boost/hana/eval.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::eval`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_EVAL_HPP
0011 #define BOOST_HANA_EVAL_HPP
0012
0013 #include <boost/hana/fwd/eval.hpp>
0014
0015 #include <boost/hana/config.hpp>
0016 #include <boost/hana/core/dispatch.hpp>
0017 #include <boost/hana/detail/wrong.hpp>
0018 #include <boost/hana/functional/id.hpp>
0019
0020
0021 namespace boost { namespace hana {
0022     //! @cond
0023     template <typename Expr>
0024     constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
0025         return eval_impl<typename hana::tag_of<Expr>::type>::apply(
0026             static_cast<Expr&&>(expr)
0027         );
0028     }
0029     //! @endcond
0030
0031     template <typename T, bool condition>
0032     struct eval_impl<T, when<condition>> : default_ {
0033         template <typename Expr>
0034         static constexpr auto eval_helper(Expr&& expr, int)
0035             -> decltype(static_cast<Expr&&>(expr)())
0036         { return static_cast<Expr&&>(expr)(); }
0037
0038         template <typename Expr>
0039         static constexpr auto eval_helper(Expr&& expr, long)
0040             -> decltype(static_cast<Expr&&>(expr)(hana::id))
0041         { return static_cast<Expr&&>(expr)(hana::id); }
0042
0043         template <typename Expr>
0044         static constexpr auto eval_helper(Expr&&, ...) {
0045             static_assert(detail::wrong<Expr>{},
0046             "hana::eval(expr) requires the expression to be a hana::lazy, "
0047             "a nullary Callable or a unary Callable that may be "
0048             "called with hana::id");
0049         }
0050
0051         template <typename Expr>
0052         static constexpr decltype(auto) apply(Expr&& expr)
0053         { return eval_helper(static_cast<Expr&&>(expr), int{}); }
0054     };
0055 }} // end namespace boost::hana
0056
0057 #endif // !BOOST_HANA_EVAL_HPP