Re: Usage of auto keyword in X3 tutorials

Baptiste Wicht <[email protected]> Sat, 12 Nov 2016 14:55:08 +0100
Newsgroups	gmane.comp.parsers.spirit.general
Message-ID	<[email protected]>
Hi Stephan,

On Fri, Nov 11, 2016 at 04:04:53PM +0100, Stephan Menzel wrote:
> Indeed. This would not be very nice. And in fact, I like generic lambdas a
> lot. They are the prime reason I wanted to check out X3 since according to
> my knowledge Spirit2 doesn't allow native lambdas in semantic actions and
> forces me to use phoenix, which has a tendency to cause problems and longer
> compile times.

I think you're right about Spirit2 not allowing native lambdas in
semantic actions.

> Moreover, auto usage in Spirit is not only related to x3. In Qi, you
> > could already use auto rules in order to improve performance.
> >
>
> Are you sure about that? I use spirit almost exclusively by defining
> grammars. Both qi and karma, mostly symmetrical. So my general pattern is
> to have a rather large set of grammars like this:
>
>
> template <typename Iterator>
> struct username_parser : qi::grammar<Iterator, std::string()> {
> username_parser() : username_parser::base_type(m_start, "username") {
>
> m_start %= ... define my stuff ...
> }
>
> qi::rule<Iterator, std::string()> m_start;
> };
>
> which I then combine to the actual larger grammars that I really need
> (identical pattern).
>
> All those are kept hidden in a header that is only included by a compile
> unit which then exposes plain functions to do the parsing in order to keep
> compile times in check by not letting anything spirit out into the rest of
> the code.
>
> Which means I use the rule definitions a lot.
>
> Are you saying that above example would be better by using auto instead of
> the rule definition? Or are you referring to the "auto" parser which
> chooses a generic parser by attribute type? Because I am using this,
> although rarely.

I was actually to this kind of rule:
http://boost-spirit.com/home/articles/qi-example/zero-to-60-mph-in-2-seconds/

They are generally more efficient than qi::rule, but you cannot use them
everywhere though. For instance, you cannot have recursive auto rules,
but there are good candidates for small sub parts of the grammar.

Baptiste

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