ceph/src/boost/libs/hana/include/boost/hana/functional/on.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::on`.
   4
   5 @copyright Louis Dionne 2013-2016
   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_FUNCTIONAL_ON_HPP
  11 #define BOOST_HANA_FUNCTIONAL_ON_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/detail/create.hpp>
  15 #include <boost/hana/functional/infix.hpp>
  16
  17 #include <utility>
  18
  19
  20 BOOST_HANA_NAMESPACE_BEGIN
  21     //! @ingroup group-functional
  22     //! Invoke a function with the result of invoking another function on
  23     //! each argument.
  24     //!
  25     //! Specifically, `on(f, g)` is a function such that
  26     //! @code
  27     //!     on(f, g)(x...) == f(g(x)...)
  28     //! @endcode
  29     //!
  30     //! For convenience, `on` also supports infix application as provided
  31     //! by `infix`.
  32     //!
  33     //!
  34     //! @note
  35     //! `on` is associative, i.e. `on(f, on(g, h))` is equivalent to
  36     //! `on(on(f, g), h)`.
  37     //!
  38     //! @internal
  39     //! ### Proof of associativity
  40     //!
  41     //! @code
  42     //!     on(f, on(g, h))(xs...) == f(on(g, h)(xs)...)
  43     //!                            == f(g(h(xs))...)
  44     //!
  45     //!     on(on(f, g), h)(xs...) == on(f, g)(h(xs)...)
  46     //!                            == f(g(h(xs))...)
  47     //! @endcode
  48     //! @endinternal
  49     //!
  50     //!
  51     //! ### Example
  52     //! @include example/functional/on.cpp
  53 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  54     constexpr auto on = infix([](auto&& f, auto&& g) {
  55         return [perfect-capture](auto&& ...x) -> decltype(auto) {
  56             return forwarded(f)(g(forwarded(x))...);
  57         };
  58     });
  59 #else
  60     template <typename F, typename G>
  61     struct on_t {
  62         F f; G g;
  63         template <typename ...X>
  64         constexpr decltype(auto) operator()(X&& ...x) const& {
  65             return f(g(static_cast<X&&>(x))...);
  66         }
  67
  68         template <typename ...X>
  69         constexpr decltype(auto) operator()(X&& ...x) & {
  70             return f(g(static_cast<X&&>(x))...);
  71         }
  72
  73         template <typename ...X>
  74         constexpr decltype(auto) operator()(X&& ...x) && {
  75             return std::move(f)(g(static_cast<X&&>(x))...);
  76         }
  77     };
  78
  79     constexpr auto on = infix(detail::create<on_t>{});
  80 #endif
  81 BOOST_HANA_NAMESPACE_END
  82
  83 #endif // !BOOST_HANA_FUNCTIONAL_ON_HPP