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grammar

Synopsis

template <class StartSymbol = BOOST_METAPARSE_STRING("S")>
struct grammar
{
  template <class S, class Pos>
  struct apply;

  template <class Name, class P>
  struct import;

  template <class S, class Action = /* unspecified */>
  struct rule;
};

Description

[Note] Note

Note that using this adds a significant overhead to your builds. When someone uses your parser, the compiler will have to build your grammar parser, use it to parse your grammar and build your parser and then it can parse the input the user would like to parse with your parser. You might consider using the parser combinators the library provides.

Parser combinator for constructing parsers based on an embedded DSL similar to EBNF. It can be used the following way:

grammar<>
  // definitions

where a definition can be a rule or an import command.

Rules look like on of the following:

::rule<BOOST_METAPARSE_STRING("name ::= def")>
::rule<BOOST_METAPARSE_STRING("name ::= def"), semantic_action>

name consists of letters, digits and the _ character. It is the name of the symbol being defined. def describes the rule. It can be

Rules take an optional semantic_action argument. It is a placeholder expression taking one argument. When this is given, this is used to transform the result of the rule.

Imports can be used to turn an arbitrary parser into a symbol available for the rules. Import definitions look like the following:

::import<BOOST_METAPARSE_STRING("name"), parser>

name is the name of the symbol, parser is the parser to bind the name to.

The start symbol of the grammar is specified by the template argument of the grammar template. This is optional, the default value is S.

Header

#include <boost/metaparse/grammar.hpp>

Example

#define BOOST_METAPARSE_LIMIT_STRING_SIZE 64

#include <boost/metaparse/grammar.hpp>
#include <boost/metaparse/token.hpp>
#include <boost/metaparse/int_.hpp>
#include <boost/metaparse/entire_input.hpp>
#include <boost/metaparse/build_parser.hpp>
#include <boost/metaparse/string.hpp>

#include <boost/mpl/front.hpp>
#include <boost/mpl/back.hpp>
#include <boost/mpl/plus.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/lambda.hpp>

using boost::metaparse::token;
using boost::metaparse::int_;
using boost::metaparse::build_parser;
using boost::metaparse::entire_input;
using boost::metaparse::grammar;

using boost::mpl::front;
using boost::mpl::back;
using boost::mpl::plus;
using boost::mpl::fold;
using boost::mpl::lambda;
using boost::mpl::_1;
using boost::mpl::_2;

template <class A, class B>
struct lazy_plus : boost::mpl::plus<typename A::type, typename B::type> {};

template <class Sequence, class State, class ForwardOp>
struct lazy_fold :
  fold<typename Sequence::type, typename State::type, typename ForwardOp::type>
{};

using plus_action =
  lazy_fold<back<_1>, front<_1>, lambda<lazy_plus<_1, back<_2>>>::type>;

using plus_grammar =
  grammar<BOOST_METAPARSE_STRING("plus_exp")>
    ::import<BOOST_METAPARSE_STRING("int_token"), token<int_>>::type

    ::rule<BOOST_METAPARSE_STRING("ws ::= (' ' | '\n' | '\r' | '\t')*")>::type
    ::rule<BOOST_METAPARSE_STRING("plus_token ::= '+' ws"), front<_1>>::type
    ::rule<BOOST_METAPARSE_STRING("plus_exp ::= int_token (plus_token int_token)*"), plus_action>::type
  ;

using plus_parser = build_parser<entire_input<plus_grammar>>;

static_assert(
  plus_parser::apply<BOOST_METAPARSE_STRING("1 + 2 + 3 + 4")>::type::value == 10,
  "Arithmetic expression should be evaluated"
);

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