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1/*=============================================================================
2    Copyright (c) 2012 Paul Fultz II
3    first_of.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_CONDITIONAL_H
9#define BOOST_HOF_GUARD_FUNCTION_CONDITIONAL_H
10
11/// first_of
12/// ========
13///
14/// Description
15/// -----------
16///
17/// The `first_of` function adaptor combines several functions together. If
18/// the first function can not be called, then it will try to call the next
19/// function. This can be very useful when overloading functions using
20/// template constraints(such as with `enable_if`).
21///
22/// Note: This is different than the [`match`](match.md) function adaptor, which
23/// can lead to ambiguities. Instead, `first_of` will call the first function
24/// that is callable, regardless if there is another function that could be
25/// called as well.
26///
27/// Synopsis
28/// --------
29///
30///     template<class... Fs>
31///     constexpr first_of_adaptor<Fs...> first_of(Fs... fs);
32///
33/// Requirements
34/// ------------
35///
36/// Fs must be:
37///
38/// * [ConstInvocable](ConstInvocable)
39/// * MoveConstructible
40///
41/// Example
42/// -------
43///
44///     #include <boost/hof.hpp>
45///     #include <iostream>
46///     using namespace boost::hof;
47///
48///     struct for_ints
49///     {
50///         void operator()(int) const
51///         {
52///             printf("Int\n");
53///         }
54///     };
55///
56///     struct for_floats
57///     {
58///         void operator()(float) const
59///         {
60///             printf("Float\n");
61///         }
62///     };
63///
64///     int main() {
65///         first_of(for_ints(), for_floats())(3.0);
66///     }
67///
68/// This will print `Int` because the `for_floats` function object won't ever be
69/// called. Due to the conversion rules in C++, the `for_ints` function can be
70/// called on floats, so it is chosen by `first_of` first, even though
71/// `for_floats` is a better match.
72///
73/// So, the order of the functions in the `first_of_adaptor` are very important
74/// to how the function is chosen.
75///
76/// References
77/// ----------
78///
79/// * [POO51](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0051r2.pdf) - Proposal for C++
80///   Proposal for C++ generic overload function
81/// * [Conditional overloading](<Conditional overloading>)
82///
83
84#include <boost/hof/reveal.hpp>
85#include <boost/hof/detail/compressed_pair.hpp>
86#include <boost/hof/detail/callable_base.hpp>
87#include <boost/hof/detail/delegate.hpp>
88#include <boost/hof/detail/join.hpp>
89#include <boost/hof/detail/seq.hpp>
90#include <boost/hof/detail/make.hpp>
91#include <boost/hof/detail/static_const_var.hpp>
92
93namespace boost { namespace hof {
94
95namespace detail {
96
97template<class F1, class F2>
98struct basic_first_of_adaptor : F1, F2
99{
100    BOOST_HOF_INHERIT_DEFAULT(basic_first_of_adaptor, F1, F2)
101
102    template<class A, class B,
103        BOOST_HOF_ENABLE_IF_CONVERTIBLE(A, F1),
104        BOOST_HOF_ENABLE_IF_CONVERTIBLE(B, F2)>
105    constexpr basic_first_of_adaptor(A&& f1, B&& f2)
106    noexcept(BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(F1, A&&) && BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(F2, B&&))
107    : F1(BOOST_HOF_FORWARD(A)(f1)), F2(BOOST_HOF_FORWARD(B)(f2))
108    {}
109
110    template<class X,
111        class=typename std::enable_if<
112        BOOST_HOF_IS_CONVERTIBLE(X, F1) &&
113        BOOST_HOF_IS_DEFAULT_CONSTRUCTIBLE(F2)
114    >::type>
115    constexpr basic_first_of_adaptor(X&& x)
116    BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(F1, X&&)
117    : F1(BOOST_HOF_FORWARD(X)(x))
118    {}
119
120    template<class... Ts>
121    struct select
122    : std::conditional
123    <
124        is_invocable<F1, Ts...>::value,
125        F1,
126        F2
127    >
128    {};
129
130    BOOST_HOF_RETURNS_CLASS(basic_first_of_adaptor);
131
132    template<class... Ts, class F=typename select<Ts...>::type>
133    constexpr BOOST_HOF_SFINAE_RESULT(typename select<Ts...>::type, id_<Ts>...)
134    operator()(Ts && ... xs) const
135    BOOST_HOF_SFINAE_RETURNS
136    (
137        BOOST_HOF_RETURNS_STATIC_CAST(const F&)(*BOOST_HOF_CONST_THIS)(BOOST_HOF_FORWARD(Ts)(xs)...)
138    );
139};
140
141template <class F1, class F2>
142constexpr const F1& which(std::true_type, const F1& f1, const F2&) noexcept
143{
144    return f1;
145}
146
147template <class F1, class F2>
148constexpr const F2& which(std::false_type, const F1&, const F2& f2) noexcept
149{
150    return f2;
151}
152
153template<class F1, class F2>
154struct conditional_kernel : compressed_pair<F1, F2>
155{
156    typedef compressed_pair<F1, F2> base;
157    BOOST_HOF_INHERIT_CONSTRUCTOR(conditional_kernel, base)
158
159    template<class... Ts>
160    struct select
161    : std::conditional
162    <
163        is_invocable<F1, Ts...>::value,
164        F1,
165        F2
166    >
167    {};
168
169    BOOST_HOF_RETURNS_CLASS(conditional_kernel);
170
171    template<class... Ts, class PickFirst=is_invocable<F1, Ts...>>
172    constexpr BOOST_HOF_SFINAE_RESULT(typename select<Ts...>::type, id_<Ts>...)
173    operator()(Ts && ... xs) const
174    BOOST_HOF_SFINAE_RETURNS
175    (
176        boost::hof::detail::which(
177            BOOST_HOF_RETURNS_CONSTRUCT(PickFirst)(),
178            BOOST_HOF_MANGLE_CAST(const F1&)(BOOST_HOF_CONST_THIS->first(xs...)),
179            BOOST_HOF_MANGLE_CAST(const F2&)(BOOST_HOF_CONST_THIS->second(xs...))
180        )
181        (BOOST_HOF_FORWARD(Ts)(xs)...)
182    );
183};
184}
185
186template<class F, class... Fs>
187struct first_of_adaptor
188: detail::conditional_kernel<F, BOOST_HOF_JOIN(first_of_adaptor, Fs...) >
189{
190    typedef first_of_adaptor fit_rewritable_tag;
191    typedef BOOST_HOF_JOIN(first_of_adaptor, Fs...) kernel_base;
192    typedef detail::conditional_kernel<F, kernel_base > base;
193
194    BOOST_HOF_INHERIT_DEFAULT(first_of_adaptor, base)
195
196    template<class X, class... Xs,
197        BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(base, X, kernel_base),
198        BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(kernel_base, Xs...)>
199    constexpr first_of_adaptor(X&& f1, Xs&& ... fs)
200    noexcept(BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(base, X&&, kernel_base) && BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(kernel_base, Xs&&...))
201    : base(BOOST_HOF_FORWARD(X)(f1), kernel_base(BOOST_HOF_FORWARD(Xs)(fs)...))
202    {}
203
204    template<class X, class... Xs,
205        BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(base, X)>
206    constexpr first_of_adaptor(X&& f1)
207    BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(base, X&&)
208    : base(BOOST_HOF_FORWARD(X)(f1))
209    {}
210
211    struct failure
212    : failure_for<F, Fs...>
213    {};
214};
215
216template<class F>
217struct first_of_adaptor<F> : F
218{
219    typedef first_of_adaptor fit_rewritable_tag;
220    BOOST_HOF_INHERIT_CONSTRUCTOR(first_of_adaptor, F);
221
222    struct failure
223    : failure_for<F>
224    {};
225};
226
227template<class F1, class F2>
228struct first_of_adaptor<F1, F2>
229: detail::conditional_kernel<F1, F2>
230{
231    typedef detail::conditional_kernel<F1, F2> base;
232    typedef first_of_adaptor fit_rewritable_tag;
233    BOOST_HOF_INHERIT_CONSTRUCTOR(first_of_adaptor, base);
234
235    struct failure
236    : failure_for<F1, F2>
237    {};
238};
239
240BOOST_HOF_DECLARE_STATIC_VAR(first_of, detail::make<first_of_adaptor>);
241
242}} // namespace boost::hof
243
244#endif
245