/*=============================================================================
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Copyright (c) 2016 Paul Fultz II
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is_unpackable.hpp
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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_IS_UNPACKABLE_HPP
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#define BOOST_HOF_GUARD_IS_UNPACKABLE_HPP
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/// is_unpackable
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/// =============
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///
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/// This is a trait that can be used to detect whether the type can be called
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/// with `unpack`.
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///
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/// Synopsis
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/// --------
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///
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/// template<class T>
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/// struct is_unpackable;
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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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/// int main() {
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/// static_assert(boost::hof::is_unpackable<std::tuple<int>>::value, "Failed");
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/// }
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///
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#include <boost/hof/unpack_sequence.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/detail/static_const_var.hpp>
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#include <boost/hof/detail/unpack_tuple.hpp>
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namespace boost { namespace hof {
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namespace detail {
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struct unpack_impl_f
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{
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template<class F, class Sequence>
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constexpr auto operator()(F&& f, Sequence&& s) const BOOST_HOF_RETURNS
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(
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boost::hof::unpack_sequence<typename std::remove_cv<typename std::remove_reference<Sequence>::type>::type>::
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apply(BOOST_HOF_FORWARD(F)(f), BOOST_HOF_FORWARD(Sequence)(s))
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);
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};
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BOOST_HOF_DECLARE_STATIC_VAR(unpack_impl, unpack_impl_f);
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#if BOOST_HOF_CHECK_UNPACK_SEQUENCE
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struct private_unpack_type {};
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template<class Sequence>
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struct unpack_impl_result
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{
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static_assert(boost::hof::is_invocable<unpack_impl_f, decltype(boost::hof::always(private_unpack_type())), Sequence>::value,
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"Unpack is invalid for this sequence. The function used to unpack this sequence is not callable."
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);
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typedef decltype(boost::hof::detail::unpack_impl(boost::hof::always(private_unpack_type()), std::declval<Sequence>())) type;
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};
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template<class Sequence>
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struct is_proper_sequence
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: std::is_same<
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private_unpack_type,
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typename unpack_impl_result<Sequence>::type
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>
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{};
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#endif
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template<class Sequence, class=void>
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struct is_unpackable_impl
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: std::true_type
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{
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#if BOOST_HOF_CHECK_UNPACK_SEQUENCE
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static_assert(is_proper_sequence<Sequence>::value,
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"Unpack is invalid for this sequence. The function used to unpack this sequence does not invoke the function."
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);
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#endif
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};
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template<class Sequence>
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struct is_unpackable_impl<Sequence, typename detail::holder<
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typename unpack_sequence<Sequence>::not_unpackable
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>::type>
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: std::false_type
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{};
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}
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template<class Sequence>
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struct is_unpackable
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: detail::is_unpackable_impl<
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typename std::remove_cv<typename std::remove_reference<Sequence>::type>::type
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>
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{
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#if BOOST_HOF_CHECK_UNPACK_SEQUENCE
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typedef detail::is_unpackable_impl<
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typename std::remove_cv<typename std::remove_reference<Sequence>::type>::type
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> base;
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typedef std::conditional<base::value, detail::is_proper_sequence<Sequence>, std::true_type> check;
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static_assert(check::type::value,
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"Unpack is invalid for this sequence. The function used to unpack this sequence does not invoke the function."
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);
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#endif
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};
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}} // namespace boost::hof
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#endif
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