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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::fold_right`. | |
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_FOLD_RIGHT_HPP | |
11 | #define BOOST_HANA_FWD_FOLD_RIGHT_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Right-fold of a structure using a binary operation and an optional | |
19 | //! initial reduction state. | |
20 | //! @ingroup group-Foldable | |
21 | //! | |
22 | //! `fold_right` is a right-associative fold using a binary operation. | |
23 | //! Given a structure containing `x1, ..., xn`, a function `f` and | |
24 | //! an optional initial state, `fold_right` applies `f` as follows | |
25 | //! @code | |
26 | //! f(x1, f(x2, f(x3, f(x4, ... f(xn-1, xn) ... )))) // without state | |
27 | //! f(x1, f(x2, f(x3, f(x4, ... f(xn, state) ... )))) // with state | |
28 | //! @endcode | |
29 | //! | |
30 | //! @note | |
31 | //! It is worth noting that the order in which the binary function should | |
32 | //! expect its arguments is reversed from `fold_left`. | |
33 | //! | |
34 | //! When the structure is empty, two things may arise. If an initial | |
35 | //! state was provided, it is returned as-is. Otherwise, if the no-state | |
36 | //! version of the function was used, an error is triggered. When the | |
37 | //! stucture contains a single element and the no-state version of the | |
38 | //! function was used, that single element is returned as is. | |
39 | //! | |
40 | //! | |
41 | //! Signature | |
42 | //! --------- | |
43 | //! Given a `Foldable` `F` and an optional initial state of tag `S`, | |
44 | //! the signatures for `fold_right` are | |
45 | //! \f[ | |
46 | //! \mathtt{fold\_right} : F(T) \times S \times (T \times S \to S) \to S | |
47 | //! \f] | |
48 | //! | |
49 | //! for the variant with an initial state, and | |
50 | //! \f[ | |
51 | //! \mathtt{fold\_right} : F(T) \times (T \times T \to T) \to T | |
52 | //! \f] | |
53 | //! | |
54 | //! for the variant without an initial state. | |
55 | //! | |
56 | //! @param xs | |
57 | //! The structure to fold. | |
58 | //! | |
59 | //! @param state | |
60 | //! The initial value used for folding. | |
61 | //! | |
62 | //! @param f | |
63 | //! A binary function called as `f(x, state)`, where `state` is the | |
64 | //! result accumulated so far and `x` is an element in the structure. | |
65 | //! For right folds without an initial state, the function is called as | |
66 | //! `f(x1, x2)`, where `x1` and `x2` are elements of the structure. | |
67 | //! | |
68 | //! | |
69 | //! Example | |
70 | //! ------- | |
71 | //! @include example/fold_right.cpp | |
72 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
73 | constexpr auto fold_right = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) { | |
74 | return tag-dispatched; | |
75 | }; | |
76 | #else | |
77 | template <typename T, typename = void> | |
78 | struct fold_right_impl : fold_right_impl<T, when<true>> { }; | |
79 | ||
80 | struct fold_right_t { | |
81 | template <typename Xs, typename State, typename F> | |
82 | constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const; | |
83 | ||
84 | template <typename Xs, typename F> | |
85 | constexpr decltype(auto) operator()(Xs&& xs, F&& f) const; | |
86 | }; | |
87 | ||
88 | constexpr fold_right_t fold_right{}; | |
89 | #endif | |
90 | BOOST_HANA_NAMESPACE_END | |
91 | ||
92 | #endif // !BOOST_HANA_FWD_FOLD_RIGHT_HPP |