bug#53749: 29.0.50; [PATCH] Xref backend for TeX buffers

[David Fussner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi Dmitry,

Once again, many thanks for the feedback. I'm still not certain I
agree about the risks involved in creating a new "thing" type, as it
really only appears in a small number of commands and then only in TeX
buffers, and generally I tried to design the code to keep out of the
way of anything outside of such buffers, but needless to say you see
further and more clearly than I can. I've been reviewing your comments
and my code, and have a few ideas and questions about how to go
forward. Though I haven't coded it yet, it's possible that the
simplest (and least intrusive) approach to follow would do something
like this:

1. Get rid of the new texsymbol "thing" and just use a buffer-local
value of find-tag-default-function and a rather more thoroughly
modified syntax table to control what "symbol" means, but _only_ in
the context of commands that use find-tag-default-function. I think
I'd lose the ability to change the behavior of
isearch-forward-thing-at-point and project-find-regexp, as I can't see
how to temporarily modify the syntax table there, though perhaps I'm
missing something.

2. Simply eliminate the TeX escape character entirely, both from tag
names in a TAGS file and from any thing-at-point in a TeX buffer. I
think this would eliminate the need to distinguish among the various
xref commands in terms of whether they can or can't handle the escape
character. It would also eliminate the need for the new user option in
etags.c, as there would no longer be any code to cope with the escape
character when finding a (thing-at-point 'symbol). This is slightly
less powerful than the default I proposed, but there are probably many
use cases where it won't matter at all (though it would for my own,
possibly eccentric, use case).

Does this sound to you like a plausible way forward?

I've tried to reach out to the AUCTeX developers to see what they
might want to do about setting the value of local variables there, and
anything they come up with should be doable.

 Thanks again.

On Thu, 14 Sept 2023 at 00:59, Dmitry Gutov <dgutov@yandex.ru> wrote:
>
> On 13/09/2023 20:01, David Fussner via Bug reports for GNU Emacs, the
> Swiss army knife of text editors wrote:
>
> >> These won't be affected either way, right? Because project-find-regexp
> >> defaults its input to (thing-at-point 'symbol t), and isearch...
> >> probably also uses "symbol" if you ask it to.
> >>
> >> So... why not just make tex-thingatpt-include-escape a boolean? What
> >> commands need to be distinguished that way? I think 'find-tag' (it's
> >> obsolete but still used sometimes) would need to obey this var as well.
> >
> > xref-find-apropos and xref-find-references don't work well (or at all)
> > with the escape char included in the search string, so I was keeping
> > that char away from them. (The buffer-local variables I manipulate for
> > project-find-regexp and isearch-forward-thing-at-point have to do with
> > ensuring they use the texsymbol thing in the first place -- see
> > tex--symbol-or-texsymbol.) Does that make sense?
>
> Hmm, I suppose I skipped over that part of the patch too quickly.
>
> Here's a potential problem with replacing the notion of "symbol": some
> other existing code (also working with TeX/LaTeX) might disagree, as it
> might have some existing notion of what a "symbol" in those modes is (as
> defined by the syntax table).
>
> In general, we change the notion of a symbol by either changing the
> mode's syntax table, or by augmenting its effect using
> syntax-propertize-function (which, for example, could propertize the
> backslashes inside the buffer as "symbol constituent"). The latter might
> actually be a change that would affect how 'M-x xref-find-references'
> works (it will likely start to consider those \tags as symbol
> occurrences together with the backslash). But like other changes of what
> is considered to be a "symbol" in a major mode, it could conflict with
> existing code.
>
> Anyway, I'm not saying you have to change the approach, but that's
> something to be aware of.
>
> And to look at it from another direction: if the default implementation
> of xref-find-references (and etags uses the very generic one) doesn't
> work for you, perhaps it would be worth it to define a TeX-specific Xref
> backend. That would perhaps take 20-30 lines of code total, most of them
> delegating to the etags backend, or the default impl. But while
> delegating, you can modify the passed argument - e.g. if it included a
> backslash, you could forward it to the default impl for "find
> references" without a backslash. Or - alternatively - call
> (project-find-regexp "...") with a more complex regexp of your choice.
> The first alternative could look like this:
>
>    (cl-defmethod xref-backend-references ((_backend (eql 'tex-etags))
> identifier)
>      (xref-backend-references 'etags (string-remove-prefix "\\"
> identifier)))
>
> > I'll look at find-tag, too; thanks for pointing that out.
>
> Doing the above choice on the level of Xref backend's methods
> would/should automatically make it work for all commands appropriately.
>
> >> Why not set the variable find-tag-default-function instead? That seems
> >> easier and more appropriate to do inside a major mode function.
> >
> > I settled on putting the symbol on the modes because I thought it was
> > simpler than setting the variable buffer-locally in all the in-tree
> > and AUCTeX modes, but I'll revisit this and see whether I can come up
> > with something better.
>
> Do AUCTeX modes inherit from tex-mode? Or all call
> tex-common-initialization? Then you could set that variable locally
> inside that function once.
>
> All in all, it might not be wise to modify the behavior of third-party
> packages from inside Emacs this way (they might have other expectations,
> or there's going to appear a new major mode that needs the same
> treatment anyway).
>
> Setting a variable to be used through mode inheritance or delegation is
> fine, but if that doesn't help, I would probably stop at defining a
> helper function or two and documenting how it should be used. And then
> maybe work with AUCTeX people to get the remaining necessary changes in
> from their side (or just leaving that up to the user, depending on how
> functional the default config ends up being).