Re: Allow xref to use other than current major-mode

[Dmitry Gutov <dgutov@yandex.ru>]

On 08/25/2015 06:31 PM, Stephen Leake wrote:

> cl-generic.el is preloaded, so we could use a cl-defgeneric, and
> dispatch on (mode (eql <major-mode>)).

A given implementation is, of course, free to use cl-defgeneric and 
dispatch on whatever it wants, internally.

> To make such a minor mode work, xref--read-identifier could be a
> cl-defgeneric that dispatches on (project <project-type).

The plan is to consolidate the various -function's as generic method, 
dispatching on a single xref-backend value. I haven't got around to this 
yet, blocked on find-file-delayed.

> But if it also
> dispatches on (mode (eql <major-mode>)), each backend would have to
> define all combinations; that's a pain.

If a backend needs to dispatch based on the current language, it will. 
It will do that inside the method implementations.

But many backends will only need to deal with one language, a couple, or 
won't really need to check the language of the current buffer.

> I've started writing such a thing, but it gets defeated by the fact that
> xref--read-identifier uses the buffer-local value of
> xref-identifier-completion-table-function, which is set by elisp-mode
> (or c-mode, etc, or by default to etags). There's no way to change that.

A backend is a combination of xref-find-function, xref-i-c-t-function 
and xref-i-a-p-function. If a minor mode changes the current backend, it 
would need to set all these variables (but see the planned change 
mentioned above).

> We could require the user to manually enable an xref minor mode in each
> buffer, but that's not friendly.

We have "globalized" minor modes, to be used in similar situations.

> I guess a global xref backend could somehow reset that variable, in all
> elisp buffers? Maybe a function on elisp-mode-hook.

A minor mode function is indeed normally added to xxx-mode-hook, whether 
it's by user's hand, or somehow automatically.

> So a better approach would be to define a way to disable emacs-lisp-mode
> (and all other major-modes) setting the xref buffer-local variables, so
> that the user choice of xref backend is respected. That would be much
> simpler than having to add a function to all the major mode hooks (most
> of which are unknown at load time).

You're speaking from the standpoint of wanting a single, global xref 
backend everywhere for the duration of the Emacs session.

As previous discussions showed, that approach isn't natural for everyone.

> We could define a new global variable xref-global-backend; if nil (the
> default),

We've discussed a very similar proposal, very recently. As long as a 
minor mode can take over major mode's setting anyway, there's no point 
in adding this kind of explicit override.

> major modes can set xref vars; if non-nil, it indicates which
> particular global backend is in use. It could be a function that returns
> a boolean indicating whether the global backend wants to control xref in
> the current buffer. So a semi-global backend could provide C and Ada
> xref, but not elisp xref, for example. It could be a list of such
> functions; only if all return nil should the major mode set the xref
> vars.

Sounds like an unnecessary complication.