bug#43601: 27.1; Macroexpansion bug in `push'

bug#43601: 27.1; Macroexpansion bug in `push'

[Sean Devlin]

Hi folks,

I believe I've found a macroexpansion bug in the `push' macro. Open a
new instance of Emacs and evaluate the following form in the scratch
buffer:

    (macroexpand '(push (list 'x)
                        (cdr my-list)))

The result (with some reformatting) is this:

    (let* ((v (list 'x))
           (v my-list))
      (setcdr v
              (cons v
                   (cdr v))))

Both values are bound to `v', so the former is shadowed by the latter.

Thanks!


In GNU Emacs 27.1 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60 Version 10.14.6 (Build 18G95))
of 2020-08-12 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.2011
System Description:  macOS 11.0

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
(x y)
See buffer *Pp Macroexpand Output*.
You can run the command ‘pp-macroexpand-last-sexp’ with M-x p-m-l RET
See buffer *Pp Macroexpand Output*.
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bug#43601: Acknowledgement (27.1; Macroexpansion bug in `push')

[Sean Devlin]

Hi folks,

Upon closer inspection, it looks like the two bindings are distinct. I guess the symbols must be created by `make-symbol’ or similar. Still, it might be nice for debugging if the symbols had distinct names.

Thanks!

bug#43601: 27.1; Macroexpansion bug in `push'

[Phil Sainty]

On 2020-09-25 10:54, Sean Devlin wrote:
> Hi folks,
> 
> I believe I've found a macroexpansion bug in the `push' macro. Open a
> new instance of Emacs and evaluate the following form in the scratch
> buffer:
> 
>     (macroexpand '(push (list 'x)
>                         (cdr my-list)))
> 
> The result (with some reformatting) is this:
> 
>     (let* ((v (list 'x))
>            (v my-list))
>       (setcdr v
>               (cons v
>                    (cdr v))))
> 
> Both values are bound to `v', so the former is shadowed by the latter.

It's actually fine.

What you're seeing is the *printed representation* of the lisp objects,
and two completely independent symbols, each with the name "v".

(setq foo (macroexpand '(push (list 'x) (cdr my-list))))
=> (let* ((v (list 'x)) (v my-list)) (setcdr v (cons v (cdr v))))

(setq vfirst (caaadr foo))
=> v

(setq vsecond (car (cadadr foo)))
=> v

(eq vfirst vsecond)
=> nil

(cdaddr (caddr foo))
=> (v (cdr v))

(eq vfirst (car (cdaddr (caddr foo))))
=> t

(eq vsecond (cadadr (cdaddr (caddr foo))))
=> t


This is like:

(eq (make-symbol "v") (make-symbol "v"))
=> nil

bug#43601: 27.1; Macroexpansion bug in `push'

[Philipp Stephani]

Am Fr., 25. Sept. 2020 um 00:56 Uhr schrieb Sean Devlin <spd@toadstyle.org>:
>
> Hi folks,
>
> I believe I've found a macroexpansion bug in the `push' macro. Open a
> new instance of Emacs and evaluate the following form in the scratch
> buffer:
>
>     (macroexpand '(push (list 'x)
>                         (cdr my-list)))
>
> The result (with some reformatting) is this:
>
>     (let* ((v (list 'x))
>            (v my-list))
>       (setcdr v
>               (cons v
>                    (cdr v))))
>
> Both values are bound to `v', so the former is shadowed by the latter.


These are different uninterned symbols:

(let ((print-gensym t) (print-circle t))
  (print (macroexpand '(push (list 'x) (cdr my-list))))
  nil)

⇒ (let* ((#2=#:v (list 'x)) (#1=#:v my-list)) (setcdr #1# (cons #2# (cdr #1#))))

In general, if you see a "weird" expansion like this, bind
print-gensym to non-nil to see the uninterned symbols.

bug#43601: 27.1; Macroexpansion bug in `push'

[Lars Ingebrigtsen]

Philipp Stephani <p.stephani2@gmail.com> writes:

> In general, if you see a "weird" expansion like this, bind
> print-gensym to non-nil to see the uninterned symbols.

True.  But we could change push to use a different name for the
gensymmed symbol to avoid confusions like this.

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

bug#43601: 27.1; Macroexpansion bug in `push'

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Philipp Stephani <p.stephani2@gmail.com> writes:
>
>> In general, if you see a "weird" expansion like this, bind
>> print-gensym to non-nil to see the uninterned symbols.
>
> True.  But we could change push to use a different name for the
> gensymmed symbol to avoid confusions like this.

I've now done so in Emacs 28.

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