[#letter]
[section letter]

[h1 Synopsis]

  struct letter;

This is a [link parser parser].

[h1 Description]

Parser accepting one character in the range `a-z` or `A-Z`. The result of the
parser is the accepted character.

[h1 Header]

  #include <boost/metaparse/letter.hpp>

[h1 Expression semantics]

The following are equivalent:

  letter
  
  accept_when<one_char, util::is_letter<>, error::letter_expected>

[h1 Example]

  #include <boost/metaparse/letter.hpp>
  #include <boost/metaparse/start.hpp>
  #include <boost/metaparse/string.hpp>
  #include <boost/metaparse/is_error.hpp>
  #include <boost/metaparse/get_result.hpp>
  
  using namespace boost::metaparse;
  
  static_assert(
    !is_error<letter::apply<BOOST_METAPARSE_STRING("a"), start>>::type::value,
    "letter should accept a letter"
  );
  
  static_assert(
    is_error<letter::apply<BOOST_METAPARSE_STRING("0"), start>>::type::value,
    "letter should reject a digit"
  );
  
  static_assert(
    get_result<
      letter::apply<BOOST_METAPARSE_STRING("z"), start>
    >::type::value == 'z',
    "the result of parsing should be the character value"
  );

[endsect]