58 lines
1.8 KiB
C++
58 lines
1.8 KiB
C++
/*!
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@file
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Defines `boost::hana::eval`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_EVAL_HPP
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#define BOOST_HANA_EVAL_HPP
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#include <boost/hana/fwd/eval.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/detail/wrong.hpp>
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#include <boost/hana/functional/id.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Expr>
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constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
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return eval_impl<typename hana::tag_of<Expr>::type>::apply(
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static_cast<Expr&&>(expr)
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);
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}
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//! @endcond
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template <typename T, bool condition>
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struct eval_impl<T, when<condition>> : default_ {
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template <typename Expr>
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static constexpr auto eval_helper(Expr&& expr, int)
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-> decltype(static_cast<Expr&&>(expr)())
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{ return static_cast<Expr&&>(expr)(); }
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template <typename Expr>
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static constexpr auto eval_helper(Expr&& expr, long)
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-> decltype(static_cast<Expr&&>(expr)(hana::id))
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{ return static_cast<Expr&&>(expr)(hana::id); }
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template <typename Expr>
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static constexpr auto eval_helper(Expr&&, ...) {
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static_assert(detail::wrong<Expr>{},
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"hana::eval(expr) requires the expression to be a hana::lazy, "
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"a nullary Callable or a unary Callable that may be "
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"called with hana::id");
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}
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template <typename Expr>
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static constexpr decltype(auto) apply(Expr&& expr)
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{ return eval_helper(static_cast<Expr&&>(expr), int{}); }
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_EVAL_HPP
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