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From: Stefan Monnier <monnier@HIDDEN>
To: Ihor Radchenko <yantar92@HIDDEN>
Subject: Re: bug#68509: 30.0.50; pcase-dolist matches backquote pattern
incorrectly
In-Reply-To: <871q98hiqa.fsf@localhost> (Ihor Radchenko's message of "Mon, 19
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> * lisp/emacs-lisp/pcase.el (pcase-dolist): Use `pcase' rather than
> `pcase-let*' to match the list elements. Update the docstring,
That changes its behavior. It *will* break code.
This is basically not the same macro any more. Instead of being a macro
that iterates over all the elements of the list, it becomes a macro
which iterates only over the matching elements.
Stefan
bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
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From: Stefan Monnier <monnier@HIDDEN>
To: Jim Porter <jporterbugs@HIDDEN>
Subject: Re: bug#68509: 30.0.50; pcase-dolist matches backquote pattern
incorrectly
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>> Consider the following
>> (pcase-dolist (`(,(and (pred stringp) a) .
>> ,(and (pred stringp) b))
>> '(("TODO") ("DONE" . "a")))
>> (warn "%S :: %S" a b))
>> Executing the above yields
>> =E2=9B=94 Warning (emacs): "TODO" :: nil
>> =E2=9B=94 Warning (emacs): "DONE" :: "a"
>> even though ("TODO") does not match the pattern.
>
> This isn't an issue with 'pcase-dolist', but rather a known/intentional
> limitation of 'pcase-let':
Indeed, I consider the above a pilot error.
`pcase-dolist` and `pcase-let` use Pcase patterns to do *destructuring*,
which is a different task than the one done by `pcase` (which decides
whether a value matches a pattern or not).
>> Each EXP should match (i.e. be of compatible structure) to its
>> respective PATTERN; a mismatch may signal an error or may go
>> undetected, binding variables to arbitrary values, such as nil.
> I do think we should fix it somehow though. This behavior is
> extremely confusing, and as much as I'm a fan of 'pcase', I'm
> emphatically *not* a fan of how this part works.
I'm quite happy with the way it works when you use it as intended.
But I'm not really satisfied either with the way it behaves when the
coder doesn't understand its semantics, nor about the way we document
that semantics.
The difficulty in resolving this can be illustrated with the following
pattern:
`(a . ,b)
This pattern leads to two tests: (consp VAL) and (eq 'a (car VAL)).
When destructuring, we want to throw away both tests (we want to throw
away most tests, except those needed to choose between two `or`
branches).
We could decide to emit a warning because we silently skip
the `eq` test, which would help the coders understand that the pattern
doesn't do what they think.
But emitting that same warning because we silently skip the `consp` test
would be really annoying because rewriting the pattern to avoid this
is impractical.
For a human, it's pretty easy to distinguish those two cases. But it's
difficult to provide a precise definition that distinguishes those two case=
s.
We could also keep the tests and emit a warning or even an error when
they fail, but if that's the behavior you want, then you should
arguably use `pcase(-exhaustive)` instead.
Stefan
bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
Full text available.Received: (at 68509) by debbugs.gnu.org; 19 Feb 2024 12:59:47 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Mon Feb 19 07:59:47 2024 Received: from localhost ([127.0.0.1]:40670 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1rc3FJ-0004l7-Lu for submit <at> debbugs.gnu.org; Mon, 19 Feb 2024 07:59:47 -0500 Received: from eggs.gnu.org ([209.51.188.92]:34838) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from <eliz@HIDDEN>) id 1rc3FF-0004kp-PK for 68509 <at> debbugs.gnu.org; Mon, 19 Feb 2024 07:59:43 -0500 Received: from fencepost.gnu.org ([2001:470:142:3::e]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from <eliz@HIDDEN>) id 1rc3En-0000vE-Lu; Mon, 19 Feb 2024 07:59:14 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=gnu.org; s=fencepost-gnu-org; h=References:Subject:In-Reply-To:To:From:Date: mime-version; bh=jXb2dk+P5wQjbr4buLF9jvXCOtDyChHe7bOMcRThoq0=; b=nFd3HF3A3B6p RWWIvDru4pCcE6+PLM2Fqpxhm5w1h7LfnQzPslGkEEqE41Pc9MA6DFy9r0TIYeDPDhIbHX2e8FejK KUCBqN7xbAPzh/QFH4IkLt+Cg3o6ACESOArg24zlkOPArtA/k/NyiWo+Iuc5uJudTpTGaZB2SVz67 EfaVLubtUXmN7XiLQxbLsu0XX1fQ2EjhKzd9z9y9DLPgjsSYy9wZHZ5zcWVcemSwdV5ncalfamaVN +rr+8Y9Ynvk+DH2rBYZmwW5qROxEG4X89ZSa5ctr/nYmAZ6eDhzrIwuFt7BLVTNsnPj8lQodkGvrc 1NhmylvfLgd9NuTXAA4/0w==; Date: Mon, 19 Feb 2024 14:59:09 +0200 Message-Id: <86sf1opq3m.fsf@HIDDEN> From: Eli Zaretskii <eliz@HIDDEN> To: Ihor Radchenko <yantar92@HIDDEN>, Stefan Monnier <monnier@HIDDEN> In-Reply-To: <871q98hiqa.fsf@localhost> (message from Ihor Radchenko on Mon, 19 Feb 2024 10:05:33 +0000) Subject: Re: bug#68509: 30.0.50; pcase-dolist matches backquote pattern incorrectly References: <87a5p5l3jm.fsf@localhost> <9bc2bd05-5fdb-9d5a-3d98-c344c7275027@HIDDEN> <87sf2xjezi.fsf@localhost> <871q98hiqa.fsf@localhost> X-Spam-Score: -4.2 (----) X-Debbugs-Envelope-To: 68509 Cc: jporterbugs@HIDDEN, 68509 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <https://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <https://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit/> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <https://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org> X-Spam-Score: -5.2 (-----) > Cc: 68509 <at> debbugs.gnu.org > From: Ihor Radchenko <yantar92@HIDDEN> > Date: Mon, 19 Feb 2024 10:05:33 +0000 > > Ihor Radchenko <yantar92@HIDDEN> writes: > > > Not sure about pcase-let, but pcase-dolist specifically may be > > simplified not to use pcase-let: > > > > (if (pcase--trivial-upat-p (car spec)) > > `(dolist ,spec ,@body) > > (let ((tmpvar (gensym "x"))) > > `(dolist (,tmpvar ,@(cdr spec)) > > (pcase ,tmpvar (,(car spec) ,@body))))) > > See the attached patch. > If the patch is acceptable, we also need to update the manual. Thanks. Adding Stefan to the discussion.
bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
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From: Ihor Radchenko <yantar92@HIDDEN>
To: Jim Porter <jporterbugs@HIDDEN>
Subject: Re: bug#68509: 30.0.50; pcase-dolist matches backquote pattern
incorrectly
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--=-=-=
Content-Type: text/plain
Ihor Radchenko <yantar92@HIDDEN> writes:
> Not sure about pcase-let, but pcase-dolist specifically may be
> simplified not to use pcase-let:
>
> (if (pcase--trivial-upat-p (car spec))
> `(dolist ,spec ,@body)
> (let ((tmpvar (gensym "x")))
> `(dolist (,tmpvar ,@(cdr spec))
> (pcase ,tmpvar (,(car spec) ,@body)))))
See the attached patch.
If the patch is acceptable, we also need to update the manual.
--=-=-=
Content-Type: text/x-patch
Content-Disposition: inline;
filename=0001-pcase-let-Skip-LIST-element-that-do-not-match-the-PA.patch
From f3e1362f7687c731e0ba4e410f005252309ffc3f Mon Sep 17 00:00:00 2001
Message-ID: <f3e1362f7687c731e0ba4e410f005252309ffc3f.1708337064.git.yantar92@HIDDEN>
From: Ihor Radchenko <yantar92@HIDDEN>
Date: Mon, 19 Feb 2024 13:02:21 +0300
Subject: [PATCH] pcase-let: Skip LIST element that do not match the PATTERN
(bug#68509)
* lisp/emacs-lisp/pcase.el (pcase-dolist): Use `pcase' rather than
`pcase-let*' to match the list elements. Update the docstring,
describing the behavior when list elements to not match the pattern.
The previous undefined behavior is removed.
* test/lisp/emacs-lisp/pcase-tests.el (pcase-tests-pcase-dolist): Add
new test.
---
lisp/emacs-lisp/pcase.el | 13 ++++++-------
test/lisp/emacs-lisp/pcase-tests.el | 12 ++++++++++++
2 files changed, 18 insertions(+), 7 deletions(-)
diff --git a/lisp/emacs-lisp/pcase.el b/lisp/emacs-lisp/pcase.el
index ae9bd87997c..8dc11b20a6f 100644
--- a/lisp/emacs-lisp/pcase.el
+++ b/lisp/emacs-lisp/pcase.el
@@ -329,21 +329,20 @@ pcase-let
(defmacro pcase-dolist (spec &rest body)
"Eval BODY once for each set of bindings defined by PATTERN and LIST elements.
PATTERN should be a `pcase' pattern describing the structure of
-LIST elements, and LIST is a list of objects that match PATTERN,
-i.e. have a structure that is compatible with PATTERN.
+LIST elements, and LIST is a list of objects.
For each element of LIST, this macro binds the variables in
PATTERN to the corresponding subfields of the LIST element, and
-then evaluates BODY with these bindings in effect. The
-destructuring bindings of variables in PATTERN to the subfields
-of the elements of LIST is performed as if by `pcase-let'.
+then evaluates BODY with these bindings in effect. When an element does
+not match the pattern, such element is skipped.
+The destructuring bindings of variables in PATTERN to the subfields
+of the elements of LIST is performed as if by `pcase'.
\n(fn (PATTERN LIST) BODY...)"
(declare (indent 1) (debug ((pcase-PAT form) body)))
(if (pcase--trivial-upat-p (car spec))
`(dolist ,spec ,@body)
(let ((tmpvar (gensym "x")))
`(dolist (,tmpvar ,@(cdr spec))
- (pcase-let* ((,(car spec) ,tmpvar))
- ,@body)))))
+ (pcase ,tmpvar (,(car spec) ,@body))))))
;;;###autoload
(defmacro pcase-setq (pat val &rest args)
diff --git a/test/lisp/emacs-lisp/pcase-tests.el b/test/lisp/emacs-lisp/pcase-tests.el
index d062965952a..241729c108a 100644
--- a/test/lisp/emacs-lisp/pcase-tests.el
+++ b/test/lisp/emacs-lisp/pcase-tests.el
@@ -160,4 +160,16 @@ pcase-tests-setq
(should-error (pcase-setq a)
:type '(wrong-number-of-arguments)))
+(ert-deftest pcase-tests-pcase-dolist ()
+ ;; Ignore non-matching elements.
+ (should
+ (equal
+ '(("DONE" . "a"))
+ (let (result)
+ (pcase-dolist (`(,(and (pred stringp) a) .
+ ,(and (pred stringp) b))
+ '(("TODO") ("DONE" . "a")))
+ (push (cons a b) result))
+ (nreverse result)))))
+
;;; pcase-tests.el ends here.
--
2.43.0
--=-=-=
Content-Type: text/plain
--
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>
--=-=-=--
bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
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From: Ihor Radchenko <yantar92@HIDDEN>
To: Jim Porter <jporterbugs@HIDDEN>
Subject: Re: bug#68509: 30.0.50; pcase-dolist matches backquote pattern
incorrectly
In-Reply-To: <9bc2bd05-5fdb-9d5a-3d98-c344c7275027@HIDDEN>
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<9bc2bd05-5fdb-9d5a-3d98-c344c7275027@HIDDEN>
Date: Tue, 16 Jan 2024 18:43:45 +0000
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Jim Porter <jporterbugs@HIDDEN> writes:
> This isn't an issue with 'pcase-dolist', but rather a known/intentional
> limitation of 'pcase-let':
>
> (pcase-let ((`(,(and (pred stringp) a) .
> ,(and (pred stringp) b))
> '("TODO")))
> (warn "%S :: %S" a b))
> -> Warning (emacs): "TODO" :: nil
>
> The 'pcase-let' docstring says this:
>
>> Each EXP should match (i.e. be of compatible structure) to its
>> respective PATTERN; a mismatch may signal an error or may go
>> undetected, binding variables to arbitrary values, such as nil.
>
> I do think we should fix it somehow though. This behavior is extremely
> confusing, and as much as I'm a fan of 'pcase', I'm emphatically *not* a
> fan of how this part works.
Not sure about pcase-let, but pcase-dolist specifically may be
simplified not to use pcase-let:
(if (pcase--trivial-upat-p (car spec))
`(dolist ,spec ,@body)
(let ((tmpvar (gensym "x")))
`(dolist (,tmpvar ,@(cdr spec))
(pcase ,tmpvar (,(car spec) ,@body)))))
--
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>
bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
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On 1/16/2024 7:07 AM, Ihor Radchenko wrote:
> Consider the following
>
> (pcase-dolist (`(,(and (pred stringp) a) .
> ,(and (pred stringp) b))
> '(("TODO") ("DONE" . "a")))
> (warn "%S :: %S" a b))
>
> Executing the above yields
>
> ⛔ Warning (emacs): "TODO" :: nil
> ⛔ Warning (emacs): "DONE" :: "a"
>
> even though ("TODO") does not match the pattern.
This isn't an issue with 'pcase-dolist', but rather a known/intentional
limitation of 'pcase-let':
(pcase-let ((`(,(and (pred stringp) a) .
,(and (pred stringp) b))
'("TODO")))
(warn "%S :: %S" a b))
-> Warning (emacs): "TODO" :: nil
The 'pcase-let' docstring says this:
> Each EXP should match (i.e. be of compatible structure) to its
> respective PATTERN; a mismatch may signal an error or may go
> undetected, binding variables to arbitrary values, such as nil.
I do think we should fix it somehow though. This behavior is extremely
confusing, and as much as I'm a fan of 'pcase', I'm emphatically *not* a
fan of how this part works.
bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
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From: Ihor Radchenko <yantar92@HIDDEN>
To: bug-gnu-emacs@HIDDEN
Subject: 30.0.50; pcase-dolist matches backquote pattern incorrectly
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Hi,
Consider the following
(pcase-dolist (`(,(and (pred stringp) a) .
,(and (pred stringp) b))
'(("TODO") ("DONE" . "a")))
(warn "%S :: %S" a b))
Executing the above yields
=E2=9B=94 Warning (emacs): "TODO" :: nil
=E2=9B=94 Warning (emacs): "DONE" :: "a"
even though ("TODO") does not match the pattern.
In contrast
(dolist (el '(("TODO") ("DONE" . "a")))
(pcase el
(`(,(and (pred stringp) a) .
,(and (pred stringp) b))
(warn "%S :: %S" a b))))
correctly displays
=E2=9B=94 Warning (emacs): "DONE" :: "a"
In GNU Emacs 30.0.50 (build 8, x86_64-pc-linux-gnu, GTK+ Version
3.24.39, cairo version 1.18.0) of 2024-01-16 built on localhost
Repository revision: 5f73258753ee91c8648e0be55332f7dbaea9fa8b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101010
System Description: Gentoo Linux
--=20
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>
Ihor Radchenko <yantar92@HIDDEN>:bug-gnu-emacs@HIDDEN.
Full text available.bug-gnu-emacs@HIDDEN:bug#68509; Package emacs.
Full text available.
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