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Subject: Re: [boost] Requesting a review of Metaparse
From: Abel Sinkovics (abel_at_[hidden])
Date: 2012-01-31 02:33:54

Hi Luke,

> Right now it is hard coded that the function object generated by apply_wrap1 takes one argument. I'm pretty sure you could build compile time logic to search for the largest _2, _4 type token and select the specialization of the wrapper that matches the number of arguments used in the expression string.
Yes, it is possible. I used the "always take 1 argument" approach to
make the example as simple as I could - the goal of this example is the
demonstration of how to build callable functions.

> You have a comment:
> /*
> * The grammar
> *
> * expression ::= plus_exp
> * plus_exp ::= prod_exp ((plus_token | minus_token) prod_exp)*
> * prod_exp ::= value_exp ((mult_token | div_token) value_exp)*
> * value_exp ::= int_token | '_'
> */
> And then implement the grammar using metaparse based metaprogramming, eventually declaring your compile time parser with this:
> typedef build_parser<entire_input<expression> > function_parser;
> I believe we could apply the metaparse library to itself to get a more natural syntax for declaring a metaparser:
> typedef build_parser<entire_input<grammar<rule<_S("expression ::= plus_exp")>,
> rule<_S("plus_exp ::= prod_exp ((plus_token | minus_token) prod_exp)*")>,
> rule<_S("value_exp ((mult_token | div_token) value_exp)*")>,
> rule<_S("value_exp ::= int_token | '_'")>
> >,
> semantic_action<_S("value_exp"), build_value>,
> semantic_action<_S("prod_exp"), build_mult>,
> semantic_action<_S("plus_exp"), build_plus>
> > > function_parser;
> This is essentially what I have in mind when I think of metaparse and constexpr based mpl string used to make metaprogramming easier.
> I'd be interested in seeing if you can get the above proposed syntax (or something similar) to compile to demonstrate the metapower of metaparse. A compile time parser for generating compile time parsers.

I've added a new example: meta_metaparse doing this. You can define the
grammar the following way:

     ::rule_<_S("plus_token"), token<lit_c<'+'>>>::type
     ::rule_<_S("minus_token"), token<lit_c<'-'>>>::type
     ::rule_<_S("mult_token"), token<lit_c<'*'>>>::type
     ::rule_<_S("div_token"), token<lit_c<'/'>>>::type
     ::rule_<_S("arg_token"), token<lit_c<'_'>>>::type
     ::rule_<_S("int_token"), token<int_>>::type

     ::rule<_S("S"), _S("plus_exp")>::type
     ::rule<_S("plus_exp"), _S("prod_exp plus_exp_*")>::type
     ::rule<_S("plus_exp_"), _S("plus_op prod_exp")>::type
     ::rule<_S("plus_op"), _S("plus_token | minus_token")>::type
     ::rule<_S("prod_exp"), _S("value_exp prod_exp_*")>::type
     ::rule<_S("prod_exp_"), _S("prod_op value_exp")>::type
     ::rule<_S("prod_op"), _S("mult_token | div_token")>::type
     ::rule<_S("value_exp"), _S("int_token | arg_token")>::type

     ::semantic_action<_S("int_token"), build_value>::type
     ::semantic_action<_S("arg_token"), build_arg>::type
     ::semantic_action<_S("prod_exp"), build_mult>::type
     ::semantic_action<_S("plus_exp"), build_plus>::type

typedef build_parser<entire_input<g>> function_parser;


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