Subject: Re: bug#36407: 27.0.50; `plist-get', `equal' etc. and circular "lists"

From: Pip Cet <[email-masked]>
Date: Fri, 28 Jun 2019 08:05:10 +0000

On Thu, Jun 27, 2019 at 10:52 PM Paul Eggert <[email-masked]> wrote:
> aren't needed. I installed that patch; thanks.

Thanks!

> > shouldn't `equal' be symmetric?
>
> Yes, on its domain. But circular lists are outside its domain, and the
> documentation doesn't promise any particular behavior on them. It's OK
> if (equal a b) signals an error and (equal b a) does not. It's even OK
> if (equal a b) signals an error and a later call (equal a b) with
> exactly the same (unchanged) arguments does not (because the stack
> happens to have more room the second time). We still have symmetry in
> the sense that (eq (equal a b) (equal b a)) always either returns t or
> signals an error; it never returns nil.

Thanks for your explanation, that makes perfect sense.

I was confused, in part, by the hash table code in internal_equal,
which appears to be designed to handle circular structures with some
generality.

On further thought, maybe that code is written for DAGs which contain
diamond-shaped subgraphs. However, those don't appear to be working
very well...

> I installed the attached doc patch to try to make this a bit clearer.

Thanks again! I'm closing this bug.