/*=============================================================================
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Copyright (c) 2014 Paul Fultz II
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result.h
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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==============================================================================*/
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#ifndef BOOST_HOF_GUARD_RESULT_H
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#define BOOST_HOF_GUARD_RESULT_H
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/// result
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/// ======
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///
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/// Description
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/// -----------
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///
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/// The `result` function adaptor sets the return type for the function, which
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/// can be useful when dealing with multiple overloads. Since the return type
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/// is no longer dependent on the parameters passed to the function, the
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/// `result_adaptor` provides a nested `result_type` that is the return type
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/// of the function.
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///
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/// Synopsis
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/// --------
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///
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/// template<class Result, class F>
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/// constexpr result_adaptor<Result, F> result(F f);
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///
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/// Requirements
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/// ------------
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///
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/// F must be:
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///
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/// * [ConstInvocable](ConstInvocable)
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/// * MoveConstructible
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///
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/// Example
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/// -------
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///
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/// #include <boost/hof.hpp>
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/// #include <cassert>
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///
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/// struct id
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/// {
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/// template<class T>
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/// T operator()(T x) const
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/// {
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/// return x;
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/// }
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/// };
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///
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/// int main() {
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/// auto int_result = boost::hof::result<int>(id());
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/// static_assert(std::is_same<decltype(int_result(true)), int>::value, "Not the same type");
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/// }
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///
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#include <boost/hof/detail/callable_base.hpp>
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#include <boost/hof/is_invocable.hpp>
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#include <boost/hof/always.hpp>
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#include <boost/hof/reveal.hpp>
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namespace boost { namespace hof {
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template<class Result, class F>
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struct result_adaptor : detail::callable_base<F>
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{
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BOOST_HOF_INHERIT_CONSTRUCTOR(result_adaptor, detail::callable_base<F>)
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typedef Result result_type;
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struct failure
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: failure_for<detail::callable_base<F>>
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{};
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template<class... Ts>
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constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const
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{
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return boost::hof::always_ref(*this)(xs...);
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}
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template<class... Ts, class=typename std::enable_if<(boost::hof::is_invocable<F, Ts...>::value)>::type>
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constexpr result_type operator()(Ts&&... xs) const
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{
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return this->base_function(xs...)(BOOST_HOF_FORWARD(Ts)(xs)...);
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};
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};
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template<class F>
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struct result_adaptor<void, F> : detail::callable_base<F>
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{
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BOOST_HOF_INHERIT_CONSTRUCTOR(result_adaptor, detail::callable_base<F>)
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typedef void result_type;
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template<class... Ts>
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constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const
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{
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return boost::hof::always_ref(*this)(xs...);
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}
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template<class... Ts, class=typename std::enable_if<(boost::hof::is_invocable<F, Ts...>::value)>::type>
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constexpr typename detail::holder<Ts...>::type operator()(Ts&&... xs) const
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{
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return (typename detail::holder<Ts...>::type)this->base_function(xs...)(BOOST_HOF_FORWARD(Ts)(xs)...);
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};
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};
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#if BOOST_HOF_HAS_VARIABLE_TEMPLATES
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namespace result_detail {
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template<class Result>
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struct result_f
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{
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template<class F>
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constexpr result_adaptor<Result, F> operator()(F f) const
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{
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return result_adaptor<Result, F>(boost::hof::move(f));
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}
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};
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}
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template<class Result>
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static constexpr auto result = result_detail::result_f<Result>{};
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#else
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template<class Result, class F>
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constexpr result_adaptor<Result, F> result(F f)
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{
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return result_adaptor<Result, F>(boost::hof::move(f));
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}
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#endif
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}} // namespace boost::hof
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#endif
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