From: "Pascal J. Bourguignon" <pjb@informatimago.com>
To: help-gnu-emacs@gnu.org
Subject: Re: `append' vs. `nconc'
Date: Thu, 31 Dec 2015 06:36:08 +0100 [thread overview]
Message-ID: <87vb7fch87.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: mailman.1402.1451534421.843.help-gnu-emacs@gnu.org
Emanuel Berg <embe8573@student.uu.se> writes:
> "Pascal J. Bourguignon" <pjb@informatimago.com>
> writes:
>
>>> OK, so for example this: (setq
>>> completion-ignored-extensions (append
>>> completion-ignored-extensions '(".bcf" ".run.xml"))
>>> ) Is better as: (nconc completion-ignored-extensions
>>> (list ".bcf" ".run.xml")) ?
>>
>> No. You should never use nconc only for effect (just
>> like delete, etc).
>
> What do you mean by "effect"?
Side effect, same thing.
(nconc nil '(1 2 3)) has no side effect!
> The reason I thought it fitting was the definitions of
> `append' and `ncons'. Here I want to change
> `completion-ignored-extensions'. `append' doesn't
> change it, which is why the assignment with `setq' was
> there. If it can't be removed (well, in the form of
> `setf'), there is still the gain of not creating the
> list unnecessarily, which in this case is a matter of
> principle but in other cases (with long lists) is
> a performance bottleneck?
>
>> Always re-assign the modified list:
>>
>> (setf completion-ignored-extensions (nconc
>> completion-ignored-extensions (list ".bcf"
>> ".run.xml")))
>>
>> otherwise it will break when the place is bound to
>> nil. (nil is a symbol and cannot be mutated into
>> a cons cell!).
>
> ?
>
> What place is bound to nil, when?
Any place you want, when it is.
(let ((mylist '()))
(nconc mylist (list 1 2 3))
mylist)
--> nil
(setf completion-ignored-extensions '())
(nconc completion-ignored-extensions (list ".o"))
completion-ignored-extensions
--> nil
--
__Pascal Bourguignon__ http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk
next prev parent reply other threads:[~2015-12-31 5:36 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 [this message]
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
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
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