Re: Old-style backquotes in cc-vars.el

[Alan Mackenzie <acm@muc.de>]

Hi, Stefan,

On Sat, Oct 06, 2007 at 06:27:00PM -0400, Stefan Monnier wrote:
> >> cc-vars uses a mix of old-style a new-style backquotes.
> > I don't think it does.

> The warning indicates that Emacs sees old-style backquotes, aqnd I'm
> pretty sure there are new style backquotes as well, so I'm pretty sure
> you're wrong.

OK.  I've played around with things a bit since yesterday.  In
particular, I've discovered the useful functions `read' and
`backquote-process'.  I'm begining to think you're right and I'm wrong.
:-)

> > He wouldn't knowingly have put in old-style BQs as late as 2001.

> Maybe he did it knowingly: he's using nested backquotes and IIRC they
> didn't always work correctly in older versions of Emacs when used with
> new-style backquotes.  Actually IIRC the problem was with nested
> new-style commas (where the second would not be recognized as
> new-style) which indeed do appear in that code.

Ah.  OK.  That's sounds like a sound reason for not upgrading that code
in cc-vars.el, if it's the case.  (Or, for introducing yet another
difference between the cc-vars.el in Emacs and the standalone CC Mode,
which retains compatibility with older (X)Emacsen.  :-(  )

Probably I'll just scrap the old code and put in Thi's new version of the
function instead (after testing it thoroughly).  It looks a lot cleaner
and easier to understand.

> > I'm not sure whether the rules for nested backquotes are clearly laid
> > out anywhere.

> The problem is not about nesting but about new-style vs old-style: the
> detailed rule about which is which is indeed only specified implicitly in
> the source code.

OK.

> > I think they're formally ambiguous (i.e., RTFS).  I tried
> > replacing "`(` (radio" with both

> >     "`( (` (radio"        (explicitly putting in an "old-style" BQ)

> > , and

> >     "`` (radio"           (replacing the alleged "old-style" BQ with a
> >                            "new-style" one)

> > , but each of these generated results different from the original
> > (checked with macroexpand).

> You need to fix the ,(, part accordingly.

Ah!  I'm still figuring out what ,(, actually means.

> Thien-Thi might be a good start.

Yes.  He's already been very helpful.

> > Who is the bytecomp expert, again?  Could we possibly ask him about the
> > semantics of  `(` (radio ?

> It's (\` (\` (radio

I managed to get that late last night with   (read "`(` (a b c))").  :-)

Thanks for all the help!

>         Stefan

-- 
Alan.

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