From: "Mattias Engdegård" <mattias.engdegard@gmail.com>
To: Pip Cet <pipcet@protonmail.com>
Cc: Stefan Monnier <monnier@iro.umontreal.ca>,
Stefan Kangas <stefankangas@gmail.com>,
Tassilo Horn <tsdh@gnu.org>, Eli Zaretskii <eliz@gnu.org>,
Anand Tamariya <atamariya@gmail.com>,
emacs-devel@gnu.org
Subject: Re: Negative nth index
Date: Wed, 25 Dec 2024 11:30:20 +0100 [thread overview]
Message-ID: <350E9109-04F6-4C20-B0F6-37C0A482DAD3@gmail.com> (raw)
In-Reply-To: <87r05wahsi.fsf@protonmail.com>
24 dec. 2024 kl. 22.33 skrev Pip Cet via Emacs development discussions. <emacs-devel@gnu.org>:
> I think the OP meant that (nth -1 '(1 2)) is 1 on current Emacs, which
> is definitely unexpected.
It's a straight consequence of (nth N L) = (car (nthcdr N L)).
>> I don't see a strong argument in favor of making it index from the end
>> rather than signal an error. What would the implementation look like?
>
> I agree, but the current implementation does not signal an error. It
> should, and I think that's what the suggestion was.
It is true that signalling an error for negative arguments might catch some bugs but doing so would also risk breaking working code that relies on the current (well-defined) behaviour.
And there's not much performance to gain from it: while a good compiler might use it for value range propagation, as in
(use (nth n lis))
(if (< n 0) ...) ; dead if nth signals on negative n
it doesn't really justify a change.
next prev parent reply other threads:[~2024-12-25 10:30 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-12-22 5:43 Negative nth index Anand Tamariya
2024-12-22 7:39 ` Eli Zaretskii
2024-12-22 9:01 ` Tassilo Horn
2024-12-23 5:40 ` Stefan Kangas
2024-12-24 5:11 ` Stefan Monnier
2024-12-24 6:42 ` Teemu Likonen
2024-12-24 6:48 ` Thierry Volpiatto
2024-12-24 13:09 ` Philip Kaludercic
2024-12-24 15:18 ` Stefan Monnier
2024-12-24 15:24 ` Sebastián Monía
2024-12-24 21:33 ` Pip Cet via Emacs development discussions.
2024-12-25 10:30 ` Mattias Engdegård [this message]
2024-12-25 10:56 ` Pip Cet via Emacs development discussions.
2024-12-25 12:14 ` Mattias Engdegård
2024-12-25 13:04 ` Pip Cet via Emacs development discussions.
2024-12-25 14:23 ` Stefan Kangas
2024-12-25 17:22 ` Stefan Monnier
2024-12-25 14:18 ` Stefan Kangas
2024-12-22 10:16 ` Andreas Schwab
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