1/*!
2@file
3Defines `boost::hana::compose`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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_COMPOSE_HPP
11#define BOOST_HANA_FUNCTIONAL_COMPOSE_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/detail/create.hpp>
15#include <boost/hana/detail/variadic/foldl1.hpp>
16
17#include <utility>
18
19
20namespace boost { namespace hana {
21 //! @ingroup group-functional
22 //! Return the composition of two functions or more.
23 //!
24 //! `compose` is defined inductively. When given more than two functions,
25 //! `compose(f, g, h...)` is equivalent to `compose(f, compose(g, h...))`.
26 //! When given two functions, `compose(f, g)` is a function such that
27 //! @code
28 //! compose(f, g)(x, y...) == f(g(x), y...)
29 //! @endcode
30 //!
31 //! If you need composition of the form `f(g(x, y...))`, use `demux` instead.
32 //!
33 //! @note
34 //! `compose` is an associative operation; `compose(f, compose(g, h))`
35 //! is equivalent to `compose(compose(f, g), h)`.
36 //!
37 //! @internal
38 //! ### Proof of associativity
39 //!
40 //! @code
41 //! compose(f, compose(g, h))(x, xs...) == f(compose(g, h)(x), xs...)
42 //! == f(g(h(x)), xs...)
43 //!
44 //! compose(compose(f, g), h)(x, xs...) == compose(f, g)(h(x), xs...)
45 //! == f(g(h(x)), xs...)
46 //! @endcode
47 //! @endinternal
48 //!
49 //! ### Example
50 //! @include example/functional/compose.cpp
51#ifdef BOOST_HANA_DOXYGEN_INVOKED
52 constexpr auto compose = [](auto&& f1, auto&& f2, ..., auto&& fn) {
53 return [perfect-capture](auto&& x, auto&& ...xs) -> decltype(auto) {
54 return forwarded(f1)(
55 forwarded(f2)(
56 ...
57 forwarded(fn)(forwarded(x))
58 ),
59 forwarded(xs)...
60 );
61 }
62 };
63#else
64 template <typename F, typename G>
65 struct _compose {
66 F f; G g;
67
68 template <typename X, typename ...Xs>
69 constexpr decltype(auto) operator()(X&& x, Xs&& ...xs) const& {
70 return f(
71 g(static_cast<X&&>(x)),
72 static_cast<Xs&&>(xs)...
73 );
74 }
75
76 template <typename X, typename ...Xs>
77 constexpr decltype(auto) operator()(X&& x, Xs&& ...xs) & {
78 return f(
79 g(static_cast<X&&>(x)),
80 static_cast<Xs&&>(xs)...
81 );
82 }
83
84 template <typename X, typename ...Xs>
85 constexpr decltype(auto) operator()(X&& x, Xs&& ...xs) && {
86 return std::move(f)(
87 std::move(g)(static_cast<X&&>(x)),
88 static_cast<Xs&&>(xs)...
89 );
90 }
91 };
92
93 struct _make_compose {
94 template <typename F, typename G, typename ...H>
95 constexpr decltype(auto) operator()(F&& f, G&& g, H&& ...h) const {
96 return detail::variadic::foldl1(detail::create<_compose>{},
97 static_cast<F&&>(f),
98 static_cast<G&&>(g),
99 static_cast<H&&>(h)...
100 );
101 }
102 };
103
104 BOOST_HANA_INLINE_VARIABLE constexpr _make_compose compose{};
105#endif
106}} // end namespace boost::hana
107
108#endif // !BOOST_HANA_FUNCTIONAL_COMPOSE_HPP
109

source code of boost/libs/hana/include/boost/hana/functional/compose.hpp