[#change_error_message]
[section change_error_message]

[h1 Synopsis]

  template <class P, class Msg>
  struct change_error_message;

This is a [link parser_combinator parser combinator].

[table Arguments
  [[Name]  [Type]]
  [[`P`]   [[link parser parser]]]
  [[`Msg`] [[link parsing_error_message parsing error message]]]
]

[h1 Description]

It applies `P` on the input. When `P` succeeds, `change_error_message` returns
the result `P` returns, otherwise `change_error_message` rejects the input and
the reason will be `Msg`.

[h1 Header]

  #include <boost/metaparse/change_error_message.hpp>

[h1 Expression semantics]

For any `p` parser and `m` parsing error message, `s` compile-time string and
`pos` source position

  change_error_message<p, msg>::apply<s, pos>

is equivalent to `p::apply<s, pos>` when `p` accepts the input.
It is equivalent to `fail<msg>::apply<s, pos>` otherwise.

[h1 Example]

  #include <boost/metaparse/change_error_message.hpp>
  #include <boost/metaparse/repeated1.hpp>
  #include <boost/metaparse/letter.hpp>
  #include <boost/metaparse/keyword.hpp>
  #include <boost/metaparse/last_of.hpp>
  #include <boost/metaparse/token.hpp>
  #include <boost/metaparse/string.hpp>
  #include <boost/metaparse/is_error.hpp>
  #include <boost/metaparse/start.hpp>
  #include <boost/metaparse/get_message.hpp>
  #include <boost/metaparse/define_error.hpp>
  
  #include <type_traits>
  
  using namespace boost::metaparse;
  
  BOOST_METAPARSE_DEFINE_ERROR(name_expected, "Name expected");
  
  using keyword_name = token<keyword<BOOST_METAPARSE_STRING("name")>>;
  using name_token = token<repeated1<letter>>;
  
  using name_parser =
    last_of<keyword_name, change_error_message<name_token, name_expected>>;
  
  static_assert(
    !is_error<
      name_parser::apply<BOOST_METAPARSE_STRING("name Bela"), start>
    >::type::value,
    "name_parser should accept \"name <a name>\""
  );
  
  static_assert(
    is_error<
      name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
    >::type::value,
    "name_parser should reject input when name is a question mark"
  );
  
  static_assert(
    std::is_same<
      get_message<
        name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
      >::type,
      name_expected
    >::type::value,
    "the error message should be the one specified by change_error_message"
  );

[endsect]

