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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::product`. | |
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_PRODUCT_HPP | |
11 | #define BOOST_HANA_FWD_PRODUCT_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | #include <boost/hana/fwd/integral_constant.hpp> | |
16 | ||
17 | ||
18 | BOOST_HANA_NAMESPACE_BEGIN | |
19 | //! Compute the product of the numbers of a structure. | |
20 | //! @ingroup group-Foldable | |
21 | //! | |
22 | //! More generally, `product` will take any foldable structure containing | |
23 | //! objects forming a Ring and reduce them using the Ring's binary | |
24 | //! operation. The initial state for folding is the identity of the | |
25 | //! Ring's operation. It is sometimes necessary to specify the Ring to | |
26 | //! use; this is possible by using `product<R>`. If no Ring is specified, | |
27 | //! the structure will use the Ring formed by the elements it contains | |
28 | //! (if it knows it), or `integral_constant_tag<int>` otherwise. | |
29 | //! Hence, | |
30 | //! @code | |
31 | //! product<R>(xs) = fold_left(xs, one<R or inferred Ring>(), mult) | |
32 | //! product<> = product<integral_constant_tag<int>> | |
33 | //! @endcode | |
34 | //! | |
35 | //! For numbers, this will just compute the product of the numbers in the | |
36 | //! `xs` structure. | |
37 | //! | |
38 | //! @note | |
39 | //! The elements of the structure are not actually required to be in the | |
40 | //! same Ring, but it must be possible to perform `mult` on any two | |
41 | //! adjacent elements of the structure, which requires each pair of | |
42 | //! adjacent element to at least have a common Ring embedding. The | |
43 | //! meaning of "adjacent" as used here is that two elements of the | |
44 | //! structure `x` and `y` are adjacent if and only if they are adjacent | |
45 | //! in the linearization of that structure, as documented by the Iterable | |
46 | //! concept. | |
47 | //! | |
48 | //! @note | |
49 | //! See the documentation for `sum` to understand why the Ring must | |
50 | //! sometimes be specified explicitly. | |
51 | //! | |
52 | //! | |
53 | //! Example | |
54 | //! ------- | |
55 | //! @include example/product.cpp | |
56 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
57 | constexpr auto product = see documentation; | |
58 | #else | |
59 | template <typename T, typename = void> | |
60 | struct product_impl : product_impl<T, when<true>> { }; | |
61 | ||
62 | template <typename R> | |
63 | struct product_t; | |
64 | ||
65 | template <typename R = integral_constant_tag<int>> | |
66 | constexpr product_t<R> product{}; | |
67 | #endif | |
68 | BOOST_HANA_NAMESPACE_END | |
69 | ||
70 | #endif // !BOOST_HANA_FWD_PRODUCT_HPP |