From: Philip Kaludercic <philipk@posteo.net>
To: Eli Zaretskii <eliz@gnu.org>
Cc: mekeor@posteo.de, monnier@iro.umontreal.ca, 71503@debbugs.gnu.org
Subject: bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected
Date: Sat, 20 Jul 2024 10:29:55 +0000 [thread overview]
Message-ID: <87ikx0mk4c.fsf@posteo.net> (raw)
In-Reply-To: <865xt0e6wc.fsf@gnu.org> (Eli Zaretskii's message of "Sat, 20 Jul 2024 12:42:43 +0300")
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Eli Zaretskii <eliz@gnu.org> writes:
> Ping! Ping!
>
>> Cc: mekeor@posteo.de, monnier@iro.umontreal.ca, 71503@debbugs.gnu.org
>> Date: Sat, 06 Jul 2024 10:38:00 +0300
>> From: Eli Zaretskii <eliz@gnu.org>
>>
>> Ping!
>>
>> > Cc: mekeor@posteo.de, 71503@debbugs.gnu.org
>> > Date: Sat, 22 Jun 2024 11:37:02 +0300
>> > From: Eli Zaretskii <eliz@gnu.org>
>> >
>> > > 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)?
Sorry, I didn't realise you were pining me. I was thinking of something
like
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diff --git a/lisp/emacs-lisp/pcase.el b/lisp/emacs-lisp/pcase.el
index 5a7f3995311..f546ba34c7c 100644
--- a/lisp/emacs-lisp/pcase.el
+++ b/lisp/emacs-lisp/pcase.el
@@ -241,9 +241,10 @@ pcase-exhaustive
;;;###autoload
(defmacro pcase-lambda (lambda-list &rest body)
"Like `lambda' but allow each argument to be a pattern.
-I.e. accepts the usual &optional and &rest keywords, but every
-formal argument can be any pattern accepted by `pcase' (a mere
-variable name being but a special case of it)."
+I.e. accepts the usual &optional and &rest keywords, but every formal
+argument can be any pattern accepted by `pcase' (a mere variable name
+being but a special case of it). Keep in mind that BODY is always
+evaluated, regardless of whether the argument-patterns match or not."
(declare (doc-string 2) (indent defun)
(debug (&define (&rest pcase-PAT) lambda-doc def-body)))
(let* ((bindings ())
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but now I notice that `pcase-let*' documents that all expressions should
match,
Each EXP should match its respective PATTERN (i.e. be of structure
compatible to PATTERN); a mismatch may signal an error or may go
undetected, binding variables to arbitrary values, such as nil.
Since `pcase-lambda' inherits these semantics we would have to propagate
this promise -- or be more specific about what happens, e.g. binding
values to nil.
>> > Thanks.
--
Philip Kaludercic on peregrine
next prev parent reply other threads:[~2024-07-20 10:29 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-06-12 8:40 bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected Mekeor Melire
2024-06-14 8:42 ` Philip Kaludercic
2024-06-14 16:08 ` Drew Adams via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-06-22 8:37 ` Eli Zaretskii
2024-07-06 7:38 ` Eli Zaretskii
2024-07-20 9:42 ` Eli Zaretskii
2024-07-20 10:29 ` Philip Kaludercic [this message]
2024-07-20 10:46 ` Eli Zaretskii
2024-07-21 11:17 ` Philip Kaludercic
2024-07-21 13:33 ` Mekeor Melire
2024-07-20 14:06 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-07-21 12:39 ` Philip Kaludercic
2024-07-21 13:51 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-08-04 7:58 ` Eli Zaretskii
2024-08-04 14:54 ` Philip Kaludercic
2024-08-04 15:08 ` Eli Zaretskii
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