GNU bug report logs - #25929
25.2; map-delete doesn't delete permanently 1st alist elt

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From: Richard Stallman <rms <at> gnu.org>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 25929 <at> debbugs.gnu.org, nicolas <at> petton.fr, tino.calancha <at> gmail.com
Subject: bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt
Date: Wed, 27 Apr 2022 23:17:20 -0400

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  > > *) It happens just for alist, other maps are fine.

  > (I'm going through old bug reports that unfortunately weren't resolved
  > at the time.)

  > Various things were proposed here, but they all had problems.  So I
  > don't think there's much to be done here except documenting this, which
  > I've now done in Emacs 29.

It is a natural consequence of list structure that deleting the first element
doesn't do it destructively, so you need to do
  (setq foo (delq elt foo))

This is a general principal of deletion in Lisp, so let's teach it to
people rather than trying to help people avoid the need to learn it.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)

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