From: "Drew Adams" <drew.adams@oracle.com>
To: "'Le Wang'" <l26wang@gmail.com>, <9469@debbugs.gnu.org>
Subject: bug#9469: buffer-local variables seem to remember previous values
Date: Sat, 10 Sep 2011 10:44:19 -0700 [thread overview]
Message-ID: <70AD30BE45C847BE828A3E8A71280A8E@us.oracle.com> (raw)
In-Reply-To: <CAM=K+ipbSAqy1PLo=qHRb_SqtKU+M6B1RRDv_LPym3P1ir2W4Q@mail.gmail.com>
> 1. emacs -Q
> 2. eval this region:
> (setq buf-a (create-file-buffer "a"))
> (setq foo nil)
> (make-variable-buffer-local 'foo)
> (defun test1 ()
> (interactive)
> (let (alist)
> (push '(:var . 0) alist)
> (with-current-buffer buf-a
> (setq foo alist))))
> (defun test2 ()
> (interactive)
> (with-current-buffer buf-a
> (setcdr (assq :var foo) 20)))
> (defun show ()
> (interactive)
> (with-current-buffer buf-a
> (format " ; foo in 'a' is %s" foo)))
> (defun test3 ()
> (interactive)
> (let (alist)
> (push `(:var . ,(+ 0)) alist)
> (with-current-buffer buf-a
> (setq foo alist))))
>
> (test1)
> (test2)
> (test1)
> (insert (show))
> (test3)
> (insert (show))
>
> Note results on both `insert' lines should be identical but the first
> insert some how remembers a previous value. I find it surprising that
> no one has ever come across this before.
No, they should not be identical. This is a classic Lisp gotcha.
(test1) sets buffer-local var `foo' to a new alist ((:var . 0)).
(test2) sets the cdr of the single element of that alist to 20.
That means that `foo' in buf-a is now ((:var . 20)).
(test1) then creates a new alist and pushes the _same_ cons, (:var . 20) onto
it.
And it sets `foo' to this new alist.
If you were to use (cons :var 0) instead of '(:var . 0) then you would not be
reusing the same cons cell.
Note that different Lisps (and different implementations of the same Lisp) can
treat a sexp such as '(a b) differently - they might or might not create a new
list each time it is read or eval'd.
To be sure to get what you expect in situations like this, do not use '(...).
Use `cons' or `list' or equivalent backquote syntax. Do not expect '(...) to
create new list structure each time it is read/eval'd.
next prev parent reply other threads:[~2011-09-10 17:44 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-09-10 17:07 bug#9469: buffer-local variables seem to remember previous values Le Wang
2011-09-10 17:44 ` Drew Adams [this message]
2011-09-11 16:57 ` Le Wang
2011-09-11 17:56 ` Drew Adams
2011-09-11 18:08 ` Andreas Schwab
2011-09-11 18:49 ` Le Wang
2011-09-11 18:54 ` Lars Magne Ingebrigtsen
2011-09-11 19:18 ` Le Wang
2011-09-11 19:54 ` Lars Magne Ingebrigtsen
2011-09-12 4:39 ` Le Wang
2011-09-12 7:57 ` Andreas Schwab
2011-09-12 8:23 ` Le Wang
2011-09-12 8:35 ` Andreas Schwab
2011-09-12 14:30 ` Drew Adams
2011-09-12 15:06 ` Le Wang
2011-09-13 12:54 ` Stefan Monnier
2011-09-13 13:12 ` Le Wang
2011-09-13 15:00 ` Helmut Eller
2011-09-13 18:02 ` Johan Bockgård
2011-09-13 18:52 ` Drew Adams
2011-09-13 20:42 ` Helmut Eller
2011-09-15 18:14 ` Le Wang
2011-09-11 19:37 ` Helmut Eller
2011-09-11 19:46 ` Andreas Schwab
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