bug#72343: [PATCH] Fix eglot-server-programs for TeX modes

["João Távora" <joaotavora@gmail.com>]

On Tue, Aug 6, 2024 at 5:38 PM Augusto Stoffel <arstoffel@gmail.com> wrote:

> > So there's no "single pick" in general.
> You don't need to explain that to me ;-).

It is you who alluded to a single pick, so I thought I had to.  Also
earlier you proposed multiple entries in eglot-server-programs, which
would impede this multi-language-per-session feature.  So I figured you
wouldn't be familiar with it.

> What I want to achieve (or, if I may, what Eglot should do) in case it
> wasn't clear yet, is: file x.tex is sent language id "latex" and file
> y.tex is sent language id "plain-tex".

Indeed it wasn't clear, so I'm very glad I asked.  I don't see
"plain-tex" in the list over at

  https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/

I just see "tex" and "latex" and "bibtex".

Now, if you want "latex" to be sent for x.tex (in latex-mode) and "tex"
to be sent for x.tex (in plain-tex-mode), there is this possibility.

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 5845aff39b7..2695aa93ffe 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -292,7 +292,7 @@ eglot-server-programs
     (scala-mode . ,(eglot-alternatives
                     '("metals" "metals-emacs")))
     (racket-mode . ("racket" "-l" "racket-langserver"))
-    ((tex-mode context-mode texinfo-mode bibtex-mode)
+    ((latex-mode tex-mode context-mode texinfo-mode bibtex-mode)
      . ,(eglot-alternatives '("digestif" "texlab")))
     (erlang-mode . ("erlang_ls" "--transport" "stdio"))
     ((yaml-ts-mode yaml-mode) . ("yaml-language-server" "--stdio"))

No need to mention plain-tex-mode at all. The "guess" logic will work
(though I still recommend :language-id cookie).

You might notice this will only work for a particular sequence of
invocations (specifically starting the server with the latex-mode file
open, then opening other files in plain-tex-mode).

But it won't for many other cases.  The fact that Emacs models
latex-mode as a descendent of tex-mode is something Eglot can't do
anything about.  It means that Emacs thinks that that every LaTex
program is a TeX program at least at some syntactic level (is this true?
no idea).  So as soon as you have the two files open, M-x
eglot-reconnect will send "tex" for _both_.  Which, according to Emacs's
view of the world, isn't wrong.

This has happened before with the js/json mode and has been fixed in the
mode relations.  I think bug#67463.  The same idea thatfixed it there
could be applied here.

Anyway, if plain-tex-mode is an attempt to fix this, by making a core
mode that is not directly related to latex-mode, then I think it should
appear in the list, but then :language-id set to to "tex".

But I see plain-tex-mode also inherits from tex-mode, so the fundamental
problem remains. Unless you remove tex-mode from the list (which is what
you did).  But then outside descendents of tex-mode would be cut off,
which is what I warned about.

I do think that's the lesser of two evils though.  So anyway this is my
suggestion:

@@ -292,7 +292,7 @@ eglot-server-programs
     (scala-mode . ,(eglot-alternatives
                     '("metals" "metals-emacs")))
     (racket-mode . ("racket" "-l" "racket-langserver"))
-    ((tex-mode context-mode texinfo-mode bibtex-mode)
+    ((latex-mode (plain-tex-mode :language-id "tex") context-mode
texinfo-mode bibtex-mode)
      . ,(eglot-alternatives '("digestif" "texlab")))
     (erlang-mode . ("erlang_ls" "--transport" "stdio"))
     ((yaml-ts-mode yaml-mode) . ("yaml-language-server" "--stdio"))

In theory, this could also be fixed by keeping tex-mode in there, and
then adjusting Eglot's heuristic, by ranking the potential ancestors and
selecting the closest one, in 'eglot--languageId'.  Could be tricky or
could be easy, feel free to give it a shot.

Yet another idea is to not reorder modes at all in 'eglot--lookup-mode'
and use an explicit order to express this ranking.  I admit I don't
remember what the reordering idea (main-mode-sym is the local var name)
is for.  it couldprecede the multiple-language-ids feature.  Or it could
be essential for some other thing: testing would be needed.

> This doesn't matter, as it's an Eglot issue.  Any TeX server will do.

This is not established, at least not yet.  You are requesting for a
"plain-tex" LSP languageId property to be sent and that is outside the
spec as far as I can tell from reading the spec.  Maybe your digestif
server supports and likes it, though.  But it might break "texlab" for
all I know.  Even the fact that "tex" is all that's being sent at the
moment may or may not be a bug -- only LaTex/Tex specialists can say
that.  And certainly a capable enough language server, which is tasked
with analysing a file syntactically, can do much the same as our own
tex-mode.el and figure out the correct language from the contents.
Might do it better than Emacs.

That said, if you really want to send "plain-tex", then there are two
options.  You can change your user value of eglot-server-programs,
obviously.  Or you can confirm with "texlab" that they don't mind, in
which case -- but only in that case -- your latest patch is acceptable.