GNU bug report logs - #55395
What does (1 2 3 . #2) mean?

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Message #8 received at 55395 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: 55395 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#55395: What does (1 2 3 . #2) mean?
Date: Fri, 13 May 2022 17:22:13 +0200

Mattias Engdegård <mattiase <at> acm.org> writes:

> It may have made more sense before the switch of cycle-detection algorithm from Floyd to Brent. This can be fixed by hand-coding the list iteration and explicitly remembering the index of the tortoise, but would that be correct? What's the spec?

I don't think I've ever considered #x to be meaningful outside of
print-circle, but I guess if we wanted to have some semantics here, I
think I would have expected the index of the tortoise?  But...

> If #N means 'Nth object from the top along the path to the current object, starting at 0' then we should have
>
> (rho 2 3) => (1 2 3 4 5 . #2)
> (list (rho 2 3)) => ((1 2 3 4 5 . #3))
>
> ie, adding the print depth to the index in the list. Do you agree?

I've added Stefan to the CCs; I'm sure he has an opinion.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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