at (point))` always returns 'html, even
when I'm inside the nodes captured by the css-query or js-query.

Some additional context: the reason I'm looking into tree-sitter (and its
functionalities to support multiple languages) is to potentially use it to
fontify markdown code blocks and to improve emacs support for python
notebooks. For markdown, I was trying a similar approach to the HTML one
described in the manual, but ran into other similar issues:
https://www.reddit.com/r/emacs/comments/1gcrv8k/syntaxhighlighting_codeblocks_in_markdown/
.
I'm just including this as context.

Let me know if any of this is not clear.

Thanks in advance for all your help!

Best,
Andrew

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