GNU bug report logs - #19790
[PATCH] destructive splicing in backquote

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Package: emacs; Reported by: Robin Templeton <robin@HIDDEN>; Keywords: patch; dated Fri, 6 Feb 2015 06:48:02 UTC; Maintainer for emacs is bug-gnu-emacs@HIDDEN.

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To: Robin Templeton <robin@HIDDEN>
Subject: Re: bug#19790: [PATCH] destructive splicing in backquote
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Robin Templeton <robin@HIDDEN> writes:

> Common Lisp and MACLISP define comma-dot syntax for destructive splicing
> in backquote expressions. The Elisp reader reads ",.x" as "(\,\. x)"
> (like ",@"), but the resulting forms are not processed by the backquote
> library. A patch follows that adds CL-compatible comma-dot support.
>
> Originally reported by at Artur Malabarba in
> <http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00624.html>.

I think the conclusion to the discussion was that we do not want to
support the ,. operation in Emacs Lisp.

(setq .foo 1)

`((,.foo))
=> (((\,\. foo)))

So we should remove this feature completely, because that's obviously a
bug...

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




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Subject: Re: bug#19790: [PATCH] destructive splicing in backquote
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> There's never been a guarantee that it would use destructive splicing
> (it's just allowed to),

Then I'd vote for just removing it.
Still, this whole thing is corner-case enough that any solution is fine with me.




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From: Stefan Monnier <monnier@HIDDEN>
To: Artur Malabarba <bruce.connor.am@HIDDEN>
Subject: Re: bug#19790: [PATCH] destructive splicing in backquote
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> I'm fine with using this syntax for destructive splicing or with
> disabling it altogether. But I think aliasing it to ",@" might be a
> little deceptive for people who actually expect destructive splicing.

There's never been a guarantee that it would use destructive splicing
(it's just allowed to), and I generally dislike destructive operations,
so I'd rather not go through extra trouble to support what I tend to see
as a misfeature.

So aliasing ,. to ,@ is OK, but making it work destructively is not.


        Stefan




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Subject: Re: bug#19790: [PATCH] destructive splicing in backquote
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Yes, I was surprised mostly for being caught of guard. :-)
After all this years I had never heard of ",." doing anything.

I agree that the current behavior is a bug, but either way we go is
fine for me (I managed to get around this limitation by simply adding
a space between the comma and the dot).

I'm fine with using this syntax for destructive splicing or with
disabling it altogether. But I think aliasing it to ",@" might be a
little deceptive for people who actually expect destructive splicing.

2015-02-08 17:23 GMT-02:00 Stefan Monnier <monnier@HIDDEN>:
>> Also, I could submit a patch to remove the special handling for ",." or
>> make it a syntax error, if either of those options would be preferable.
>> But the current behavior seems to be a bug.
>
> Hmm... indeed recently Artur bumped into this (he uses identifiers
> starting with "." and was surprised that ,<ident> didn't work in that
> case).
>
> Maybe removing this special syntax is the best option.  Tho at least the
> current situation leaves both choices still open ;-)
>
>
>         Stefan




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From: Stefan Monnier <monnier@HIDDEN>
To: Robin Templeton <robin@HIDDEN>
Subject: Re: bug#19790: [PATCH] destructive splicing in backquote
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> Also, I could submit a patch to remove the special handling for ",." or
> make it a syntax error, if either of those options would be preferable.
> But the current behavior seems to be a bug.

Hmm... indeed recently Artur bumped into this (he uses identifiers
starting with "." and was surprised that ,<ident> didn't work in that
case).

Maybe removing this special syntax is the best option.  Tho at least the
current situation leaves both choices still open ;-)


        Stefan




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Robin Templeton <robin@HIDDEN> writes:

> Stefan Monnier <monnier@HIDDEN> writes:
>
>>> library.  A patch follows that adds CL-compatible comma-dot support.
>>
>> Other than CL-compatibility, what is the use of this?
>
> It's an improvement over having the feature half-implemented, as it is
> now. Read syntax for comma-dot was added in 1995 (commit
> 176348460d640ae96b0b21567df0de1457aa962b) without corresponding support
> in backquote.el, and it has only been mentioned online a few times since
> then, so it seemed harmless to implement what was likely originally
> intended.

Also, I could submit a patch to remove the special handling for ",." or
make it a syntax error, if either of those options would be preferable.
But the current behavior seems to be a bug.

-- 
Inteligenta persono lernas la lingvon Esperanton rapide kaj facile.
Esperanto estas moderna, kultura lingvo por la mondo. Simpla, fleksebla,
belsona, Esperanto estas la praktika solvo de la problemo de universala
interkompreno. Lernu la interlingvon Esperanton!





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Stefan Monnier <monnier@HIDDEN> writes:

>> library.  A patch follows that adds CL-compatible comma-dot support.
>
> Other than CL-compatibility, what is the use of this?

It's an improvement over having the feature half-implemented, as it is
now. Read syntax for comma-dot was added in 1995 (commit
176348460d640ae96b0b21567df0de1457aa962b) without corresponding support
in backquote.el, and it has only been mentioned online a few times since
then, so it seemed harmless to implement what was likely originally
intended.

-- 
Inteligenta persono lernas la lingvon Esperanton rapide kaj facile.
Esperanto estas moderna, kultura lingvo por la mondo. Simpla, fleksebla,
belsona, Esperanto estas la praktika solvo de la problemo de universala
interkompreno. Lernu la interlingvon Esperanton!





Information forwarded to bug-gnu-emacs@HIDDEN:
bug#19790; Package emacs. Full text available.

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> library.  A patch follows that adds CL-compatible comma-dot support.

Other than CL-compatibility, what is the use of this?


        Stefan




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Common Lisp and MACLISP define comma-dot syntax for destructive splicing
in backquote expressions. The Elisp reader reads ",.x" as "(\,\. x)"
(like ",@"), but the resulting forms are not processed by the backquote
library. A patch follows that adds CL-compatible comma-dot support.

Originally reported by at Artur Malabarba in
<http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00624.html>.

-- >8 --
Subject: [PATCH] destructive splicing in backquote

Allow Common Lisp-compatible destructive splicing in backquote
expressions using ",." syntax.

* lisp/emacs-lisp/backquote.el (backquote-destructive-splice-symbol):
  New variable.
  (backquote-process): Allow backquote-destructive-splice-symbol as a
  synonym for backquote-splice-symbol.
---
 lisp/ChangeLog               | 7 +++++++
 lisp/emacs-lisp/backquote.el | 6 +++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index c18a8ff..fd643ed 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,10 @@
+2015-02-06  Robin Templeton  <robin@HIDDEN>
+
+	* emacs-lisp/backquote.el (backquote-destructive-splice-symbol):
+	New variable.
+	(backquote-process): Allow backquote-destructive-splice-symbol as
+	a synonym for backquote-splice-symbol.
+
 2015-02-03  Artur Malabarba  <bruce.connor.am@HIDDEN>
 
 	* emacs-lisp/package.el (package-delete): Document NOSAVE.
diff --git a/lisp/emacs-lisp/backquote.el b/lisp/emacs-lisp/backquote.el
index d5cdca2..b6e1792 100644
--- a/lisp/emacs-lisp/backquote.el
+++ b/lisp/emacs-lisp/backquote.el
@@ -90,6 +90,9 @@ For example (backquote-list* 'a 'b 'c) => (a b . c)"
 (defconst backquote-splice-symbol '\,@
   "Symbol used to represent a splice inside a backquote.")
 
+(defconst backquote-destructive-splice-symbol '\,.
+  "Symbol used to represent a destructive splice inside a backquote.")
+
 (defmacro backquote (structure)
   "Argument STRUCTURE describes a template to build.
 
@@ -160,7 +163,8 @@ LEVEL is only used internally and indicates the nesting level:
          (t (cons (if (eq (car-safe (nth 1 s)) 'quote) 0 1)
                   (nth 1 s))))
       (backquote-delay-process s (1- level))))
-   ((eq (car s) backquote-splice-symbol)
+   ((or (eq (car s) backquote-splice-symbol)
+        (eq (car s) backquote-destructive-splice-symbol))
     (if (<= level 0)
         (if (> (length s) 2)
             ;; (cons 2 `(append . ,(cdr s)))
-- 
2.1.4






Acknowledgement sent to Robin Templeton <robin@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-gnu-emacs@HIDDEN. Full text available.
Report forwarded to bug-gnu-emacs@HIDDEN:
bug#19790; Package emacs. Full text available.
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