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

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Philip Kaludercic <philipk <at> posteo.net>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#71503: closed (30.0.50; pcase-lambda (with "and" and "guard")
 does not work as expected)
Date: Sun, 04 Aug 2024 14:56:02 +0000

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Your message dated Sun, 04 Aug 2024 14:54:44 +0000
with message-id <87bk28z6bv.fsf <at> posteo.net>
and subject line Re: bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected
has caused the debbugs.gnu.org bug report #71503,
regarding 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected
to be marked as done.

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From: Mekeor Melire <mekeor <at> posteo.de>
To: bug-gnu-emacs <at> gnu.org
Subject: 30.0.50; pcase-lambda (with "and" and "guard") does not work as
 expected
Date: Wed, 12 Jun 2024 08:40:54 +0000

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?

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.41, cairo version 1.16.0).

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From: Philip Kaludercic <philipk <at> posteo.net>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: mekeor <at> posteo.de, Stefan Monnier <monnier <at> iro.umontreal.ca>,
 71503-done <at> debbugs.gnu.org
Subject: Re: bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does
 not work as expected
Date: Sun, 04 Aug 2024 14:54:44 +0000

Eli Zaretskii <eliz <at> gnu.org> writes:

> Ping!  can we please make progress with this issue, one way or
> another?

I've pushed the change in 0756f308 that Mekeor previously acknowledged
as fine.  Closing.

>> From: Stefan Monnier <monnier <at> iro.umontreal.ca>
>> Cc: Eli Zaretskii <eliz <at> gnu.org>,  mekeor <at> posteo.de,  71503 <at> debbugs.gnu.org
>> Date: Sun, 21 Jul 2024 09:51:04 -0400
>> 
>> > Another confusing behaviour that we should document is that in
>> >
>> > (let ((a nil))
>> >   (funcall
>> >    (pcase-lambda (a (and (guard a) b))
>> >      (list a b))
>> >    1 2))
>> >
>> > I would expect that the (guard a) would use the nil binding from the
>> > `let' form, but instead it is ignored and b is bound to 2.  I am not
>> > sure if this is related to the "destructing" point or not.
>> 
>> It most definitely is: the `guard` controls here whether the pattern
>> matches or not, but since it's a destructuring match `pcase` takes it
>> for granted that the pattern does match, so the guard is ignored.
>> 
>> What might be more confusing is the scoping, e.g.:
>> 
>>     (macroexpand '(pcase-lambda ((and (guard a) b) a) (FOO)))
>> =>
>>     #'(lambda (arg0 a) (pcase-let* (((and (guard a) b) arg0)) (FOO)))
>> 
>> so the `a` in the guard refers to the argument that follows rather than
>> to a surrounding `a` binding.
>> 
>> 
>>         Stefan
>> 
>> 

-- 
	Philip Kaludercic on peregrine

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