Intelligent cursor restoration after auto-formatting about elisp-tree-sitter

[ubolonton]

There's a macro in 0.5.0 for this: tree-sitter-save-excursion. It keeps the cursor on the same node, and on the same vertical position.

The following works for me in Python (provided that tree-sitter-mode is turned on in the buffer), with Black formatter:

(require 'tree-sitter-extras)

(define-advice format-all-buffer (:around (f &rest args) tree-sitter-save-excursion)
  (if tree-sitter-mode
      (tree-sitter-save-excursion
        (apply f args))
    (apply f args)))

I think it's better to combine this with the diff-based approach mentioned in the issue you linked, though.

[ubolonton]

This is a nice idea! I think it can be done as followed:

• Before formatting, store the "path" leading from the root node to the current node.
• After formatting, use that path to search for the corresponding node in the new syntax tree.

[leungbk]

@raxod502, who wrote apheleia.el, is knowledgeable about this. If we use Needleman-Wunsch as with apheleia, it's not clear to me how we'd combine this with a tree-sitter-based approach.

[raxod502]

Yes, there are basically two good options for handling this issue:

• The smart approach, which is to have an understanding of the AST and the cursor's position within a specific node
• The dumb approach, which is to use a string alignment or regexp-based algorithm

If you've got the resources available to do the first approach, it'll probably work better. The reason I think the second approach is good is because the first approach requires a lot of work for each language / formatter, whereas the second approach works for all languages and formatters.

The disadvantages of the second approach are:

• it needs some heuristics to optimize for large diff regions, because the dynamic programming algorithm scales quadratically (radian-software/apheleia#4)
• it needs some heuristics to nudge the cursor to the most appropriate position sometimes, because the dynamic programming algorithm can select one of several different string alignments which score equally well but which do not seem equally appropriate to a user (radian-software/apheleia#2)