[#one_char]
[section one_char]

[h1 Synopsis]

  struct one_char;

This is a [link parser parser].

[h1 Description]

`one_char` accepts one character. The result of parsing is the accepted
character. It fails for empty input.

[h1 Header]

  #include <boost/metaparse/one_char.hpp>

[h1 Expression semantics]

For any `s` non-empty compile-time string and `pos` source position the
following are equivalent

  get_result<one_char::apply<s, pos>>
  
  boost::mpl::front<s>

  get_remaining<one_char::apply<s, pos>>
  
  boost::mpl::pop_front<s>

The value of `get_position<one_char::apply<s, pos>>` depends on the first
character of the sequence and the value of the one parsed before that (which is
stored in the source position). `one_char` updates the source position's `col`
and `line` values based on the new line characters. It detects Linux (`\n`),
Windows (`\r\n`) and Mac (`\r`) line endings.

`one_char::apply<BOOST_METAPARSE_STRING(""), pos>` returns an error for every
`pos` source position.

[h1 Example]

  #include <boost/metaparse/one_char.hpp>
  #include <boost/metaparse/start.hpp>
  #include <boost/metaparse/string.hpp>
  #include <boost/metaparse/get_result.hpp>
  #include <boost/metaparse/get_remaining.hpp>
  #include <boost/metaparse/is_error.hpp>
  
  #include <type_traits>
  
  using namespace boost::metaparse;
  
  static_assert(
    get_result<
      one_char::apply<BOOST_METAPARSE_STRING("foo"), start>
    >::type::value == 'f',
    "the result of parsing should be the first character of the input"
  );
  
  static_assert(
    std::is_same<
      BOOST_METAPARSE_STRING("oo"),
      get_remaining<one_char::apply<BOOST_METAPARSE_STRING("foo"), start>>::type
    >::type::value,
    "one_char should consume the first character of the input"
  );
  
  static_assert(
    is_error<one_char::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
    "it should return an error for empty input"
  );

[endsect]