1 /*=============================================================================
2     Copyright (c) 2015 Paul Fultz II
3     eval.h
4     Distributed under the Boost Software License, Version 1.0. (See accompanying
5     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 ==============================================================================*/
7 
8 #ifndef BOOST_HOF_GUARD_EVAL_H
9 #define BOOST_HOF_GUARD_EVAL_H
10 
11 /// eval
12 /// ====
13 ///
14 /// Description
15 /// -----------
16 ///
17 /// The `eval` function will evaluate a "thunk". This can be either a nullary
18 /// function or it can be a unary function that takes the identity function as
19 /// the first parameter(which is helpful to delay compile-time checking).
20 /// Also, additional parameters can be passed to `eval` to delay
21 /// compiliation(so that result can depend on template parameters).
22 ///
23 /// Synopsis
24 /// --------
25 ///
26 ///     template<class F, class... Ts>
27 ///     constexpr auto eval(F&& f, Ts&&...);
28 ///
29 /// Requirements
30 /// ------------
31 ///
32 /// F must be:
33 ///
34 /// * [EvaluatableFunctionObject](EvaluatableFunctionObject)
35 ///
36 /// Example
37 /// -------
38 ///
39 ///     #include <boost/hof.hpp>
40 ///     #include <cassert>
41 ///
42 ///     int main() {
43 ///         assert(boost::hof::eval([]{ return 3; }) == 3);
44 ///     }
45 ///
46 /// References
47 /// ----------
48 ///
49 /// * [POO51](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0051r2.pdf) - Proposal for C++
50 ///   Proposal for C++ generic overload function
51 /// * [static_if](static_if)
52 /// * [Ordering evaluation of arguments](<Ordering evaluation of arguments>)
53 ///
54 
55 #include <boost/hof/always.hpp>
56 #include <boost/hof/identity.hpp>
57 #include <boost/hof/first_of.hpp>
58 #include <boost/hof/detail/result_of.hpp>
59 
60 namespace boost { namespace hof {
61 
62 namespace detail {
63 
64 struct simple_eval
65 {
66     template<class F, class... Ts>
67     constexpr BOOST_HOF_SFINAE_RESULT(F)
68     operator()(F&& f, Ts&&...xs) const BOOST_HOF_SFINAE_RETURNS
69     (boost::hof::always_ref(f)(xs...)());
70 };
71 
72 struct id_eval
73 {
74     template<class F, class... Ts>
75     constexpr BOOST_HOF_SFINAE_RESULT(F, id_<decltype(boost::hof::identity)>)
76     operator()(F&& f, Ts&&...xs) const BOOST_HOF_SFINAE_RETURNS
77     (boost::hof::always_ref(f)(xs...)(boost::hof::identity));
78 };
79 
80 }
81 
82 BOOST_HOF_DECLARE_STATIC_VAR(eval, boost::hof::first_of_adaptor<detail::simple_eval, detail::id_eval>);
83 
84 }} // namespace boost::hof
85 
86 #endif
87