Harmonize Emacs indentation and LSP indentation

[brosasaki]

If you have a major-mode which sets its own formatting parameters (i.e. uses its own version of tab-width), then there is no way to communicate that to eglot so that requests to format the file, etc. use the proper formatting parameters.

Reproduction Recipe

1. Ensure R and its languageserver package are installed along with the latest version of ess.
2. Open a .R script and start the language server.
3. Use M-x ess-set-style or C-c C-e s to set the style to something with an ess-indent-offset of something other than tab-width (by default, this is 'RRR', with a 4-wide indent and call-aligned function arguments--very different from tab-width's default 8).
4. Indent your code with indent-region or indent-paragraph*.
5. Use M-x eglot-format-buffer and watch it change all of the indentation again*.

*There is a bug in languageserver right now in which the indentation linter is hardcoded to a 2-space indent, so it may be marked as incorrect indentation, even after the same language server (which the linter is a part of) just formatted it. But that is a langageserver problem.

Solution Pointers

Looking over the source, it looks like maybe changing these lines to refer to new package options which could be specified per-major-mode (e.g. in eglot-server-programs) would allow for users to specify which variables are the correct formatting parameters for each major mode. But, since I do not understand the codebase well, I am hesitant to write the patch myself for now.

[joaotavora]

There is no real harmony between Elisp indent-region and LSP "formatting". There never was, and I doubt if it is even possible. Elisp's indentation is about inserting or removing spaces or tabs at the beginning of the line, whereas LSP's is about adding/removing arbitrary whitespace (or maybe even more than that...).

The best you can hope for is baking an indent-region-function, but that didn't work very well last time I tried it: https://mail.gnu.org/archive/html/bug-gnu-emacs/2023-07/msg01541.html

[brosasaki]

Yes, in my own investigation of the issue I've found that one of the issues preventing this is that LSPs are not required to make formatting decisions without context as indent-region does on RET, e.g. a code block or multiline function call might not get indented until it is closed. How do other editors do it? Or do they have "indentation wars" too?

[joaotavora]

How do other editors do it? Or do they have "indentation wars" too?

🤷

[brosasaki]

It still may be useful to expose those variables to bridge the gap a little. Even if it isn't perfect, a separate plist (listing major-modes and formatting parameters) could be made to keep things sanitary if it ever gets changed in the future. For now, I've written a small advice hack for init.el for myself and anyone else interested:

(advice-add 'eglot-format :around
	    (lambda (fn &rest r)
	      (let ((tab-width
		     (cond 
		      ((member major-mode '(ess-mode ess-r-mode)) ess-indent-offset))))
		(apply fn r))))

You can add more major modes by adding another clause to the cond call.

[joaotavora]

You can propose anything to [email protected], I won't veto it. But I think this is just making a useless indirection. Why not just set tab-width buffer-locally in your ers-mode thing?