GNU bug report logs -
#71503
30.0.50; pcase-lambda (with "and" and "guard") does not work as expected
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Reported by: Mekeor Melire <mekeor <at> posteo.de>
Date: Wed, 12 Jun 2024 08:42:01 UTC
Severity: normal
Found in version 30.0.50
Done: Philip Kaludercic <philipk <at> posteo.net>
Bug is archived. No further changes may be made.
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> Cc: mekeor <at> posteo.de, 71503 <at> debbugs.gnu.org
> Date: Sat, 22 Jun 2024 11:37:02 +0300
> From: Eli Zaretskii <eliz <at> gnu.org>
>
> > Cc: 71503 <at> debbugs.gnu.org
> > From: Philip Kaludercic <philipk <at> posteo.net>
> > Date: Fri, 14 Jun 2024 08:42:25 +0000
> >
> > Mekeor Melire <mekeor <at> posteo.de> writes:
> >
> > > I was expecting these two expressions to evaluate to the same value.
> > > First, we call `pcase' on a value with a pattern involving `and' and
> > > `guard':
> > >
> > > (pcase "value"
> > > ((and v (guard (string= "not-value" v))) v))
> > > ;; => nil
> > >
> > > Second, let's use the same value and pattern, but this time using
> > > `pcase-lambda':
> > >
> > > (funcall
> > > (pcase-lambda
> > > ((and v (guard (string= "not-value" v)))) v)
> > > "value")
> > > ;; => "value"
> > >
> > > Am I missing something or is this a bug?
> >
> > The difference is that pcase-lambda doesn't do case-distinction, but
> > just pattern matching/destruncting. So if the pattern-matching fails,
> > then the variable is just not bound, instead of the entire expression
> > falling back to returning no value/nil. I am guessing you wanted to
> > have something like Scheme's `case-lambda'[0]? Or we could clarify this
> > point in the docstring.
>
> Would you mind suggesting a clarification for the doc string (and the
> ELisp manual as well)?
>
> Thanks.
>
>
>
>
This bug report was last modified 287 days ago.
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