Re: [Qemu-devel] [PATCH v2] Add .dir-locals.el file to configure emacs coding style

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Sep 15, 2015 at 09:31:08AM -0600, Eric Blake wrote:
> ping
> 
> On 08/24/2015 06:15 PM, John Snow wrote:
> > 
> > 
> > On 06/18/2015 10:05 AM, Markus Armbruster wrote:
> >> Peter Maydell <peter.maydell@linaro.org> writes:
> >>
> >>> On 18 June 2015 at 10:28, Markus Armbruster <armbru@redhat.com> wrote:
> >>>> However, I can't see how I could define a new C style there without
> >>>> pushing the "local variables" feature well beyond its intended use, and
> >>>> triggering the confirmation prompts.
> >>>
> >>> We wouldn't want to define a new C style, but in general the items
> >>> I have in my config over the Stroustrup defaults are going to be there
> >>> because I've noticed something where Stroustrup doesn't indent right...
> >>
> >> As far as I can see, the difference bwteen stroustrup style and yours is
> >> a few tweaks to c-offsets-alist and c-hanging-braces-alist.  I'm not
> >> sure how to do that from .dir-locals.el.
> >>
> >>>> If we take Dan's patch, every Emacs user who hasn't already configured a
> >>>> suitable style profits.  Users who have may have to adjust their
> >>>> configuration to work with or around Dan's patch.
> >>>
> >>> Is there some way to tell whether your emacs has picked up the local
> >>> style info rather than the one you have in your .emacs ?
> >>
> >> The obvious way is to check the buffer-local variables.  Buffer-local
> >> variable c-indentation-style is the name of the style in use.  Without
> >> anything in .emacs or local variables, it's "gnu".  With your
> >> (c-add-style "qemu" qemu-c-style) in .emacs, it should be "qemu".  With
> >> Dan's .dir-locals.el, it should be "stroustrup".
> >>
> >> When you can't or don't want to create or modify a .dir-locals.el, you
> >> can do something like this instead:
> >>
> >>     (dir-locals-set-class-variables 'qemu '((c-mode . ((c-file-style . "qemu")))))
> >>     (dir-locals-set-directory-class "~/work/qemu" 'qemu)
> >>
> >> This *overrides* .dir-locals.el in my testing.
> >>
> > 
> > ping -- any love for this for 2.5?

So just to re-cap.....

My original proposed .dir-locals.el was pretty short:

  ((c-mode . ((c-file-style . "stroustrup")
             (indent-tabs-mode . nil))))

IIUC, the quote above says it is possible to override this with more
developer custom styles if desired, so it shouldn't cause any obvious
disadvantage / problem to contributors to have this set by default.

I have tried an alternative .dir-locals.el that contains all of
Peter's rules (from http://wiki.linaro.org/PeterMaydell/QemuEmacsStyle)
which looks like this:


  ;; Based on https://wiki.linaro.org/PeterMaydell/QemuEmacsStyle
  ;; which aims to apply CODING_STYLE guidelines
  ((c-mode . (
      (c-file-style . "stroustrup")
      (indent-tabs-mode . nil)
      (tab-width . 8)
      (c-comment-only-line-offset . 1)
      (c-hanging-braces-alist . ((substatement-open before after)))
      (c-offsets-alist . (
          (statement-block-intro . +)
          (substatement-open . 0)
          (label . 0)
          (statement-cont . +)
          (innamespace . 0)
          (inline-open . 0)
      ))
      (c-hanging-braces-alist . (
          (brace-list-open)
          (brace-list-intro)
          (brace-list-entry)
          (brace-list-close)
          (brace-entry-open)
          (block-close . c-snug-do-while)
          ;; structs have hanging braces on open
          (class-open . (after))
          ;; ditto if statements
          (substatement-open . (after))
          ;; and no auto newline at the end
          (class-close)
      ))
  )))

The main downside/problem I found with having this more comprehensive
.dir-locals.el file, is that emacs will raise a warning at launch time:

"The local variables list in /home/berrange/src/virt/qemu
 contains vlaues that may not be safe (*).

 Do you want to apply it? You can type
  y -- to apply the local variables list.
  n -- to ignore the local variables list.
  ! -- to apply the local variables list and permanently
       mark these values (*) as safe (in the future, they
       will be set automatically)"

It will ask this every time it launches unless you say "!"
whereupon it modifies your $HOME/.emacs to disable the warning

Personally I be fine with both the minimal approach or the more
comprehensive approach of Peter's, but I'd probably tend towards
the minimal approach to avoid the warnings problem.

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|