1/*!
2@file
3Forward declares `boost::hana::extract`.
4
5Copyright Louis Dionne 2013-2022
6Distributed under the Boost Software License, Version 1.0.
7(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
8 */
9
10#ifndef BOOST_HANA_FWD_EXTRACT_HPP
11#define BOOST_HANA_FWD_EXTRACT_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/core/when.hpp>
15
16
17namespace boost { namespace hana {
18 //! Extract a value in a given comonadic context.
19 //! @ingroup group-Comonad
20 //!
21 //! Given a value inside a comonadic context, extract it from that
22 //! context, performing whatever effects are mandated by that context.
23 //! This can be seen as the dual operation to the `lift` method of the
24 //! Applicative concept.
25 //!
26 //!
27 //! Signature
28 //! ---------
29 //! Given a Comonad `W`, the signature is
30 //! \f$
31 //! \mathtt{extract} : W(T) \to T
32 //! \f$
33 //!
34 //! @param w
35 //! The value to be extracted inside a comonadic context.
36 //!
37 //!
38 //! Example
39 //! -------
40 //! @include example/extract.cpp
41#ifdef BOOST_HANA_DOXYGEN_INVOKED
42 constexpr auto extract = [](auto&& w) -> decltype(auto) {
43 return tag-dispatched;
44 };
45#else
46 template <typename W, typename = void>
47 struct extract_impl : extract_impl<W, when<true>> { };
48
49 struct extract_t {
50 template <typename W_>
51 constexpr decltype(auto) operator()(W_&& w) const;
52 };
53
54 BOOST_HANA_INLINE_VARIABLE constexpr extract_t extract{};
55#endif
56}} // end namespace boost::hana
57
58#endif // !BOOST_HANA_FWD_EXTRACT_HPP
59

source code of boost/libs/hana/include/boost/hana/fwd/extract.hpp