boost/hana/flatten.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::flatten`.
   4
   5 @copyright Louis Dionne 2013-2017
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_FLATTEN_HPP
  11 #define BOOST_HANA_FLATTEN_HPP
  12
  13 #include <boost/hana/fwd/flatten.hpp>
  14
  15 #include <boost/hana/concept/monad.hpp>
  16 #include <boost/hana/concept/sequence.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/core/make.hpp>
  20 #include <boost/hana/detail/unpack_flatten.hpp>
  21 #include <boost/hana/functional/id.hpp>
  22 #include <boost/hana/fwd/chain.hpp>
  23
  24 #include <cstddef>
  25 #include <utility>
  26
  27
  28 BOOST_HANA_NAMESPACE_BEGIN
  29     //! @cond
  30     template <typename Xs>
  31     constexpr auto flatten_t::operator()(Xs&& xs) const {
  32         using M = typename hana::tag_of<Xs>::type;
  33         using Flatten = BOOST_HANA_DISPATCH_IF(flatten_impl<M>,
  34             hana::Monad<M>::value
  35         );
  36
  37 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  38         static_assert(hana::Monad<M>::value,
  39         "hana::flatten(xs) requires 'xs' to be a Monad");
  40 #endif
  41
  42         return Flatten::apply(static_cast<Xs&&>(xs));
  43     }
  44     //! @endcond
  45
  46     template <typename M, bool condition>
  47     struct flatten_impl<M, when<condition>> : default_ {
  48         template <typename Xs>
  49         static constexpr auto apply(Xs&& xs)
  50         { return hana::chain(static_cast<Xs&&>(xs), hana::id); }
  51     };
  52
  53     template <typename S>
  54     struct flatten_impl<S, when<Sequence<S>::value>> {
  55         template <typename Xs>
  56         static constexpr auto apply(Xs&& xs) {
  57             return detail::unpack_flatten(static_cast<Xs&&>(xs), hana::make<S>);
  58         }
  59     };
  60 BOOST_HANA_NAMESPACE_END
  61
  62 #endif // !BOOST_HANA_FLATTEN_HPP