nested backquotes

nested backquotes

[Matthieu Moy]

Hi,

I'm experiencing problems with nested backquotes in Emacs lisp:

Emacs 21 says:

ELISP> ``( ,,(+ 1 2))
(cons
 (+ 1 2))

Whereas XEmacs says:
ELISP> ``( ,,(+ 1 2))
(list 3)

Question 1: Who is right? (I'd say XEmacs here. I don't understand
Emacs's behavior)

Question 2: Is there a portable way to use nested backquotes in Emacs
Lisp?

Thanks,

-- 
Matthieu

Re: nested backquotes

[rgb]

> I'm experiencing problems with nested backquotes in Emacs lisp:
>
> Emacs 21 says:
>
> ELISP> ``( ,,(+ 1 2))
> (cons
>  (+ 1 2))

On 22.0.50 I'm getting (list 3)

Re: nested backquotes

[Matthieu Moy]

"rgb" <rbielaws@i1.net> writes:

>> I'm experiencing problems with nested backquotes in Emacs lisp:
>>
>> Emacs 21 says:
>>
>> ELISP> ``( ,,(+ 1 2))
>> (cons
>>  (+ 1 2))
>
> On 22.0.50 I'm getting (list 3)

Surprisingly, backquote.el hasn't changed much. Loading backquote.el
from CVS on my Emacs 21 "works" (in the sense that it doesn't break my
Emacs), but still gets the same result.

So, I'm still looking for a portable solution ... :-(

The goal would be to implement this:

  https://mail.gna.org/public/xtla-el-dev/2004-11/msg00078.html

Or the variant

  https://mail.gna.org/public/xtla-el-dev/2004-12/msg00001.html

in Xtla, without droping support for GNU Emacs 22.

-- 
Matthieu

Re: nested backquotes

[Thierry Emery]

Hi,

Matthieu Moy <MatthieuNOSPAM.Moy@imag.fr.invalid> writes:

> "rgb" <rbielaws@i1.net> writes:
>
>> > I'm experiencing problems with nested backquotes in Emacs lisp:
>> > 
>> > Emacs 21 says:
>> > 
>> > ELISP> ``( ,,(+ 1 2))
>> > (cons
>> >  (+ 1 2))
>> 
>> On 22.0.50 I'm getting (list 3)
>
> Surprisingly, backquote.el hasn't changed much. Loading backquote.el
> from CVS on my Emacs 21 "works" (in the sense that it doesn't break my
> Emacs), but still gets the same result.
>
> So, I'm still looking for a portable solution ... :-(

(` (` ( (, (, (+ 1 2))))))

tested on GNU Emacs 21.4 and 22.0.50, and on XEmacs 21.4.17,
consistently returns :
(list 3)

Hoping this helps,

Thierry
-- 
thierry |dot| emery |at| free |dot| fr

Re: nested backquotes

[Stefan Monnier]

>> On 22.0.50 I'm getting (list 3)

> Surprisingly, backquote.el hasn't changed much. Loading backquote.el

The bug fixed in Emacs-CVS was in the Lisp reader that handles the backquote
syntax, not the backquote expansion.


        Stefan

Re: nested backquotes

[Stefan Monnier]

> The goal would be to implement this:
>   https://mail.gna.org/public/xtla-el-dev/2004-11/msg00078.html
> Or the variant
>   https://mail.gna.org/public/xtla-el-dev/2004-12/msg00001.html
> in Xtla, without droping support for GNU Emacs 22.

Maybe I'm way off-base, but lexical-let sounds like a better alternative.
When capturing things like the current buffer, you can just replace

     `(lambda (...) ... ,(current-buffer) ...)
with
     (lexical-let ((buf (current-buffer)))
       (lambda (...) ... buf ...))


-- Stefan

Re: nested backquotes

[Matthieu Moy]

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

> Maybe I'm way off-base, but lexical-let sounds like a better alternative.
> When capturing things like the current buffer, you can just replace
>
>      `(lambda (...) ... ,(current-buffer) ...)
> with
>      (lexical-let ((buf (current-buffer)))
>        (lambda (...) ... buf ...))

With the "capture" version, you can also do

(tla--lambda-with-capture (x y z)
  (... foo ... (capture foo) ... (capture (current-buffer))))

which I find more readable. (I've started to look at the lexical-let
option too)

Thanks for your help anyway.

-- 
Matthieu

Re: nested backquotes

[Stefan Monnier]

> With the "capture" version, you can also do

> (tla--lambda-with-capture (x y z)
>   (... foo ... (capture foo) ... (capture (current-buffer))))

Could you explain what this does?

> which I find more readable. (I've started to look at the lexical-let
> option too)

The advantage of lexical-let is that it uses a completely standard concept
(lexical scoping and closures) which exists in many other languages (where
their `let' behaves pretty much like Emacs's lexical-let).


        Stefan

Re: nested backquotes

[Matthieu Moy]

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

>> With the "capture" version, you can also do
>
>> (tla--lambda-with-capture (x y z)
>>   (... foo ... (capture foo) ... (capture (current-buffer))))
>
> Could you explain what this does?

It expands to something like

`(lambda (x y z)
   (let ((tla1 foo)
         (tla2 (current-buffer)))
     (funcall (lambda () (... foo ... tla1 ... tla2)))))

So, we still have the backquoted lambda, but containing only a let and
a funcall, the actual body being byte-compiled.

The translation from our backquote + comma version is roughly

s/`(lambda/(tla--lambda-with-capture/g
s/,.*/(capture \1)/

-- 
Matthieu

Re: nested backquotes

[Matthieu Moy]

Matthieu Moy <MatthieuNOSPAM.Moy@imag.fr.invalid> writes:

> `(lambda (x y z)
>    (let ((tla1 ,foo)
                 ^
>          (tla2 ,(current-buffer)))
                 ^
>      (funcall (lambda () (... foo ... tla1 ... tla2)))))

I had forgotten the commas, sorry.

-- 
Matthieu