Re: Not able to use x3::skip(skipper)[some recursive rule] in its rule definition

On 07/25/2017 03:40 AM, Han Wang wrote:
> I've posted the same question in stackoverflow.
> 
> https://stackoverflow.com/questions/45282293/spirit-not-able-to-use-x3skipskippersome-recursive-rule-in-its-rule-defin
> 
> Let's say we want to parse a recursive block like this. When 
> "|skip_comments_tag|" is prefixed to the block, we skip all 
> comments(|/*...*/|) within this block recursively.
> 
> |{{}{skip_comments_tag{{}/*comments*/{/*comments*/}}}}|
> 
> It's easy to come up with a recursive parser as in Coliru 
> <http://coliru.stacked-crooked.com/a/31fb17064091fae3>.
> 
> |namespaceParser{autoconstruleComment =x3::lit("/*")>>*(x3::char_ 
> -"*/")>>"*/"|x3::space;x3::rule<structSBlockId>construleBlock;autoconstruleBlock_def 
> =x3::lit('{')>>*(ruleBlock 
> |"skip_comments_tag">>x3::skip(ruleComment)[ruleBlock])>>'}';BOOST_SPIRIT_DEFINE(ruleBlock)}|
> 
> But it doesn't compile (when the |parse| function is called) because it 
> will generate an infinite context (by |x3::make_context| in 
> |x3::skip_directive|). |x3::no_case| and |x3::with| also have this 
> problem because they all use |x3::make_context| in the implementation.
> 
> Questions:
> 
>  1. Is there always a better way to write parsers for this kind of
>     questions to avoid such compile error and how?
>  2. Or is the |x3::make_context| implementation considered to be flawed
>     for this kind of questions?
> 
> BTW: TU seperation doesn't solve this.
> 
Hi Han.

You might try the get_rhs branch of a spirit fork here:

https://github.com/cppljevans/spirit/tree/get_rhs

and then #define BOOST_SPIRIT_GET_RHS_CRTP.  As you can see from here:

https://github.com/cppljevans/spirit/blob/get_rhs/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp#L140

that will disable the x3::make_context calls.

Please let me know if that works.  If so, I might be
encouraged enough to go to the trouble of creating a pull request
despite the need (as pointed out in a private email) that
I'll need to document the rationale for the pull request.

HTH.

-regards,
Larry



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