GNU bug report logs - #15538
24.3; Incorrect Expansion of Nested Backquote

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

From: 白井 彰 <okshirai <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Cc: okshirai <at> gmail.com
Subject: 24.3; Incorrect Expansion of Nested Backquote
Date: Sun, 6 Oct 2013 20:23:08 +0900

The expansion of nested backquote like

``(,,@q)

or

``(,@,@q)

is incorrect.

Please see the following results.


; The followings are from CLtL2 Appendix C. Backquote

(require 'cl-lib)
cl-lib


(setq q '(r s))
(r s)

(defun r (x) (cl-reduce #'* x))
r

(setq r '(3 5))
(3 5)

(setq s '(4 6))
(4 6)


``(,,q)
(\` ((\, (r s))))
(24)                                    ; Correct

``(,@,q)
(\` ((\,@ (r s))))
24                                      ; Correct

``(,,@q)
(\` ((\, r s)))
((3 5))                                 ; Incorrect: This should be  
((3 5) (4 6))

``(,@,@q)
(\` ((\,@ r s)))
(3 5)                                   ; Incorrect: This should be (3  
5 4 6)


(setq x '(b c))
(b c)

(setq y '(a b))
(a b)

(setq p '(cl-union x y))
(cl-union x y)

(setq q '((cl-union x y) (list 'sqrt 9)))
((cl-union x y) (list (quote sqrt) 9))

(setq r '(cl-union x y))
(cl-union x y)

(setq s '((cl-union x y)))
((cl-union x y))


``(foo ,,p)
(\` (foo (\, (cl-union x y))))
(foo (a b c))                           ; Correct

``(foo ,,@q)
(\` (foo (\, (cl-union x y) (list (quote sqrt) 9))))
(foo (a b c))                           ; Incorrect: This should be  
(foo (a b c) (sqrt 9))

``(foo ,',r)
(\` (foo (\, (quote (cl-union x y)))))
(foo (cl-union x y))                    ; Correct

``(foo ,',@s)
(\` (foo (\, (quote (cl-union x y)))))
(foo (cl-union x y))                    ; Correct

``(foo ,@,p)
(\` (foo (\,@ (cl-union x y))))
(foo a b c)                             ; Correct

``(foo ,@,@q)
(\` (foo (\,@ (cl-union x y) (list (quote sqrt) 9))))
(foo a b c)                             ; Incorrect: This should be  
(foo a b c sqrt 9)

``(foo ,@',r)
(\` (foo (\,@ (quote (cl-union x y)))))
(foo cl-union x y)                      ; Correct

``(foo ,@',@s)
(\` (foo (\,@ (quote (cl-union x y)))))
(foo cl-union x y)                      ; Correct


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 of 2013-04-30 on mini2.local
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Message #8 received at 15538 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: 白井 彰 <okshirai <at> gmail.com>
Cc: 15538 <at> debbugs.gnu.org
Subject: Re: bug#15538: 24.3; Incorrect Expansion of Nested Backquote
Date: Tue, 08 Oct 2013 00:30:45 -0400

> ``(,,@q)
> (\` ((\, r s)))
> ((3 5))                                 ; Incorrect: This should be ((3 5)
> (4 6))

Indeed, it's incorrect, but I think the intention of the current code is
to consider the above expression as invalid, so the error is to return
a value at step 2 (ignoring the second argument `s' to the unquote)
instead of signalling an error.

If you really want that result, you can do

 ``(,@(list ,@q))


> ``(,@,@q)

And here, you'd have to use

  ``(,@(append ,@q))

I installed a patch into trunk which catches the above problems and
signals an error if/when encountering such situations.
Thank you,


        Stefan

Message #13 received at 15538 <at> debbugs.gnu.org (full text, mbox):

From: Nathan Trapuzzano <nbtrap <at> nbtrap.com>
To: 15538 <at> debbugs.gnu.org
Subject: RE: bug#15538: 24.3; Incorrect Expansion of Nested Backquote 
Date: Mon, 28 Oct 2013 18:47:22 -0400

Stefan,

Is there any reason why double unquote/unquote-splicing can't work in
Emacs?  A comment in your commit seems to suggest it's doable, so may I
ask why you decided not to implement it?  I suspect many macro-writing
macros would be much more readable if they could be written as they
often are in Common Lisp.

Nathan

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