1 /*!
2 @file
3 Defines `boost::hana::on`.
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_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
__anon980a07fd0102(auto&& f, auto&& g) 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
84