Re: xref overrides user xref-backend-functions

[Dmitry Gutov <dgutov@yandex.ru>]

On 12/01/2015 08:15 AM, Stephen Leake wrote:

>      (progn
>       (setq xref-backend-functions (lambda () 'my-xref-backend))
>       (require 'xref)
>       xref-backend-functions)

I don't think we've ever discussed being able to override the backend 
from the init file like that.

> This gives:
>
> (etags--xref-backend (lambda nil (quote my-xref-backend)))
>
> which is not what I want.

There's nothing surprising about it, though.

> This needs a comment on why etags--xref-backend is not set in the
> default value of the variable; it's not clear to me based on the bug
> report, and the bug report is not referenced from the code.

Either we do it like this, or we ask every piece of code that wants to 
(add-hook 'xref-backend-functions ...), to (require 'xref) first. And if 
it doesn't do that, the default value of xref-backend-functions is lost.

Almost none hook variables have default values, but many add to it after 
the definition using `add-hook'. So I did the same.

> I think a good fix is to not add the hook if already set:
>
> (unless xref-backend-functions
>   (add-hook 'xref-backend-functions #'etags--xref-backend))
>
> This works for my use case, and for bug#21964.

This will make things worse when someone else writes (add-hook 
'xref-backend-functions 'bar-backend) with the intention of seeing the 
element added to the beginning of the list, but still fall back to etags 
when their function fails.

> An alternate work-around is to require xref before setting
> xref-backend-functions.

That's one valid approach.

How about this, though?

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 8a69b07..cddb017 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -210,7 +210,7 @@ xref-backend-functions

  ;; We make the etags backend the default for now, until something
  ;; better comes along.
-(add-hook 'xref-backend-functions #'etags--xref-backend)
+(add-hook 'xref-backend-functions #'etags--xref-backend t)

  ;;;###autoload
  (defun xref-find-backend ()