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/*!
@file
Defines `boost::hana::eval`.
@copyright Louis Dionne 2013-2016
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_EVAL_HPP
#define BOOST_HANA_EVAL_HPP
#include <boost/hana/fwd/eval.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/detail/wrong.hpp>
#include <boost/hana/functional/id.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! @cond
template <typename Expr>
constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
return eval_impl<typename hana::tag_of<Expr>::type>::apply(
static_cast<Expr&&>(expr)
);
}
//! @endcond
template <typename T, bool condition>
struct eval_impl<T, when<condition>> : default_ {
template <typename Expr>
static constexpr auto eval_helper(Expr&& expr, int)
-> decltype(static_cast<Expr&&>(expr)())
{ return static_cast<Expr&&>(expr)(); }
template <typename Expr>
static constexpr auto eval_helper(Expr&& expr, long)
-> decltype(static_cast<Expr&&>(expr)(hana::id))
{ return static_cast<Expr&&>(expr)(hana::id); }
template <typename Expr>
static constexpr auto eval_helper(Expr&&, ...) {
static_assert(detail::wrong<Expr>{},
"hana::eval(expr) requires the expression to be a hana::lazy, "
"a nullary Callable or a unary Callable that may be "
"called with hana::id");
}
template <typename Expr>
static constexpr decltype(auto) apply(Expr&& expr)
{ return eval_helper(static_cast<Expr&&>(expr), int{}); }
};
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_EVAL_HPP
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