Re: Update on tree-sitter structure navigation

[Ihor Radchenko <yantar92@posteo.net>]

Yuan Fu <casouri@gmail.com> writes:

> In the months after wrapping up tree-sitter stuff in emacs-29, I was
> thinking about how to implement structural navigation and extracting
> information from the parser with tree-sitter. In emacs-29 we have
> things like treesit-beginning/end-of-defun, and treesit-defun-name. I
> was thinking maybe we can generalize this to support getting arbitrary
> “thing” at point, move around them, and getting information like the
> name of a defun, its arglist, parent of a class, type of an variable
> declaration, etc, in a language-agnostic way.

Note that Org mode also does all of these using
https://orgmode.org/worg/dev/org-element-api.html

It would be nice if we could converge to more consistent interface
across all the modes. For example, by extending `thing-at-point' to handle
parsed elements, not just simplistic regexp-based "thing" boundaries
exposed by `thing-at-point' now.

Org approaches getting name/begin/end/arguments using a common API:

(org-element-property :begin NODE)
(org-element-property :end NODE)
(org-element-property :contents-begin NODE)
(org-element-property :contents-end NODE)
(org-element-property :name NODE)
(org-element-property :args NODE)

Language-agnostic "thing"s will certainly be welcome, especially given
that tree-sitter grammars use inconsistent naming schemes, which have to
be learned separately, and may even change with grammar versions.

I think that both NODE types and attributes can be standardized.

> Also, at the time, we only support defining things by a regexp
> matching a node’s type, which is often not enough.
>
> And it would be nice to somehow take advantage of the tree-sitter
> queries for the features I mentioned above. Tree-sitter query is what
> every other editor are using for virtually all tree-sitter related
> features. But in Emacs, we mostly only use it for font-lock.

I recall one user asking about something like VIM's textobjects via
tree-sitter queries. Example:
https://github.com/nvim-treesitter/nvim-treesitter-textobjects/blob/master/queries/cpp/textobjects.scm

> Here’s the progress as of now:
>
> - Functions like treesit-search-forward, treesit-induce-sparse-tree,
> treesit-thing-at-point, treesit--navigate-thing, etc, support a richer
> set of predicates now. Besides regexp matching the type, the predicate
> can also be a predication function, or (REGEP . FUNC), or compound
> predicates like (or PRED PRED) or (not PRED).

Slightly unrelated, but do you have any idea if it can be faster to use
Emacs' regexp search combined with treesit-thing-at-point vs. pure
tree-sitter query?

> - There’s now a variable treesit-thing-settings, which holds
> definition for things. Then, instead of passing the predicate to the
> functions I mentioned above, you can save the predicate in
> treesit-thing-settings under a symbol, say ‘sexp', and pass the symbol
> instead, just like thing-at-point.el. (We’ll work on integrating with
> thing-at-point.el later.)

This sounds similar to textobjects I linked above.
One question: how will it integrate with multiple parsers in one buffer?

> - I can’t think of a good way to integrate tree-sitter queries with
> the navigation functions we have right now. Most importantly,
> tree-sitter query always search top-down, and you can’t limit the
> depth it searches. OTOH, our navigation functions work by traversing
> the tree node-to-node.

May you elaborate about the difficulties you encountered?

> Some other things on the TODO list that people can take a jab at:
>
> - Solve the grammar versioning/breaking-change problem: tree-sitter grammar don’t have a version number, so every time the author changes the grammar, our queries break, and loading the mode only produces a giant error.

May we somehow get a hash of the library? That way, we can at least
detect if something has changed.

> - Major mode fallback/inheritance, this has been discussed many times, no good solution emerged.

I think that integration of tree-sitter with navigation functions might
be a step towards solving this problem. If common Emacs commands can
automatically choose between tree-sitter and classic implementations, it
might become easier to unify foo-ts-mode with foo-mode.

> - Isolated ranges. For many embedded languages, each blocks should be independent from another, but currently all the embedded blocks are connected together and parsed by a single parser. We probably need to spawn a parser for each block. I’ll probably work on this one next.

Do you mean that a single parser sees subsequent block as a continuation
of the previous?

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>