1/*!
2@file
3Forward declares `boost::hana::product`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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
18namespace boost { namespace hana {
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 template <typename Xs>
65 constexpr decltype(auto) operator()(Xs&& xs) const;
66 };
67
68 template <typename R = integral_constant_tag<int>>
69 BOOST_HANA_INLINE_VARIABLE constexpr product_t<R> product{};
70#endif
71}} // end namespace boost::hana
72
73#endif // !BOOST_HANA_FWD_PRODUCT_HPP
74

source code of boost/libs/hana/include/boost/hana/fwd/product.hpp