1/*=============================================================================
2 Copyright (c) 2015 Paul Fultz II
3 flow.h
4 Distributed under the Boost Software License, Version 1.0. (See accompanying
5 file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6==============================================================================*/
7
8#ifndef BOOST_HOF_GUARD_FUNCTION_FLOW_H
9#define BOOST_HOF_GUARD_FUNCTION_FLOW_H
10
11/// flow
12/// ====
13///
14/// Description
15/// -----------
16///
17/// The `flow` function adaptor provides function composition. It is useful as
18/// an alternative to using the pipe operator `|` when chaining functions. It is
19/// similiar to [`compose`](compose.md) except the evaluation order is
20/// reversed. So, `flow(f, g)(0)` is equivalent to `g(f(0))`.
21///
22///
23/// Synopsis
24/// --------
25///
26/// template<class... Fs>
27/// constexpr flow_adaptor<Fs...> flow(Fs... fs);
28///
29/// Semantics
30/// ---------
31///
32/// assert(flow(f, g)(xs...) == g(f(xs...)));
33///
34/// Requirements
35/// ------------
36///
37/// Fs must be:
38///
39/// * [ConstInvocable](ConstInvocable)
40/// * MoveConstructible
41///
42/// Example
43/// -------
44///
45/// #include <boost/hof.hpp>
46/// #include <cassert>
47/// using namespace boost::hof;
48///
49/// struct increment
50/// {
51/// template<class T>
52/// T operator()(T x) const
53/// {
54/// return x + 1;
55/// }
56/// };
57///
58/// struct decrement
59/// {
60/// template<class T>
61/// T operator()(T x) const
62/// {
63/// return x - 1;
64/// }
65/// };
66///
67/// int main() {
68/// int r = flow(increment(), decrement(), increment())(3);
69/// assert(r == 4);
70/// }
71///
72/// References
73/// ----------
74///
75/// * [Function composition](https://en.wikipedia.org/wiki/Function_composition)
76///
77
78#include <boost/hof/detail/callable_base.hpp>
79#include <boost/hof/always.hpp>
80#include <boost/hof/detail/delegate.hpp>
81#include <boost/hof/detail/compressed_pair.hpp>
82#include <boost/hof/detail/join.hpp>
83#include <tuple>
84#include <boost/hof/detail/move.hpp>
85#include <boost/hof/detail/make.hpp>
86#include <boost/hof/detail/result_type.hpp>
87#include <boost/hof/detail/static_const_var.hpp>
88
89namespace boost { namespace hof { namespace detail {
90
91template<class F1, class F2>
92struct flow_kernel : detail::compressed_pair<detail::callable_base<F1>, detail::callable_base<F2>>, compose_function_result_type<F2, F1>
93{
94 typedef detail::compressed_pair<detail::callable_base<F1>, detail::callable_base<F2>> base_type;
95
96 BOOST_HOF_INHERIT_CONSTRUCTOR(flow_kernel, base_type)
97
98 BOOST_HOF_RETURNS_CLASS(flow_kernel);
99
100 template<class... Ts>
101 constexpr BOOST_HOF_SFINAE_RESULT(const detail::callable_base<F2>&, result_of<const detail::callable_base<F1>&, id_<Ts>...>)
102 operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
103 (
104 BOOST_HOF_MANGLE_CAST(const detail::callable_base<F2>&)(BOOST_HOF_CONST_THIS->second(xs...))(
105 BOOST_HOF_MANGLE_CAST(const detail::callable_base<F1>&)(BOOST_HOF_CONST_THIS->first(xs...))(BOOST_HOF_FORWARD(Ts)(xs)...)
106 )
107 );
108};
109}
110
111template<class F, class... Fs>
112struct flow_adaptor : detail::flow_kernel<F, BOOST_HOF_JOIN(flow_adaptor, Fs...)>
113{
114 typedef flow_adaptor fit_rewritable_tag;
115 typedef BOOST_HOF_JOIN(flow_adaptor, Fs...) tail;
116 typedef detail::flow_kernel<F, tail> base_type;
117
118 BOOST_HOF_INHERIT_DEFAULT(flow_adaptor, base_type)
119
120 template<class X, class... Xs,
121 BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(detail::callable_base<F>, X),
122 BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(tail, Xs...)
123 >
124 constexpr flow_adaptor(X&& f1, Xs&& ... fs)
125 BOOST_HOF_NOEXCEPT(BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(base_type, X&&, tail) && BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(tail, Xs&&...))
126 : base_type(BOOST_HOF_FORWARD(X)(f1), tail(BOOST_HOF_FORWARD(Xs)(fs)...))
127 {}
128
129 template<class X,
130 BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(detail::callable_base<F>, X)
131 >
132 constexpr flow_adaptor(X&& f1)
133 BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(base_type, X&&)
134 : base_type(BOOST_HOF_FORWARD(X)(f1))
135 {}
136};
137
138template<class F>
139struct flow_adaptor<F> : detail::callable_base<F>
140{
141 typedef flow_adaptor fit_rewritable_tag;
142 BOOST_HOF_INHERIT_DEFAULT(flow_adaptor, detail::callable_base<F>)
143
144 template<class X, BOOST_HOF_ENABLE_IF_CONVERTIBLE(X, detail::callable_base<F>)>
145 constexpr flow_adaptor(X&& f1)
146 BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(detail::callable_base<F>, X&&)
147 : detail::callable_base<F>(BOOST_HOF_FORWARD(X)(f1))
148 {}
149
150};
151
152template<class F1, class F2>
153struct flow_adaptor<F1, F2>
154: detail::flow_kernel<detail::callable_base<F1>, detail::callable_base<F2>>
155{
156 typedef flow_adaptor fit_rewritable_tag;
157 typedef detail::flow_kernel<detail::callable_base<F1>, detail::callable_base<F2>> base_type;
158
159 BOOST_HOF_INHERIT_CONSTRUCTOR(flow_adaptor, base_type)
160};
161
162BOOST_HOF_DECLARE_STATIC_VAR(flow, detail::make<flow_adaptor>);
163
164}} // namespace boost::hof
165
166#endif
167

source code of boost/libs/hof/include/boost/hof/flow.hpp