63e88f80
 Hu Chunming
提交三方库
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/*!
@file
Forward declares `boost::hana::duplicate`.
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_DUPLICATE_HPP
#define BOOST_HANA_FWD_DUPLICATE_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Add an extra layer of comonadic context to a comonadic value.
//! @ingroup group-Comonad
//!
//! Given a value already in a comonadic context, `duplicate` wraps this
//! value with an additional layer of comonadic context. This can be seen
//! as the dual operation to `flatten` from the Monad concept.
//!
//!
//! Signature
//! ---------
//! Given a Comonad `W`, the signature is
//! \f$
//! \mathtt{duplicate} : W(T) \to W(W(T))
//! \f$
//!
//! @param w
//! The value to wrap in an additional level of comonadic context.
//!
//!
//! Example
//! -------
//! @include example/duplicate.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto duplicate = [](auto&& w) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename W, typename = void>
struct duplicate_impl : duplicate_impl<W, when<true>> { };
struct duplicate_t {
template <typename W_>
constexpr decltype(auto) operator()(W_&& w) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr duplicate_t duplicate{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_DUPLICATE_HPP
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