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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::unpack`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
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_FWD_UNPACK_HPP | |
11 | #define BOOST_HANA_FWD_UNPACK_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Invoke a function with the elements of a Foldable as arguments. | |
19 | //! @ingroup group-Foldable | |
20 | //! | |
21 | //! Given a function and a foldable structure whose length can be known at | |
22 | //! compile-time, `unpack` invokes the function with the contents of that | |
23 | //! structure. In other words, `unpack(xs, f)` is equivalent to `f(x...)`, | |
24 | //! where `x...` are the elements of the structure. The length of the | |
25 | //! structure must be known at compile-time, because the version of `f`'s | |
26 | //! `operator()` that will be compiled depends on the number of arguments | |
27 | //! it is called with, which has to be known at compile-time. | |
28 | //! | |
29 | //! To create a function that accepts a foldable instead of variadic | |
30 | //! arguments, see `fuse` instead. | |
31 | //! | |
32 | //! | |
33 | //! @param xs | |
34 | //! The structure to expand into the function. | |
35 | //! | |
36 | //! @param f | |
37 | //! A function to be invoked as `f(x...)`, where `x...` are the elements | |
38 | //! of the structure as-if they had been linearized with `to<tuple_tag>`. | |
39 | //! | |
40 | //! | |
41 | //! Example | |
42 | //! ------- | |
43 | //! @include example/unpack.cpp | |
44 | //! | |
45 | //! | |
46 | //! Rationale: `unpack`'s name and parameter order | |
47 | //! ---------------------------------------------- | |
48 | //! It has been suggested a couple of times that `unpack` be called | |
49 | //! `apply` instead, and that the parameter order be reversed to match | |
50 | //! that of the [proposed std::apply function][1]. However, the name | |
51 | //! `apply` is already used to denote normal function application, an use | |
52 | //! which is consistent with the Boost MPL library and with the rest of | |
53 | //! the world, especially the functional programming community. | |
54 | //! Furthermore, the author of this library considers the proposed | |
55 | //! `std::apply` to have both an unfortunate name and an unfortunate | |
56 | //! parameter order. Indeed, taking the function as the first argument | |
57 | //! means that using `std::apply` with a lambda function looks like | |
58 | //! @code | |
59 | //! std::apply([](auto ...args) { | |
60 | //! use(args...); | |
61 | //! }, tuple); | |
62 | //! @endcode | |
63 | //! | |
64 | //! which is undeniably ugly because of the trailing `, tuple)` part | |
65 | //! on the last line. On the other hand, taking the function as a | |
66 | //! second argument allows one to write | |
67 | //! @code | |
68 | //! hana::unpack(tuple, [](auto ...args) { | |
69 | //! use(args...); | |
70 | //! }); | |
71 | //! @endcode | |
72 | //! | |
73 | //! which looks much nicer. Because of these observations, the author | |
74 | //! of this library feels justified to use `unpack` instead of `apply`, | |
75 | //! and to use a sane parameter order. | |
76 | //! | |
77 | //! [1]: http://en.cppreference.com/w/cpp/experimental/apply | |
78 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
79 | constexpr auto unpack = [](auto&& xs, auto&& f) -> decltype(auto) { | |
80 | return tag-dispatched; | |
81 | }; | |
82 | #else | |
83 | template <typename T, typename = void> | |
84 | struct unpack_impl : unpack_impl<T, when<true>> { }; | |
85 | ||
86 | struct unpack_t { | |
87 | template <typename Xs, typename F> | |
88 | constexpr decltype(auto) operator()(Xs&& xs, F&& f) const; | |
89 | }; | |
90 | ||
91 | constexpr unpack_t unpack{}; | |
92 | #endif | |
93 | BOOST_HANA_NAMESPACE_END | |
94 | ||
95 | #endif // !BOOST_HANA_FWD_UNPACK_HPP |