1 // Copyright Louis Dionne 2013-2017
2 // Distributed under the Boost Software License, Version 1.0.
3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
4
5 #include <boost/hana/ap.hpp>
6 #include <boost/hana/assert.hpp>
7 #include <boost/hana/bool.hpp>
8 #include <boost/hana/config.hpp>
9 #include <boost/hana/equal.hpp>
10 #include <boost/hana/if.hpp>
11 #include <boost/hana/lift.hpp>
12 #include <boost/hana/optional.hpp>
13 namespace hana = boost::hana;
14
15
16 template <char op>
17 constexpr auto function = hana::nothing;
18
19 template <>
__anona30cd7910102(auto x, auto y) 20 BOOST_HANA_CONSTEXPR_LAMBDA auto function<'+'> = hana::just([](auto x, auto y) {
21 return x + y;
22 });
23
24 template <>
__anona30cd7910202(auto x, auto y) 25 BOOST_HANA_CONSTEXPR_LAMBDA auto function<'-'> = hana::just([](auto x, auto y) {
26 return x - y;
27 });
28
29 // and so on...
30
31 template <char n>
32 constexpr auto digit = hana::if_(hana::bool_c<(n >= '0' && n <= '9')>,
33 hana::just(static_cast<int>(n - 48)),
34 hana::nothing
35 );
36
37 template <char x, char op, char y>
38 BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = hana::ap(function<op>, digit<x>, digit<y>);
39
main()40 int main() {
41 BOOST_HANA_CONSTEXPR_CHECK(evaluate<'1', '+', '2'> == hana::just(1 + 2));
42 BOOST_HANA_CONSTEXPR_CHECK(evaluate<'4', '-', '2'> == hana::just(4 - 2));
43
44 BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '+', '2'> == hana::nothing);
45 BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '?', '2'> == hana::nothing);
46 BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '+', '?'> == hana::nothing);
47 BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '?', '?'> == hana::nothing);
48
49 static_assert(hana::lift<hana::optional_tag>(123) == hana::just(123), "");
50 }
51