GNU bug report logs - #19734
25.0.50; nested backquotes: recent changes not backward compatible

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: michael_heerdegen <at> web.de
Subject: bug#19734: closed (Re: bug#19734: 25.0.50; nested backquotes:
 recent changes not backward compatible)
Date: Fri, 30 Jan 2015 21:03:02 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#19734: 25.0.50; nested backquotes: recent changes not backward compatible

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 19734 <at> debbugs.gnu.org.

-- 
19734: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19734
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 19734-done <at> debbugs.gnu.org
Subject: Re: bug#19734: 25.0.50;
 nested backquotes: recent changes not backward compatible
Date: Fri, 30 Jan 2015 16:01:55 -0500

>     * lisp/emacs-lisp/backquote.el (backquote-process): Optimize away
>       the ,' case.
> were not backwards compatible.  If this was intended (?), it should be
> mentioned in the news I think.

As the commit message indicates, this was supposed to be an
optimization, i.e. no change in observable behavior.

> In trunk, the backquote expression evals to something different now, and
>     ,,'(my-make-feh-cmd)
> from above must now be
>       ,(my-make-feh-cmd)

Yes, your rewrite is correct and desirable, but indeed we had a bug
which I've just fixed in master.  Thanks.


        Stefan

[Message part 3 (message/rfc822, inline)]

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: bug-gnu-emacs <at> gnu.org
Subject: 25.0.50; nested backquotes: recent changes not backward compatible
Date: Fri, 30 Jan 2015 18:49:52 +0100

Hello,

Recent changes in backquote.el, I guess especially in this commit:

86009dd5d886f1101358990e4f8f69a5d1467eb8
Author: Stefan Monnier <monnier <at> iro.umontreal.ca>
Date:   Sat Nov 15 23:59:50 2014 -0500

    * lisp/emacs-lisp/backquote.el (backquote-process): Optimize away
      the ,' case.

were not backwards compatible.  If this was intended (?), it should be
mentioned in the news I think.

Here is an example.  In Emacs 24, I had something like this in my config
(massively shortened):


--8<---------------cut here---------------start------------->8---
(defun my-make-feh-cmd ()
  "Return a feh command line suitble for dired.
Take the current screen resolution at account."
  (if (not window-system)
      "feh"
    (let* ((display-pixel-width (display-pixel-width))
           (width (- display-pixel-width (frame-pixel-width))))
      (format "feh -Z -g %sx%s+%s *"
              width
              (- (x-display-pixel-height) 30)
              (min (+ (frame-pixel-width) 2) (- display-pixel-width width))))))

(require 'dired-x)

(let ((programs '("vlc *" "smplayer *")))
  (setq-default
   dired-guess-shell-alist-user
   `((".*" (if (file-directory-p file)
               `(,,'(my-make-feh-cmd) ;compute at runtime!
                 ,@',programs)
             ',programs)))))
--8<---------------cut here---------------end--------------->8---


In trunk, the backquote expression evals to something different now, and
AFAICT

    ,,'(my-make-feh-cmd)

from above must now be

      ,(my-make-feh-cmd)

That looks less frightening, but I had to find this out myself because
my code was broken and there was no news entry.


Thanks,

Michael.




In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.5)
 of 2015-01-29 on drachen
Repository revision: 1dc1959ebf50401c69adeb6d182950b59835f94d
Windowing system distributor `The X.Org Foundation', version 11.0.11602901
System Description:	Debian GNU/Linux 8.0 (jessie)

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GSETTINGS NOTIFY
LIBXML2 FREETYPE XFT ZLIB

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