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Re: How to parse attributes?

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David Brown <david.brown@hesbynett.no> writes:

> On 22/11/2019 05:41, Keith Thompson wrote:
<cut>
>> There is precedent in C++ for dealing with nested <> template
>> delimiters, but that was needed because >> is already a token.  Any
>> problems with [[ and ]] didn't require such a solution.
>
> For a long time (until C++11), you needed to use "> >" to close the
> nested template.  In C++11, ">>" was allowed for closing the nested
> templates.  That meant that any C++ compiler supporting C++11 had to
> find a solution to this problem, which may have meant changing their
> parser or tokeniser significantly.  C++ front ends do not use lex/yacc
> parsers any more - this is an example of why they don't.
>
> So when attributes were included in C++, the reason there was no
> problems in using "]]" to end them despite "]]" being interpreted as "]
> ]" in other situations, is that C++ compilers already had the facilities
> in place to deal with that.

Oh!  Are [[ and/or ]] single tokens in C++'s attributes?  If so, what a
shame there is now another difference with C.  If not, I don't see the
connection with >> since neither [[ nor ]] are single tokens. There is,
in C, no need for any special mechanism at all.

Attributes in C just have the grammar (very roughly):

 '[' '[' <stuff with balanced parens (of all sorts) ']' ']'

C++ could have done it that way too, without invoking any special
treatment for ]].

The only oddity is that (in C) you'd need to either switch the lexer
into a special mode, or introduce a grammar rule for an "attribute
identifier" that can be either a keyword or an ordinary identifier.

>>> But I am not 100% sure you have to do this.  What is your source for
>>> keywords being regarded as identifiers?  In the usual C grammar, the
>>> rule for "identifier" is very general (the usual letters and digits
>>> definitions), but keywords are considered to be a separate category.
>>> Thus the grammar item "identifier" really means "things that have the
>>> right form but which aren't keywords".
>> 
>> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2455.pdf (C20 draft)
>> 6.7.11.1p3:
>> 
>>     An attribute specifier that contains no attributes has no
>>     effect. The order in which attribute tokens appear in an
>>     attribute list is not significant. If a keyword (6.4.1) that
>>     satisfies the syntactic requirements of an identifier (6.4.2) is
>>     contained in an attribute token, it is considered an identifier.
>>     A strictly conforming program using a standard attribute remains
>>     strictly conforming in the

Keith:
Thanks for the quote.  I'd not read the standard but I'd hoped they were
just using "identifier" as it was used elsewhere in the C grammar.
Obviously not.

-- 
Ben.

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Thread

How to parse attributes? Philipp Klaus Krause <pkk@spth.de> - 2019-11-20 09:41 +0100
  Re: How to parse attributes? David Brown <david.brown@hesbynett.no> - 2019-11-20 10:05 +0100
    Re: How to parse attributes? Philipp Klaus Krause <pkk@spth.de> - 2019-11-20 11:05 +0100
      Re: How to parse attributes? David Brown <david.brown@hesbynett.no> - 2019-11-20 12:35 +0100
  Re: How to parse attributes? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-20 11:06 +0000
  Re: How to parse attributes? Öö Tiib <ootiib@hot.ee> - 2019-11-20 23:16 -0800
    Re: How to parse attributes? Philipp Klaus Krause <pkk@spth.de> - 2019-11-21 10:39 +0100
      Re: How to parse attributes? Öö Tiib <ootiib@hot.ee> - 2019-11-21 02:51 -0800
        Re: How to parse attributes? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-21 05:00 -0800
      Re: How to parse attributes? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-22 02:21 +0000
        Re: How to parse attributes? Keith Thompson <kst-u@mib.org> - 2019-11-21 20:41 -0800
          Re: How to parse attributes? Philipp Klaus Krause <pkk@spth.de> - 2019-11-22 09:15 +0100
          Re: How to parse attributes? David Brown <david.brown@hesbynett.no> - 2019-11-22 09:48 +0100
            Re: How to parse attributes? "A. K." <minforth@arcor.de> - 2019-11-22 02:00 -0800
              Re: How to parse attributes? David Brown <david.brown@hesbynett.no> - 2019-11-22 11:41 +0100
              Re: How to parse attributes? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-24 14:02 +0000
                Re: How to parse attributes? raltbos@xs4all.nl (Richard Bos) - 2019-11-25 16:21 +0000
            Re: How to parse attributes? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-22 17:46 +0000
              Re: How to parse attributes? Keith Thompson <kst-u@mib.org> - 2019-11-22 10:53 -0800
                Re: How to parse attributes? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-23 00:08 +0000
    Re: How to parse attributes? scott@slp53.sl.home (Scott Lurndal) - 2019-11-21 15:57 +0000
    Re: How to parse attributes? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-22 08:21 +0100
    Re: How to parse attributes? Bart <bc@freeuk.com> - 2019-11-22 12:11 +0000

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