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/*!
@file
Forward declares `boost::hana::empty`.
Copyright Louis Dionne 2013-2022
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_EMPTY_HPP
#define BOOST_HANA_FWD_EMPTY_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Identity of the monadic combination `concat`.
//! @ingroup group-MonadPlus
//!
//! Signature
//! ---------
//! Given a MonadPlus `M`, the signature is
//! @f$ \mathtt{empty}_M : \emptyset \to M(T) @f$.
//!
//! @tparam M
//! The tag of the monadic structure to return. This must be
//! a model of the MonadPlus concept.
//!
//!
//! Example
//! -------
//! @include example/empty.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <typename M>
constexpr auto empty = []() {
return tag-dispatched;
};
#else
template <typename M, typename = void>
struct empty_impl : empty_impl<M, when<true>> { };
template <typename M>
struct empty_t {
constexpr auto operator()() const;
};
template <typename M>
BOOST_HANA_INLINE_VARIABLE constexpr empty_t<M> empty{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_EMPTY_HPP
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