Re: reusing partial parsed and not matched prefix in the next alternative extended parser

On 13/06/2018 3:39 PM, Marian Klein wrote:
> Hi Spirit X3
> 
> I am using your x3 spirit library for my little project.
> 
> I would like to know if there is a way to pass an attribute from failed ,
> but partially parsed text to the next alternative parser.
> 
> Example:
> 
> auto  parser=P1|P2;
> 
> some parsed text matches P2 fully, but only prefix of parsed text matches P1 . Once P1 is 
> parsed , it will fail, backtrack and goes onto trying to parse P2 from the beginning 
> redundantly
> repeating parsing of  P1 prefix.
> It is suboptimal and unneccesarilly waste of CPU cycles.
> If parser P2  parse function had signature like

Before anything else, allow me to ask a question:
How are you supposed to know what "fully" means in the context of P1? End of
line? End of input?

> P2::parse(P1_Attr&& parsed_attr,Iterator It_begin,Iterator It_end,Attr& attr)
> 
> It could reused already parsed text prefix.
> 
> Is it difficult to implement such extension in  a generic way to the framework?
> 
> I understand you can achieve similar effect by optional suffix,but it is less elegant,
> as this is not meant to express logical grammar, it is rather technical optimization so it 
> should be hidden and visible.
> 
> Another example:
> I want to pass any number real or integer.
> auto parser=int_ | double_;
> parse text is 1234567.890
> 
> It would be great if double_ parser could reuse value parsed in int_ 1234567 which was not 
> match.
> double_::parse(int prefix,Iterator it_beg,Iterator it_end,double& val)

Well, you can do something like that using the sequence operator for the
common parts. E.g.

     auto parser = int_ >> -('.' >> uint_);

Regards,
-- 
Joel de Guzman
http://www.ciere.com
http://boost-spirit.com
http://www.cycfi.com/

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