Synopsis

template <class P, class StateP, class ForwardOp>
struct foldl_start_with_parser;

This is a parser combinator.

Description

The same as foldl, but before folding it applies a parser, StateP on the input. If it fails, foldl_start_with_parser fails. If it succeeds, foldl_start_with_parser works as foldl taking the result of StateP as the initial state.

Here is a diagram showing how foldl_start_with_parser works by example:

using int_token = token<int_>;
using plus_token = token<lit_c<'+'>>;
using plus_int = last_of<plus_token, int_token>;
using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;

Further details can be found in the Introducing foldl_start_with_parser section of the User Manual.

Header

#include <boost/metaparse/foldl_start_with_parser.hpp>

Expression semantics

For any p parser, pt class, f metafunction class taking two arguments, s compile-time string and pos source position

foldl_start_with_parser<p, pt, f>::apply<s, pos>

is equivalent to

pt::apply<s, pos>

when the above expression returns a parsing error. It is

foldl<p, get_result<pt::apply<s, pos>>::type, f>::apply<
  get_remaining<pt::apply<s, pos>>::type,
  get_position<pt::apply<s, pos>>::type
>

otherwise.

Example

#include <boost/metaparse/foldl_start_with_parser.hpp>
#include <boost/metaparse/lit_c.hpp>
#include <boost/metaparse/last_of.hpp>
#include <boost/metaparse/token.hpp>
#include <boost/metaparse/int_.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/get_result.hpp>
#include <boost/metaparse/is_error.hpp>

#include <boost/mpl/lambda.hpp>
#include <boost/mpl/plus.hpp>

using namespace boost::metaparse;

using int_token = token<int_>;
using plus_token = token<lit_c<'+'>>;
using plus_int = last_of<plus_token, int_token>;
using sum_op =
  boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;

using ints = foldl_start_with_parser<plus_int, int_token, sum_op>;

static_assert(
  get_result<
    ints::apply<BOOST_METAPARSE_STRING("11 + 13 + 3 + 21"), start>
  >::type::value == 48,
  "ints should sum the numbers"
);

static_assert(
  is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
  "when no numbers are provided, it should be an error"
);