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From: Ihor Radchenko <yantar92@posteo.net>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: Jim Porter <jporterbugs@gmail.com>, 68509@debbugs.gnu.org
Subject: bug#68509: 30.0.50; pcase-dolist matches backquote pattern incorrectly
Date: Mon, 19 Feb 2024 18:14:24 +0000	[thread overview]
Message-ID: <8734to8gov.fsf@localhost> (raw)
In-Reply-To: <jwvv86kv4cs.fsf-monnier+emacs@gnu.org>

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> * lisp/emacs-lisp/pcase.el (pcase-dolist): Use `pcase' rather than
>> `pcase-let*' to match the list elements.  Update the docstring,
>
> That changes its behavior.  It *will* break code.
> This is basically not the same macro any more.  Instead of being a macro
> that iterates over all the elements of the list, it becomes a macro
> which iterates only over the matching elements.

Currently, the docstring states:

    PATTERN should be a `pcase' pattern describing the structure of
    LIST elements, and LIST is a list of objects that match PATTERN,
    i.e. have a structure that is compatible with PATTERN.

It is undefined what happens when LIST objects do not match PATTERN.

So, any code relying upon iterating over non-matching elements is
relying upon undefined, undocumented behaviour.

Moreover, current implementation binds /partially matching/ list
elements, which is even more fragile. From `pcase-let' docstring:

    a mismatch may signal an error or may go
    undetected, binding variables to arbitrary values, such as nil.

What I suggest is to change what happens when PATTERN does not much more
explicitly.

In my patch, I proposed to skip such elements.
An alternative could be throwing an error.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>





  reply	other threads:[~2024-02-19 18:14 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-01-16 15:07 bug#68509: 30.0.50; pcase-dolist matches backquote pattern incorrectly Ihor Radchenko
2024-01-16 16:52 ` Jim Porter
2024-01-16 18:43   ` Ihor Radchenko
2024-02-19 10:05     ` Ihor Radchenko
2024-02-19 12:59       ` Eli Zaretskii
2024-02-19 15:53       ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-19 18:14         ` Ihor Radchenko [this message]
2024-02-20  2:41           ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-20 13:40             ` Ihor Radchenko
2024-02-20 14:35               ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-20 15:16                 ` Ihor Radchenko
2024-02-20 17:51                   ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-21 11:23                     ` Ihor Radchenko
2024-02-21 14:17                       ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-23 14:05                         ` Ihor Radchenko
2024-02-23 14:58                           ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-24 13:28                             ` Ihor Radchenko
2024-02-24 14:57                               ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-19 15:51   ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-02-19 18:16     ` Ihor Radchenko
2024-02-20  2:46       ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors

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