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From: Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
To: Ihor Radchenko <yantar92@posteo.net>
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 21:46:12 -0500	[thread overview]
Message-ID: <jwvfrxnuaa3.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <87zfvw720q.fsf@localhost> (Ihor Radchenko's message of "Mon, 19 Feb 2024 18:16:37 +0000")

>> The difficulty in resolving this can be illustrated with the following
>> pattern:
>>
>>     `(a . ,b)
>>
>> This pattern leads to two tests: (consp VAL) and (eq 'a (car VAL)).
>> When destructuring, we want to throw away both tests (we want to throw
>> away most tests, except those needed to choose between two `or`
>> branches).
>>
>> We could decide to emit a warning because we silently skip
>> the `eq` test, which would help the coders understand that the pattern
>> doesn't do what they think.
>> But emitting that same warning because we silently skip the `consp` test
>> would be really annoying because rewriting the pattern to avoid this
>> is impractical.
>
> May you please elaborate how this example is related to destructuring
> from user perspective? It appears to me that you are talking about
> implementation details of `pcase', which are elusive to me.

I'm just reading aloud what the `(a . ,b) pattern means: it means "VAL
has to be a cons, and its car should be equal to `a`" (and in addition
to that, we'll extract the `cdr` and put it into the `b` variable).

We might want to warn the user that such a pattern in `pcase-let` and
`pcase-dolist` is probably an error because the `a` won't actually be
checked, so it will behave just like the `(,_ . ,b) pattern.

But warning the user that the `consp` check will also be dropped would
be annoying because it's not like the users can easily write a different
pattern that still behaves the same but does not imply such
a `consp` check.


        Stefan






      reply	other threads:[~2024-02-20  2:46 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
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 [this message]

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