GNU bug report logs - #12314
24.2.50; `add-to-history': use `setq' with `delete'

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From: Wolfgang Jenkner <wjenkner <at> inode.at>
To: "Drew Adams" <drew.adams <at> oracle.com>
Cc: 12314 <at> debbugs.gnu.org, 'Eli Zaretskii' <eliz <at> gnu.org>, cyd <at> gnu.org
Subject: bug#12314: 24.2.50; `add-to-history': use `setq' with `delete'
Date: Mon, 10 Sep 2012 13:54:40 +0200

On Sun, Sep 09 2012, Drew Adams wrote:

>> Variables that point to that list structure will point to something
>> whose value is unpredictable, a.k.a. "garbage".  It is enough to say
>> that the old value is garbage and shouldn't be used, IMO.
>
> No.  It all depends.  Lisp programs that use list modification do so sometimes
> for performance in calculating the new list, but more often they do so in order
> to take advantage of SHARING list structure.
>
> Consider the example I gave before:
>
> (setq a  '(1 2 3 4))
> (setq b  (cddr a))
>
> a => (1 2 3 4)
> b => (3 4)
>
> (delq 4 b)
>
> a => (1 2 3)
> b => (3)

Though using `delete' and friends this way would have unpredictable
results in ANSI Common Lisp[1] or Scheme (delete! in srfi1[2]), hence
may be regarded as questionable programming style in Emacs Lisp as well,
which I understand is Eli's point.

The following should work in both CL (except for the missing DEFVARs)
and Emacs Lisp, but sort of defeats the point about easy sharing ;-)

(setq a (list 1 2 3 4))
(setq b (cddr a))
(setf (cddr a) (setq b (delete 4 b)))

[1] http://www.lispworks.com/documentation/HyperSpec/Body/f_rm_rm.htm#delete
    http://www.lispworks.com/documentation/HyperSpec/Issues/iss293_w.htm

[2] http://srfi.schemers.org/srfi-1/srfi-1.html#LinearUpdateProcedures

Wolfgang

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