[#is_error]
[section is_error]

[h1 Synopsis]

  template <class C>
  struct is_error;


This is a [link lazy_metafunction lazy template metafunction] that supports
[link currying currying].

[table Arguments
  [[Name] [Type]]
  [[`C`]  [[link accept accept] or [link reject reject] value]]
]

[h1 Description]

Determines if `C` is a parsing error or not. Returns a [link boxed_value boxed]
boolean value.

[h1 Header]

  #include <boost/metaparse/is_error.hpp>

[h1 Expression semantics]

For any `e` parsing error `is_error<c>::type` is a wrapped compile-time `true`
value, for any other `c` class `is_error<c>::type` is a wrapped compile-time
`false` value.

[h1 Example]

  #include <boost/metaparse/is_error.hpp>
  #include <boost/metaparse/accept.hpp>
  #include <boost/metaparse/reject.hpp>
  #include <boost/metaparse/string.hpp>
  #include <boost/metaparse/start.hpp>
  #include <boost/metaparse/define_error.hpp>
  
  #include <type_traits>
  
  using namespace boost::metaparse;
  
  BOOST_METAPARSE_DEFINE_ERROR(sample_error, "Sample error message");
  
  struct returns_reject
  {
    typedef reject<sample_error, start> type;
  };
  
  static_assert(
    !is_error<
      accept<
        std::integral_constant<int, 13>,
        BOOST_METAPARSE_STRING("foo"),
        start
      >
    >::type::value,
    "an accept should not be an error"
  );
  
  static_assert(
    is_error<reject<sample_error, start>>::type::value,
    "an reject should be an error"
  );
  
  static_assert(
    is_error<>::type::apply<reject<sample_error, start>>::type::value,
    "it should support currying"
  );
  
  static_assert(
    is_error<returns_reject>::type::value,
    "it should support lazy evaluation"
  );

[endsect]