[#always]
[section always]

[h1 Synopsis]

  template <class P, class T>
  struct always;

This is a [link parser_combinator parser combinator].

[table Arguments
  [[Name] [Type]]
  [[`P`]  [[link parser parser]]]
  [[`T`]  [[link metaprogramming_value template metaprogramming value]]]
]

[h1 Description]

It accepts an input if and only if `P` accepts it, but the result of parsing
will be `T` instead of the result `P` returned.

[h1 Header]

  #include <boost/metaparse/always.hpp>

[h1 Expression semantics]

For any `p` parser and `t` class the following are equivalent:

  always<p, t>
  
  last_of<p, return_<t>>

[h1 Example]

  #include <boost/metaparse/always.hpp>
  #include <boost/metaparse/lit_c.hpp>
  #include <boost/metaparse/start.hpp>
  #include <boost/metaparse/string.hpp>
  #include <boost/metaparse/is_error.hpp>
  #include <boost/metaparse/get_result.hpp>
  
  #include <type_traits>
  
  using namespace boost::metaparse;
  
  using always13 = always<lit_c<'x'>, std::integral_constant<int, 13>>;
  
  static_assert(
    !is_error<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
    "always13 should accept x"
  );
  
  static_assert(
    std::is_same<
      get_result<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type,
      std::integral_constant<int, 13>
    >::type::value,
    "the result of parsing should be the integral_constant type"
  );
  
  static_assert(
    is_error<always13::apply<BOOST_METAPARSE_STRING("y"), start>>::type::value,
    "always13 should reject characters other than x"
  );

[endsect]

