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Diffstat (limited to 'boost/hana/fwd/extract.hpp')
-rw-r--r-- | boost/hana/fwd/extract.hpp | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/boost/hana/fwd/extract.hpp b/boost/hana/fwd/extract.hpp new file mode 100644 index 0000000000..8a9917bc66 --- /dev/null +++ b/boost/hana/fwd/extract.hpp @@ -0,0 +1,58 @@ +/*! +@file +Forward declares `boost::hana::extract`. + +@copyright Louis Dionne 2013-2016 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_FWD_EXTRACT_HPP +#define BOOST_HANA_FWD_EXTRACT_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Extract a value in a given comonadic context. + //! @ingroup group-Comonad + //! + //! Given a value inside a comonadic context, extract it from that + //! context, performing whatever effects are mandated by that context. + //! This can be seen as the dual operation to the `lift` method of the + //! Applicative concept. + //! + //! + //! Signature + //! --------- + //! Given a Comonad `W`, the signature is + //! \f$ + //! \mathtt{extract} : W(T) \to T + //! \f$ + //! + //! @param w + //! The value to be extracted inside a comonadic context. + //! + //! + //! Example + //! ------- + //! @include example/extract.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + constexpr auto extract = [](auto&& w) -> decltype(auto) { + return tag-dispatched; + }; +#else + template <typename W, typename = void> + struct extract_impl : extract_impl<W, when<true>> { }; + + struct extract_t { + template <typename W_> + constexpr decltype(auto) operator()(W_&& w) const; + }; + + constexpr extract_t extract{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_EXTRACT_HPP |