Re: Tree-sitter integration on feature/tree-sitter

[Yuan Fu]

> On May 7, 2022, at 11:48 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Sat, 7 May 2022 11:27:11 -0700
>> Cc: emacs-devel@gnu.org
>> 
>>> Would you please suggest how to "play with it"?  What features to turn
>>> on and how, what commands and displays to try, etc.  Posting such
>>> instructions should allow all of us get to testing this branch much
>>> sooner and easier.
>> 
>> Some suggestions on top of my head:
>> 
>> - Load the attached treesit-demo.el which contains a very basic C major 
>> mode, ts-c-mode, that gives you some font-locking and indentation using 
>> tree-sitter features.
>> - Get a C file, change its extension to .tsc, and open it
>> - The major mode should be ts-c-mode, and it should have some basic 
>> fontification.
>> - Type M-x treesit-show-buffer-tree RET to see the AST of the C file, 
>> probably want to use this on a smaller C file.
>> - Type M-x treesit-inspect-mode RET to display the AST of the node at point 
>> in the mode-line, see more explanation in its doctoring.
>> - Set treesit--indent-verbose to t, and try to indent some lines, and see 
>> which indentation rule is matched.
>> 
>> Is this the sort of things you are looking for?
> 
> Yes, thanks.
> 
> However, I hoped that the font-lock and indentation are already
> integrated with tree-sitter, and all that is needed for a given
> major-mode to use it is to flip some variable.  I didn't expect to
> need a completely separate major mode for C sources, for example.
> 
> It sounds like we are farther from the goal that I thought we were,
> and some serious work is still ahead of us to get this integrated into
> the existing major modes.

I used a separate major mode because I don’t want to touch the existing one, 
and this is only a simple demo. For the author of a major-mode, they only need 
to port their font-lock-defaults and indent function to use tree-sitter 
features. If you look at the definition of ts-c-mode, its roughly

(if (treesit-should-enable-p)
    (set tree-sitter font-lock and indent variables)
  (set font-lock-defaults etc))

Then a user could choose whether to turn on tree-sitter for a particular major 
mode by changing treesit-disabled-modes. Maybe it should be 
treesit-enabled-modes, or something more elaborate, but that’s another topic.

Of course, if a major-mode author pleases, they could also define separate 
major modes, as Theodor pointed out.

> However, I hoped that the font-lock and indentation are already
> integrated with tree-sitter, and all that is needed for a given
> major-mode to use it is to flip some variable.

That’s too much magic to ask from tree-sitter. It is just an 
incremental-parser, work is still needed from major mode authors to use the 
parsed AST to font-lock/indent/etc. But at least I’ve made that process as easy 
as possible.

Yuan