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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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