GNU bug report logs - #71503
30.0.50; pcase-lambda (with "and" and "guard") does not work as expected

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From: Eli Zaretskii <eliz <at> gnu.org>
To: philipk <at> posteo.net
Cc: mekeor <at> posteo.de, monnier <at> iro.umontreal.ca, 71503 <at> debbugs.gnu.org
Subject: bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected
Date: Sat, 06 Jul 2024 10:38:00 +0300

Ping!

> 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.
> 
> 
> 
> 

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