GNU bug report logs - #9469
buffer-local variables seem to remember previous values

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From: Helmut Eller <eller.helmut <at> gmail.com>
To: 9469 <at> debbugs.gnu.org
Subject: bug#9469: buffer-local variables seem to remember previous values
Date: Sun, 11 Sep 2011 21:37:25 +0200

* Lars Magne Ingebrigtsen [2011-09-11 18:54] writes:

> Le Wang <l26wang <at> gmail.com> writes:
>
>>> This has nothing to do with the use of quote, but with the use of
>>> destructive functions (setcdr) on a shared data structure.
>>
>> Of course you're technically right, but should the manual mention in
>> the "quoting" section that using quote the way I did results in a
>> shared data structure?  This part was surprising to me.
>
> If I read your code correctly, Andreas is correct and precise as usual,
> if a bit terse.  :-)
>
> The thing you're seeing has absolutely nothing to do with the use of
> quote.
>
> You're destructively altering a single list and expecting to see the
> list be different in various buffers.

Well, quote is tricky!  It wouldn't hurt to point out that modifying a
quoted list often means modifying your code.  E.g.

(defun foo () 
  (let ((list '(1 2 3)))
    (setcdr list (cddr list))))

(equal (copy-tree (symbol-function 'foo))
       (progn
	 (foo)
	 (symbol-function 'foo)))

returns nil.  I think few beginners will see that using setcdr here not
only modifies the value stored in the variable list but also modifies
the s-exp stored in 'foo.

Helmut

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