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/*!
@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
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