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/*!
@file
Forward declares `boost::hana::extend`.
@copyright Louis Dionne 2013-2017
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_EXTEND_HPP
#define BOOST_HANA_FWD_EXTEND_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Comonadic application of a function to a comonadic value.
//! @ingroup group-Comonad
//!
//! Given a comonadic value and a function accepting a comonadic input,
//! `extend` returns the result of applying the function to that input
//! inside the comonadic context.
//!
//!
//! Signature
//! ---------
//! Given a Comonad `W` and a function of type \f$ W(T) \to U \f$, the
//! signature is
//! \f$
//! \mathtt{extend} : W(T) \times (W(T) \to U) \to W(U)
//! \f$
//!
//! @param w
//! A comonadic value to call the function with.
//!
//! @param f
//! A function of signature \f$ W(T) \to U \f$ to be applied to its
//! comonadic argument inside the comonadic context.
//!
//!
//! Example
//! -------
//! @include example/extend.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto extend = [](auto&& w, auto&& f) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename W, typename = void>
struct extend_impl : extend_impl<W, when<true>> { };
struct extend_t {
template <typename W_, typename F>
constexpr decltype(auto) operator()(W_&& w, F&& f) const;
};
constexpr extend_t extend{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_EXTEND_HPP
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