59 lines
1.6 KiB
C++
59 lines
1.6 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::extract`.
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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_EXTRACT_HPP
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#define BOOST_HANA_FWD_EXTRACT_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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//! Extract a value in a given comonadic context.
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//! @ingroup group-Comonad
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//!
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//! Given a value inside a comonadic context, extract it from that
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//! context, performing whatever effects are mandated by that context.
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//! This can be seen as the dual operation to the `lift` method of the
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//! Applicative concept.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a Comonad `W`, the signature is
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//! \f$
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//! \mathtt{extract} : W(T) \to T
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//! \f$
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//!
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//! @param w
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//! The value to be extracted inside a comonadic context.
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//!
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//!
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//! Example
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//! -------
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//! @include example/extract.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto extract = [](auto&& w) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename W, typename = void>
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struct extract_impl : extract_impl<W, when<true>> { };
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struct extract_t {
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template <typename W_>
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constexpr decltype(auto) operator()(W_&& w) const;
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};
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constexpr extract_t extract{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_EXTRACT_HPP
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