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bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work
From: |
Eli Zaretskii |
Subject: |
bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected |
Date: |
Sat, 22 Jun 2024 11:37:02 +0300 |
> Cc: 71503@debbugs.gnu.org
> From: Philip Kaludercic <philipk@posteo.net>
> Date: Fri, 14 Jun 2024 08:42:25 +0000
>
> Mekeor Melire <mekeor@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.