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/*=============================================================================
Copyright (c) 2015 Paul Fultz II
eval.h
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
==============================================================================*/
#ifndef BOOST_HOF_GUARD_EVAL_H
#define BOOST_HOF_GUARD_EVAL_H
/// eval
/// ====
///
/// Description
/// -----------
///
/// The `eval` function will evaluate a "thunk". This can be either a nullary
/// function or it can be a unary function that takes the identity function as
/// the first parameter(which is helpful to delay compile-time checking).
/// Also, additional parameters can be passed to `eval` to delay
/// compiliation(so that result can depend on template parameters).
///
/// Synopsis
/// --------
///
/// template<class F, class... Ts>
/// constexpr auto eval(F&& f, Ts&&...);
///
/// Requirements
/// ------------
///
/// F must be:
///
/// * [EvaluatableFunctionObject](EvaluatableFunctionObject)
///
/// Example
/// -------
///
/// #include <boost/hof.hpp>
/// #include <cassert>
///
/// int main() {
/// assert(boost::hof::eval([]{ return 3; }) == 3);
/// }
///
/// References
/// ----------
///
/// * [POO51](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0051r2.pdf) - Proposal for C++
/// Proposal for C++ generic overload function
/// * [static_if](static_if)
/// * [Ordering evaluation of arguments](<Ordering evaluation of arguments>)
///
#include <boost/hof/always.hpp>
#include <boost/hof/identity.hpp>
#include <boost/hof/first_of.hpp>
#include <boost/hof/detail/result_of.hpp>
namespace boost { namespace hof {
namespace detail {
struct simple_eval
{
template<class F, class... Ts>
constexpr BOOST_HOF_SFINAE_RESULT(F)
operator()(F&& f, Ts&&...xs) const BOOST_HOF_SFINAE_RETURNS
(boost::hof::always_ref(f)(xs...)());
};
struct id_eval
{
template<class F, class... Ts>
constexpr BOOST_HOF_SFINAE_RESULT(F, id_<decltype(boost::hof::identity)>)
operator()(F&& f, Ts&&...xs) const BOOST_HOF_SFINAE_RETURNS
(boost::hof::always_ref(f)(xs...)(boost::hof::identity));
};
}
BOOST_HOF_DECLARE_STATIC_VAR(eval, boost::hof::first_of_adaptor<detail::simple_eval, detail::id_eval>);
}} // namespace boost::hof
#endif
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