Re: [Boost-users] [Spirit 2] Questions related to generating grammars meta-dynamically

Subject: Re: [Boost-users] [Spirit 2] Questions related to generating grammars meta-dynamically
From: Max Motovilov (max_at_[hidden])
Date: 2010-11-29 17:40:49

> locals<>  are - well - local to the rule they are defined for. Each
> invocation of that rule creates a new instance of the local variables, very
> much like local vaiables in functions.

So if I have something like this (a very much contrived example):

struct myGrammar: grammar< .... >
{
     rule< .... locals< types > .... > some_rule;

     myGrammar() : base_type( .... ) {
        some_rule = parser_1[ _a = _1 ] << parser_2( _a );
     }
};

then "_a" would indeed refer to "some_rule"'s local variable? Are the 
grammar's local variables effectively identical to its start rule's 
local variables? Ditto _val and _rN?

> I must admit I don't fully understand what you're trying to achieve. But one
> thing I do know for sure: you shouldn't need to even look at proto, not even
> talking about using it. Everything should be accessible by utilizing
> Spirit's own facilities.

And so I was hoping too :) Let me try and elaborate, perhaps I am not 
even approaching the problem from the right side.

Basically, I'd like to map a subset of JSON schema specification 
(http://tools.ietf.org/html/draft-zyp-json-schema-03; 
http://www.json-schema.org) to a set of type declarations (that can be 
compiled from the actual schema) and have the serializers/deserializers 
for these types build their grammars automatically. I realize that the 
alternative is to use a fixed grammar and customized attribute 
conversion but this seems to require temporary dynamic data structures 
that will be built by the Kleene star and such. I was hoping to have my 
parsers push the data directly into user-defined objects.

>
> If you need to invoke an arbitrary parser at runtime, try to look at the
> lazy() construct, which takes whatever is passed to it and evaluates it as a
> parser expression. See here for an example:
> http://boost-spirit.com/home/articles/qi-example/nabialek-trick/

This looks very similar to what I'm trying to do, thank you very much! 
Unfortunately I have next to no experience with Spirit (or Fusion... or 
Phoenix... :) ) even though I've been using many other Boost libraries 
for ages, so once I can't resolve something by looking in the docs I 
don't really know what to google for...

Thanks again for your help!
...Max...