74 lines
2.6 KiB
C++
74 lines
2.6 KiB
C++
/*!
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@file
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Forward declares `boost::hana::product`.
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@copyright Louis Dionne 2013-2017
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_PRODUCT_HPP
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#define BOOST_HANA_FWD_PRODUCT_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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#include <boost/hana/fwd/integral_constant.hpp>
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namespace boost { namespace hana {
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//! Compute the product of the numbers of a structure.
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//! @ingroup group-Foldable
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//!
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//! More generally, `product` will take any foldable structure containing
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//! objects forming a Ring and reduce them using the Ring's binary
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//! operation. The initial state for folding is the identity of the
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//! Ring's operation. It is sometimes necessary to specify the Ring to
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//! use; this is possible by using `product<R>`. If no Ring is specified,
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//! the structure will use the Ring formed by the elements it contains
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//! (if it knows it), or `integral_constant_tag<int>` otherwise.
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//! Hence,
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//! @code
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//! product<R>(xs) = fold_left(xs, one<R or inferred Ring>(), mult)
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//! product<> = product<integral_constant_tag<int>>
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//! @endcode
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//!
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//! For numbers, this will just compute the product of the numbers in the
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//! `xs` structure.
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//!
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//! @note
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//! The elements of the structure are not actually required to be in the
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//! same Ring, but it must be possible to perform `mult` on any two
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//! adjacent elements of the structure, which requires each pair of
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//! adjacent element to at least have a common Ring embedding. The
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//! meaning of "adjacent" as used here is that two elements of the
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//! structure `x` and `y` are adjacent if and only if they are adjacent
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//! in the linearization of that structure, as documented by the Iterable
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//! concept.
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//!
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//! @note
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//! See the documentation for `sum` to understand why the Ring must
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//! sometimes be specified explicitly.
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//!
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//!
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//! Example
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//! -------
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//! @include example/product.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto product = see documentation;
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#else
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template <typename T, typename = void>
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struct product_impl : product_impl<T, when<true>> { };
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template <typename R>
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struct product_t {
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template <typename Xs>
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constexpr decltype(auto) operator()(Xs&& xs) const;
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};
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template <typename R = integral_constant_tag<int>>
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BOOST_HANA_INLINE_VARIABLE constexpr product_t<R> product{};
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#endif
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}} // end namespace boost::hana
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#endif // !BOOST_HANA_FWD_PRODUCT_HPP
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