From: Michael Heerdegen <michael_heerdegen@web.de>
To: Augusto Stoffel <arstoffel@gmail.com>
Cc: 62117@debbugs.gnu.org
Subject: bug#62117: 29.0.60; cl-letf on a map place has side-effects
Date: Sun, 12 Mar 2023 01:00:12 +0100 [thread overview]
Message-ID: <87sfeaj0tv.fsf@web.de> (raw)
In-Reply-To: <875yb7vijp.fsf@gmail.com> (Augusto Stoffel's message of "Sat, 11 Mar 2023 08:44:26 +0100")
Augusto Stoffel <arstoffel@gmail.com> writes:
> Consider this example:
>
> (require 'cl-lib)
> (defun f (map)
> (cl-letf (((map-elt map 'a) 1))
> map))
>
> (let ((map '(b 2)))
> (f map)
> map)
> => (b 2 a nil)
>
> (let ((map (make-hash-table)))
> (f map)
> (map-length map))
> => 1
>
>
> I would expect `f' to have no side effects, so get (b 2) and 0
> respectively in the two examples.
This is a symptom of a general limitation of `cl-letf'. Currently you can't
rely on a "no side effect" behavior. There are other examples like that
(`alist-get') and cases that are worse (binding `buffer-local-value' of
a variable in a buffer with no buffer local binding doesn't remove the
buffer-localness - that's one reason why that gv had been deprecated).
> Of course it's usual to treat a nil entry and no entry as equivalent in
> Lisp, but this behavior can be a problem e.g. when constructing data to
> pass to other programs.
I would say: if it is a problem, map.el is the wrong abstraction for
your case. That's the genuine idea of map.el: that the inner structure
of a map doesn't matter.
So I would close this one - unless you have some enlightening idea that
would be an obvious improvement with no downsides and backward
compatibility problems.
Thanks,
Michael.
next prev parent reply other threads:[~2023-03-12 0:00 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-11 7:44 bug#62117: 29.0.60; cl-letf on a map place has side-effects Augusto Stoffel
2023-03-12 0:00 ` Michael Heerdegen [this message]
2023-03-12 6:09 ` Augusto Stoffel
2023-03-15 2:13 ` Michael Heerdegen
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87sfeaj0tv.fsf@web.de \
--to=michael_heerdegen@web.de \
--cc=62117@debbugs.gnu.org \
--cc=arstoffel@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.