[#accept_when]
[section accept_when]

[h1 Synopsis]

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

This is a [link parser_combinator parser combinator].

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

[h1 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.

[h1 Header]

  #include <boost/metaparse/accept_when.hpp>

[h1 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>

[h1 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"
  );

[endsect]