Synopsis

template <class P, class Pred, class Msg>
struct accept_when;

This is a parser combinator.

Description

It parses the input with P. When P rejects the input, accept_when rejects it as well. When P accepts it, accept_when evaluates Pred with the result of parsing the input with P. When Pred returns true, accept_when accepts the input and the result of parsing will be what P returned. Otherwise accept_when rejects the input and Msg is used as the error reason.

Header

#include <boost/metaparse/accept_when.hpp>

Expression semantics

For any p parser, pred predicate, msg parsing error message, s compile-time string and pos source position

accept_when<p, pred, msg>i::apply<s, pos>::type

is equivalent to

p::apply<s, pos>::type

when p::apply<s, pos> doesn't return an error and pred::apply<get_result<p::apply<s, pos>>>::type is true. Otherwise it is equivalent to

fail<msg>

Example

#include <boost/metaparse/accept_when.hpp>
#include <boost/metaparse/one_char.hpp>
#include <boost/metaparse/util/is_digit.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/is_error.hpp>
#include <boost/metaparse/get_result.hpp>
#include <boost/metaparse/define_error.hpp>

using namespace boost::metaparse;

BOOST_METAPARSE_DEFINE_ERROR(digit_expected, "Digit expected");

using accept_digit = accept_when<one_char, util::is_digit<>, digit_expected>;

static_assert(
  !is_error<
    accept_digit::apply<BOOST_METAPARSE_STRING("0"), start>
  >::type::value,
  "accept_digit should accept a digit"
);

static_assert(
  get_result<
    accept_digit::apply<BOOST_METAPARSE_STRING("0"), start>
  >::type::value == '0',
  "the result of parsing should be the character value"
);

static_assert(
  is_error<
    accept_digit::apply<BOOST_METAPARSE_STRING("x"), start>
  >::type::value,
  "accept_digit should reject a character that is not a digit"
);