feat():initial version

install/boost_1_75_0/include/boost/hof/eval.hpp

@@ -0,0 +1,86 @@
+/*=============================================================================
+    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