64 lines
1.7 KiB
C++
64 lines
1.7 KiB
C++
/*!
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@file
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Forward declares `boost::hana::flatten`.
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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_FLATTEN_HPP
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#define BOOST_HANA_FWD_FLATTEN_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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//! Collapse two levels of monadic structure into a single level.
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//! @ingroup group-Monad
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//!
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//! Given a monadic value wrapped into two levels of monad, `flatten`
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//! removes one such level. An implementation of `flatten` must satisfy
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//! @code
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//! flatten(xs) == chain(xs, id)
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//! @endcode
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//!
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//! For `Sequence`s, this simply takes a `Sequence` of `Sequence`s, and
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//! returns a (non-recursively) flattened `Sequence`.
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//!
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//!
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//! Signature
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//! ---------
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//! For a `Monad` `M`, the signature of `flatten` is
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//! @f$
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//! \mathtt{flatten} : M(M(T)) \to M(T)
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//! @f$
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//!
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//! @param xs
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//! A value with two levels of monadic structure, which should be
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//! collapsed into a single level of structure.
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//!
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//!
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//! Example
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//! -------
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//! @include example/flatten.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto flatten = [](auto&& xs) {
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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 flatten_impl : flatten_impl<M, when<true>> { };
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struct flatten_t {
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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const;
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};
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constexpr flatten_t flatten{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_FLATTEN_HPP
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