summaryrefslogtreecommitdiff
path: root/boost/hof/flow.hpp
blob: 643189c64cd0c26ae3a2a52b17ef1f5951aded27 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
/*=============================================================================
    Copyright (c) 2015 Paul Fultz II
    flow.h
    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
==============================================================================*/

#ifndef BOOST_HOF_GUARD_FUNCTION_FLOW_H
#define BOOST_HOF_GUARD_FUNCTION_FLOW_H

/// flow
/// ====
/// 
/// Description
/// -----------
/// 
/// The `flow` function adaptor provides function composition. It is useful as
/// an alternative to using the pipe operator `|` when chaining functions. It is
/// similiar to [`compose`](compose.md) except the evaluation order is
/// reversed. So, `flow(f, g)(0)` is equivalent to `g(f(0))`.
/// 
/// 
/// Synopsis
/// --------
/// 
///     template<class... Fs>
///     constexpr flow_adaptor<Fs...> flow(Fs... fs);
/// 
/// Semantics
/// ---------
/// 
///     assert(flow(f, g)(xs...) == g(f(xs...)));
/// 
/// Requirements
/// ------------
/// 
/// Fs must be:
/// 
/// * [ConstInvocable](ConstInvocable)
/// * MoveConstructible
/// 
/// Example
/// -------
/// 
///     #include <boost/hof.hpp>
///     #include <cassert>
///     using namespace boost::hof;
/// 
///     struct increment
///     {
///         template<class T>
///         T operator()(T x) const
///         {
///             return x + 1;
///         }
///     };
/// 
///     struct decrement
///     {
///         template<class T>
///         T operator()(T x) const
///         {
///             return x - 1;
///         }
///     };
/// 
///     int main() {
///         int r = flow(increment(), decrement(), increment())(3);
///         assert(r == 4);
///     }
/// 
/// References
/// ----------
/// 
/// * [Function composition](https://en.wikipedia.org/wiki/Function_composition)
/// 

#include <boost/hof/detail/callable_base.hpp>
#include <boost/hof/always.hpp>
#include <boost/hof/detail/delegate.hpp>
#include <boost/hof/detail/compressed_pair.hpp>
#include <boost/hof/detail/join.hpp>
#include <tuple>
#include <boost/hof/detail/move.hpp>
#include <boost/hof/detail/make.hpp>
#include <boost/hof/detail/result_type.hpp>
#include <boost/hof/detail/static_const_var.hpp>

namespace boost { namespace hof { namespace detail {

template<class F1, class F2>
struct flow_kernel : detail::compressed_pair<detail::callable_base<F1>, detail::callable_base<F2>>, compose_function_result_type<F2, F1>
{
    typedef detail::compressed_pair<detail::callable_base<F1>, detail::callable_base<F2>> base_type;

    BOOST_HOF_INHERIT_CONSTRUCTOR(flow_kernel, base_type)

    BOOST_HOF_RETURNS_CLASS(flow_kernel);

    template<class... Ts>
    constexpr BOOST_HOF_SFINAE_RESULT(const detail::callable_base<F2>&, result_of<const detail::callable_base<F1>&, id_<Ts>...>) 
    operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
    (
        BOOST_HOF_MANGLE_CAST(const detail::callable_base<F2>&)(BOOST_HOF_CONST_THIS->second(xs...))(
            BOOST_HOF_MANGLE_CAST(const detail::callable_base<F1>&)(BOOST_HOF_CONST_THIS->first(xs...))(BOOST_HOF_FORWARD(Ts)(xs)...)
        )
    );
};
}

template<class F, class... Fs>
struct flow_adaptor : detail::flow_kernel<F, BOOST_HOF_JOIN(flow_adaptor, Fs...)>
{
    typedef flow_adaptor fit_rewritable_tag;
    typedef BOOST_HOF_JOIN(flow_adaptor, Fs...) tail;
    typedef detail::flow_kernel<F, tail> base_type;

    BOOST_HOF_INHERIT_DEFAULT(flow_adaptor, base_type)

    template<class X, class... Xs, 
        BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(detail::callable_base<F>, X), 
        BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(tail, Xs...)
    >
    constexpr flow_adaptor(X&& f1, Xs&& ... fs) 
    BOOST_HOF_NOEXCEPT(BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(base_type, X&&, tail) && BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(tail, Xs&&...))
    : base_type(BOOST_HOF_FORWARD(X)(f1), tail(BOOST_HOF_FORWARD(Xs)(fs)...))
    {}

    template<class X,
        BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(detail::callable_base<F>, X)
    >
    constexpr flow_adaptor(X&& f1) 
    BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(base_type, X&&)
    : base_type(BOOST_HOF_FORWARD(X)(f1))
    {}
};

template<class F>
struct flow_adaptor<F> : detail::callable_base<F>
{
    typedef flow_adaptor fit_rewritable_tag;
    BOOST_HOF_INHERIT_DEFAULT(flow_adaptor, detail::callable_base<F>)

    template<class X, BOOST_HOF_ENABLE_IF_CONVERTIBLE(X, detail::callable_base<F>)>
    constexpr flow_adaptor(X&& f1) 
    BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(detail::callable_base<F>, X&&)
    : detail::callable_base<F>(BOOST_HOF_FORWARD(X)(f1))
    {}

};

template<class F1, class F2>
struct flow_adaptor<F1, F2>
: detail::flow_kernel<detail::callable_base<F1>, detail::callable_base<F2>>
{
    typedef flow_adaptor fit_rewritable_tag;
    typedef detail::flow_kernel<detail::callable_base<F1>, detail::callable_base<F2>> base_type;

    BOOST_HOF_INHERIT_CONSTRUCTOR(flow_adaptor, base_type)
};

BOOST_HOF_DECLARE_STATIC_VAR(flow, detail::make<flow_adaptor>);

}} // namespace boost::hof

#endif