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

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Message #14 received at 55395 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 55395 <at> debbugs.gnu.org,
 Mattias Engdegård <mattiase <at> acm.org>
Subject: Re: bug#55395: What does (1 2 3 . #2) mean?
Date: Fri, 13 May 2022 13:20:51 -0400

>> 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...

Agreed (and agreed with Andreas as well).
The main goal is to avoid inf-looping and the #NNN chosen is
somewhat arbitrary.

I don't think it's worth it to try and make those #NNN more precise.
If the arbitrariness of the specific #NNN chosen is a problem, we could
replace it with a fixed `#¡cycle!` or something like that (or if we
want it to be more user-readable we could print something like
#cycle:<OBJ> where OBJ is the first N layers of the rest of the cycle,
like (rho 0 2) => (1 2 1 2 . #cycle:(1 2 1 ...))

I'm personally more bothered by the fact that those #NNN use exactly the
same syntax as used with `print-circle` yet they don't have the
same semantics.


        Stefan

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