From: Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>
To: help-gnu-emacs@gnu.org
Subject: Re: Easy to add with push but not to the end of a list
Date: Tue, 06 Dec 2022 08:56:09 -0500 [thread overview]
Message-ID: <jwvtu28vdy1.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: 871qpdgsql.fsf@dataswamp.org
>> That supports generalized variables; you don't need to do
>> that (unless you want to).
>
> Just wonder why mine looks so different ...
Because when you want to do:
(push V (aref A (cl-incf I)))
the naive expansion gives:
(setf (aref A (cl-incf I))
(cons V (aref A (cl-incf I))))
which will increment I twice :-(
So the expansion need to look like:
(let ((tmp (cl-incf I)))
(setf (aref A tmp)
(cons V (aref A tmp))))
And of course, the same holds if A is not just a variable reference but
say a call to function that's costly or that is not pure.
Furthermore, the above performs the `cl-incf` before computing V, so if
V is an expression that uses I it changes the expected result compared
to the usual left-to-right evaluation order provided by ELisp.
So in the end you need something like:
(let ((tmp1 V)
(tmp2 A)
(tmp3 (cl-incf I)))
(setf (aref tmp2 tmp3)
(cons tmp1 (aref tmp2 tmp3))))
-- Stefan
next prev parent reply other threads:[~2022-12-06 13:56 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-28 2:26 Easy to add with push but not to the end of a list Heime
2022-11-28 2:43 ` [External] : " Drew Adams
2022-11-28 3:45 ` Heime
2022-11-28 6:11 ` Drew Adams
2022-11-28 20:00 ` Emanuel Berg
2022-11-28 5:18 ` Stefan Monnier via Users list for the GNU Emacs text editor
2022-11-28 20:19 ` Emanuel Berg
2022-11-28 21:56 ` [External] : " Drew Adams
2022-11-28 22:45 ` Emanuel Berg
2022-11-28 22:01 ` Heime
2022-11-28 22:24 ` Stefan Monnier via Users list for the GNU Emacs text editor
2022-11-28 22:58 ` Emanuel Berg
2022-11-30 14:10 ` tomas
2022-11-30 16:12 ` Emanuel Berg
2022-11-28 22:46 ` Emanuel Berg
2022-11-28 22:50 ` Emanuel Berg
2022-11-29 5:23 ` tomas
2022-11-29 5:32 ` Emanuel Berg
2022-11-29 7:56 ` Heime
2022-11-29 8:33 ` Marcin Borkowski
2022-11-29 10:00 ` Emanuel Berg
2022-11-29 10:05 ` Emanuel Berg
2022-11-29 10:15 ` Heime
2022-11-29 8:38 ` tomas
2022-11-29 9:54 ` Emanuel Berg
2022-11-29 8:17 ` Marcin Borkowski
2022-11-29 8:44 ` tomas
2022-11-29 12:08 ` Dr Rainer Woitok
2022-11-29 19:54 ` Of cars and mice [was: Easy to add with push but not to the end of a list] tomas
2022-11-30 1:17 ` Emanuel Berg
2022-11-30 1:13 ` Easy to add with push but not to the end of a list Emanuel Berg
2022-11-28 19:59 ` Emanuel Berg
2022-11-28 21:56 ` [External] : " Drew Adams
2022-11-28 22:18 ` Heime
2022-11-28 22:41 ` Drew Adams
2022-11-29 11:17 ` Dr Rainer Woitok
2022-11-30 17:10 ` Michael Heerdegen
2022-11-30 18:17 ` [External] : " Drew Adams
2022-11-30 19:30 ` Emanuel Berg
2022-12-01 15:11 ` Michael Heerdegen
2022-12-01 17:25 ` Emanuel Berg
2022-12-03 21:29 ` Michael Heerdegen
2022-12-05 23:35 ` Emanuel Berg
2022-12-06 1:36 ` Michael Heerdegen
2022-12-06 1:43 ` Emanuel Berg
2022-12-06 2:17 ` Michael Heerdegen
2022-12-06 2:40 ` Emanuel Berg
2022-12-06 13:56 ` Stefan Monnier via Users list for the GNU Emacs text editor [this message]
2022-12-06 14:45 ` Michael Heerdegen
2022-12-06 2:52 ` Emanuel Berg
2022-12-06 17:22 ` Michael Heerdegen
2022-12-06 22:30 ` Emanuel Berg
2022-12-08 23:15 ` Michael Heerdegen
2022-12-28 23:52 ` Emanuel Berg
2022-12-31 16:57 ` Michael Heerdegen
2022-12-31 22:09 ` Emanuel Berg
2023-01-01 12:16 ` Michael Heerdegen
2023-01-08 4:40 ` Emanuel Berg
2023-01-09 6:32 ` Emanuel Berg
2023-01-14 12:05 ` Michael Heerdegen
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