From: <tomas@tuxteam.de>
To: help-gnu-emacs@gnu.org
Subject: Re: `append' vs. `nconc'
Date: Thu, 31 Dec 2015 10:30:45 +0100 [thread overview]
Message-ID: <20151231093045.GB19900@tuxteam.de> (raw)
In-Reply-To: <87si2jqee2.fsf@debian.uxu>
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
On Thu, Dec 31, 2015 at 08:13:41AM +0100, Emanuel Berg wrote:
> "Pascal J. Bourguignon" <pjb@informatimago.com>
> writes:
>
> > You must remember the literal/immutable status of
> > each item at each level.
> >
> > Since you are incapable of remembering it, you
> > should assume that the whole input data is immutable
> > and write purely functional code (ie. use append,
> > not nconc) in general.
>
> OK, so you use `nconc' when you yourself create the
> lists by hand and thus know they are not empty, all
> the while using `list' and not ', and when done you
> assign the result to a variable associated with a list
> INSTEAD of using `nconc' directly because that
> variable can hold the empty list, i.e. nil, which
> `nconc' contrary to `append' cannot handle. And you do
> this to save time!
Or, to squint differently at it: you use nconc when "nobody
is looking", i.e. when you *know* that there are no references
to the data being mutated (unless you *want* that the world
changes for those other watchers too).
OTOH, nconc is "just an optimization", because GC isn't ideal.
So a good rule of thumb:
- it's very obvious
It's a local value you are keeping in your little scope.
(One might argue that then, it's an idiomatic way to
emphasize this fact to the (human) reader...)
- you are really pressed for time
It's somewhere deep in many nested loops.
Otherwise... just don't use it.
Regards
- -- tomás
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.12 (GNU/Linux)
iEYEARECAAYFAlaE9cUACgkQBcgs9XrR2kZmSQCfRKgjktl8mIPS6/Eh/XkkHNiE
0E8AoIFZKrKi9bB1q0Ujul/IdXad8WX6
=Q8vS
-----END PGP SIGNATURE-----
next prev parent reply other threads:[~2015-12-31 9:30 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-28 16:35 Error: Setting the shell in sh-mode via a local variable does not work Johannes Kastl
2015-12-29 1:32 ` Emanuel Berg
2015-12-29 18:33 ` Johannes Kastl
2015-12-30 14:53 ` Emanuel Berg
2015-12-30 15:15 ` Robert Thorpe
2015-12-30 15:30 ` Emanuel Berg
2015-12-30 17:42 ` Johannes Kastl
2015-12-30 15:19 ` Teemu Likonen
2015-12-30 15:41 ` `append' vs. `nconc' (was: Re: Error: Setting the shell in sh-mode via a local variable does not work) Emanuel Berg
2015-12-30 16:37 ` `append' vs. `nconc' Teemu Likonen
2015-12-31 3:37 ` Emanuel Berg
[not found] ` <mailman.1400.1451533083.843.help-gnu-emacs@gnu.org>
2015-12-31 3:45 ` Pascal J. Bourguignon
2015-12-31 4:00 ` Emanuel Berg
2015-12-31 9:22 ` tomas
2015-12-31 18:48 ` Emanuel Berg
[not found] ` <mailman.1402.1451534421.843.help-gnu-emacs@gnu.org>
2015-12-31 5:36 ` Pascal J. Bourguignon
2015-12-31 3:50 ` Emanuel Berg
[not found] ` <mailman.1401.1451533833.843.help-gnu-emacs@gnu.org>
2015-12-31 5:37 ` Pascal J. Bourguignon
2015-12-31 7:13 ` Emanuel Berg
2015-12-31 9:30 ` tomas [this message]
2015-12-31 17:56 ` side effects, list sharing [was: `append' vs. `nconc'] Drew Adams
2015-12-31 19:00 ` Emanuel Berg
2015-12-31 19:15 ` Drew Adams
2016-01-01 13:20 ` Emanuel Berg
2015-12-31 18:51 ` `append' vs. `nconc' Emanuel Berg
[not found] ` <mailman.1440.1451588113.843.help-gnu-emacs@gnu.org>
2016-01-01 2:53 ` Barry Margolin
2016-01-01 13:26 ` Emanuel Berg
[not found] ` <mailman.1406.1451546036.843.help-gnu-emacs@gnu.org>
2015-12-31 8:54 ` Pascal J. Bourguignon
2015-12-31 7:31 ` Teemu Likonen
2015-12-31 18:35 ` Emanuel Berg
2015-12-31 20:04 ` Teemu Likonen
2016-01-01 13:23 ` Emanuel Berg
2016-01-01 14:02 ` Teemu Likonen
2016-01-01 18:31 ` Emanuel Berg
[not found] ` <mailman.1438.1451586967.843.help-gnu-emacs@gnu.org>
2016-01-01 6:25 ` Pascal J. Bourguignon
2016-01-01 13:31 ` Emanuel Berg
2016-01-01 15:04 ` Drew Adams
[not found] ` <mailman.1353.1451490125.843.help-gnu-emacs@gnu.org>
2015-12-30 16:18 ` Pascal J. Bourguignon
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20151231093045.GB19900@tuxteam.de \
--to=tomas@tuxteam.de \
--cc=help-gnu-emacs@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.