77 lines
2.5 KiB
C++
77 lines
2.5 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::cycle`.
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@copyright Louis Dionne 2013-2016
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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_CYCLE_HPP
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#define BOOST_HANA_FWD_CYCLE_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Combine a monadic structure with itself `n` times.
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//! @ingroup group-MonadPlus
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//!
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//! Given a monadic structure `xs` and a non-negative number `n`,
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//! `cycle` returns a new monadic structure which is the result of
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//! combining `xs` with itself `n` times using the `concat` operation.
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//! In other words,
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//! @code
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//! cycle(xs, n) == concat(xs, concat(xs, ... concat(xs, xs)))
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//! // ^^^^^ n times total
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//! @endcode
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//!
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//! Also note that since `concat` is required to be associative, we
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//! could also have written
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//! @code
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//! cycle(xs, n) == concat(concat(... concat(xs, xs), xs), xs)
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//! // ^^^^^ n times total
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//! @endcode
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//!
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//! If `n` is zero, then the identity of `concat`, `empty`, is returned.
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//! In the case of sequences, this boils down to returning a sequence
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//! containing `n` copies of itself; for other models it might differ.
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//!
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//!
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//! Signature
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//! ---------
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//! Given an `IntegralConstant` `C` and a `MonadPlus` `M`, the signature is
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//! @f$ \mathrm{cycle} : M(T) \times C \to M(T) @f$.
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//!
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//! @param xs
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//! A monadic structure to combine with itself a certain number of times.
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//!
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//! @param n
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//! A non-negative `IntegralConstant` representing the number of times to
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//! combine the monadic structure with itself. If `n` is zero, `cycle`
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//! returns `empty`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/cycle.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto cycle = [](auto&& xs, auto const& n) {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct cycle_impl : cycle_impl<M, when<true>> { };
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struct cycle_t {
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template <typename Xs, typename N>
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constexpr auto operator()(Xs&& xs, N const& n) const;
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};
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constexpr cycle_t cycle{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_CYCLE_HPP
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