Subject: Re: bug#73404: 30.0.50; [forward/kill/etc]-sexp commands do not behave as expected in tree-sitter modes

From: Juri Linkov <[email-masked]>
Date: Tue, 01 Oct 2024 20:49:39 +0300

> The user can modify treesit-thing-settings to alter the behavior of
> sexp navigation, they don’t necessarily need to use
> js--treesit-sexp-nodes.  Maybe we should add a test for
> (treesit-thing-defined-p 'sexp nil) in treesit-forward-sexp?

I tried to do something like this, and everything works nicely with:

  @@ -2289,11 +2289,10 @@ treesit-forward-sexp
           (node-at-point
            (treesit-node-at (point) (treesit-language-at (point)))))
       (or (when (and node-at-point
  -                   ;; Make sure point is strictly inside node.
  -                   (< (treesit-node-start node-at-point)
  -                      (point)
  -                      (treesit-node-end node-at-point))
  -                   (treesit-node-match-p node-at-point 'text t))
  +                   (or (treesit-node-match-p node-at-point 'text t)
  +                       (not (treesit-thing-at
  +                             (if (> arg 0) (point) (1- (point)))
  +                             (treesit-thing-definition 'sexp nil)))))
             (forward-sexp-default-function arg)
             t)
           (if (> arg 0)

The new logic is the following: if there is no sexp thing defined at point,
then fall back to 'forward-sexp-default-function'.

Then after (setq js--treesit-sexp-nodes '("binary_expression"))
'C-M-f' in e.g.

  export const add = (a, b) => -!-a + b;

moves point to

  export const add = (a, b) => a + b-!-;

The condition (if (> arg 0) (point) (1- (point))) above
is necessary to allow 'C-M-b' to move back to:

  export const add = (a, b) => -!-a + b;

Also the condition to make sure point is strictly inside node
was removed to handle the case when point was at the beginning
of the buffer:

  -!-
  export const add = (a, b) => a + b;

to move after

  export-!- const add = (a, b) => a + b;

by 'forward-sexp-default-function'.

> Your second example sounds useful, but right now the premise of tree-sitter
> sexp movement is to use the parse tree primarily, and only use the default
> sexp movement for comments and strings. What you envisioned seems to be the
> other way around: use default sexp movement by default, and only use
> tree-sitter movement under certain conditions. Is that few lines of change
> able to make such big difference in the logic?

I think we need to support both ways:

1. opt-out - where sexp-thing definition is used by default,
   and only text-thing allows users to override it;

2. opt-in - where 'forward-sexp-default-function' is used by default,
   and user can explicitly define what sexp-things are preferable
   for navigation by treesit.

Then in the latter case the users could prefer to use
treesit sexp navigation only for constructions with
"invisible parens".  For example, in Ruby there are
two interchangeable syntaxes for code blocks:

1. curly braces {...} that are already handled
   by 'forward-sexp-default-function';

2. do...end that can't be handled by 'forward-sexp-default-function',
   so treesit is coming to the rescue for the case of such
   implicit braces.