GNU bug report logs - #26960
26.0.50; Complaints about unused variable in cl-destructuring-bind

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Philipp Stephani <p.stephani2 <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 26.0.50; Complaints about unused variable in cl-destructuring-bind
Date: Wed, 17 May 2017 11:23:27 +0200

Create a file with the following contents (here /tmp/destr.el):

;;; -*- lexical-binding: t; -*-
(require 'cl-lib)
(cl-destructuring-bind (&whole a b &rest _) '(1 2)
  (print (list a b)))

Then run

emacs -Q -batch -f batch-byte-compile /tmp/destr.el

The byte compiler will complain:

In toplevel form:
/tmp/destr.el:3:1:Warning: variable ‘_’ not left unused

even though the variable is clearly unused.


In GNU Emacs 26.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.10.8)
 of 2017-05-17 built on localhost
Repository revision: f7c07930b581b1bcfdfb1874b6883233516bdf11
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:	Ubuntu 14.04 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --without-pop --with-mailutils --enable-checking
 --enable-check-lisp-object-type --with-modules 'CFLAGS=-O0 -ggdb3''

Configured features:
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Important settings:
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  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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Memory information:
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 (symbols 48 20116 1)
 (miscs 40 39 145)
 (strings 32 17446 3603)
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Message #10 received at 26960 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Helmut Eller <eller.helmut <at> gmail.com>
Cc: 26960 <at> debbugs.gnu.org, 8711 <at> debbugs.gnu.org,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#26960: 26.0.50; Complaints about unused variable in
 cl-destructuring-bind
Date: Sun, 08 May 2022 14:33:49 +0200

Helmut Eller <eller.helmut <at> gmail.com> writes:

> There seems be no way to avoid a warning in code like this:
>
> ;; -*- lexical-binding: t -*-
> (eval-when-compile (require 'cl))
> (defun foo (x) (destructuring-bind (_) x))
> (defun bar (x) (destructuring-bind (_) x (ignore _)))
>
> In foo the compiler complains because _ is never used and
> it bar it complains because it is used.

(I'm going through old bug reports that unfortunately weren't resolved
at the time.)

I can reproduce this in Emacs 25.1, but not in Emacs 29.  That is,
there's no warning for `foo'.  There's still a warning for `bar', but
that's correct, isn't it?  `ignore' is just a normal function, so _ is
used there.

However, this example from a merged bug report is giving a warning:

> ;;; -*- lexical-binding: t; -*-
> (require 'cl-lib)
> (cl-destructuring-bind (&whole a b &rest _) '(1 2)
>   (print (list a b)))

And that seems like a bug?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Message #13 received at 26960 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 26960 <at> debbugs.gnu.org, 8711 <at> debbugs.gnu.org,
 Helmut Eller <eller.helmut <at> gmail.com>
Subject: Re: bug#26960: 26.0.50; Complaints about unused variable in
 cl-destructuring-bind
Date: Sun, 08 May 2022 09:32:01 -0400

> I can reproduce this in Emacs 25.1, but not in Emacs 29.  That is,
> there's no warning for `foo'.  There's still a warning for `bar', but
> that's correct, isn't it?

Yes.

> `ignore' is just a normal function, so _ is used there.

It's not 100% normal, but yes.

> However, this example from a merged bug report is giving a warning:
>
>> ;;; -*- lexical-binding: t; -*-
>> (require 'cl-lib)
>> (cl-destructuring-bind (&whole a b &rest _) '(1 2)
>>   (print (list a b)))
>
> And that seems like a bug?

Indeed.  If you look at the macroexpanded code, you see that
`cl-destructuring-bind` uses:

    (let* ((_ '(1 2))
           (a _)
           (b (if _ (pop _)
                (signal 'wrong-number-of-arguments
                        (list '(&whole a b &rest _) (length _))))))
      (print (list a b)))

so, you can see that the problem is that `cl-destructuring-bind` tries
to avoid using gensym and "abuses" the &rest var as the "iterator"
variable while parsing the list.

It might have been useful back in the dynbinding days because `setq`
might have been a bit cheaper than `let`, but with lexical scoping
`let` byte-compiles to virtually nothing.


        Stefan

Message #16 received at 26960 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 26960 <at> debbugs.gnu.org, 8711 <at> debbugs.gnu.org,
 Helmut Eller <eller.helmut <at> gmail.com>
Subject: Re: bug#26960: 26.0.50; Complaints about unused variable in
 cl-destructuring-bind
Date: Mon, 09 May 2022 11:25:38 +0200

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

> Indeed.  If you look at the macroexpanded code, you see that
> `cl-destructuring-bind` uses:
>
>     (let* ((_ '(1 2))
>            (a _)
>            (b (if _ (pop _)
>                 (signal 'wrong-number-of-arguments
>                         (list '(&whole a b &rest _) (length _))))))
>       (print (list a b)))
>
> so, you can see that the problem is that `cl-destructuring-bind` tries
> to avoid using gensym and "abuses" the &rest var as the "iterator"
> variable while parsing the list.

Hm, right.

I had a quick peek at `cl--do-arglist' (which I guess is what's creating
this form), and...  I think I'll leave this for somebody else to fix.  🫠

In any case, I'm unmerging these two bug reports, and closing the one
that's fixed.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Message #21 received at 26960 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 26960 <at> debbugs.gnu.org, 8711 <at> debbugs.gnu.org,
 Helmut Eller <eller.helmut <at> gmail.com>
Subject: Re: bug#26960: 26.0.50; Complaints about unused variable in
 cl-destructuring-bind
Date: Mon, 09 May 2022 08:26:21 -0400

> I had a quick peek at `cl--do-arglist' (which I guess is what's creating
> this form), and...  I think I'll leave this for somebody else to fix.  🫠

I did the same some years ago, FWIW :-)


        Stefan

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