is_tag

Synopsis

template <class Tag, class Value>
struct is_tag
{
  // unspecified
};

Description

Metafunction checking the type of a value. Both arguments are lazy and the metafunction supports currying, thus when only Value is provided it returns a metafunction class expecting Tag as its argument.

The metafunction checks if Tag can be evaluated, and works when it is not a nullary metafunction. It is added to support tags coming from Boost.MPL.

Header

#include <mpllibs/metamonad/is_tag.hpp>

Expression semantics

For any v1, v2 and p angle-bracket expressions, where the tag of the result of p is the result of v1 and v2 evaluates to something else the following evaluate to true:

is_tag<v1, p>::type
boost::mpl::not_<is_tag<v2, p>::type>::type
apply<is_tag<v1>, p>::type
boost::mpl::not_<apply<is_tag<v2>, p>::type>::type

Example

using boost::mpl::int_;
using namespace mpllibs::metamonad::name;

catch_just<
  syntax<may_fail2<may_fail1<int_<13>>>>,
  is_tag<list_tag>,
  lambda_c<e, int_<11>>
>::type

Copyright Abel Sinkovics (abel at elte dot hu) 2011. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt

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