hana/fwd/lift.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::lift`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_LIFT_HPP
0011 #define BOOST_HANA_FWD_LIFT_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Lift a value into an `Applicative` structure.
0019     //! @ingroup group-Applicative
0020     //!
0021     //! `lift<A>` takes a normal value and embeds it into a structure whose
0022     //! shape is represented by the `A` `Applicative`. Note that the value
0023     //! may be a function, in which case the created structure may be
0024     //! `ap`plied to another `Applicative` structure containing values.
0025     //!
0026     //!
0027     //! Signature
0028     //! ---------
0029     //! Given an Applicative `A`, the signature is
0030     //! @f$ \mathtt{lift}_A : T \to A(T) @f$.
0031     //!
0032     //! @tparam A
0033     //! A tag representing the `Applicative` into which the value is lifted.
0034     //!
0035     //! @param x
0036     //! The value to lift into the applicative.
0037     //!
0038     //!
0039     //! Example
0040     //! -------
0041     //! @include example/lift.cpp
0042 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0043     template <typename A>
0044     constexpr auto lift = [](auto&& x) {
0045         return tag-dispatched;
0046     };
0047 #else
0048     template <typename A, typename = void>
0049     struct lift_impl : lift_impl<A, when<true>> { };
0050
0051     template <typename A>
0052     struct lift_t {
0053         template <typename X>
0054         constexpr auto operator()(X&& x) const;
0055     };
0056
0057     template <typename A>
0058     BOOST_HANA_INLINE_VARIABLE constexpr lift_t<A> lift{};
0059 #endif
0060 }} // end namespace boost::hana
0061
0062 #endif // !BOOST_HANA_FWD_LIFT_HPP