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From: | Paul Eggert |
Subject: | bug#36407: 27.0.50; `plist-get', `equal' etc. and circular "lists" |
Date: | Thu, 27 Jun 2019 15:51:37 -0700 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 |
I don't understand why the last two lines are there.
I thought those lines were needed to avoid an infinite loop in pathological cyclic cases. But on further thought you're right, they aren't needed. I installed that patch; 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.
I installed the attached doc patch to try to make this a bit clearer.
0001-Improve-equal-and-array-doc.patch
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