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/assert.hpp>
6 #include <boost/hana/config.hpp>
7 #include <boost/hana/contains.hpp>
8 #include <boost/hana/equal.hpp>
9 #include <boost/hana/if.hpp>
10 #include <boost/hana/monadic_compose.hpp>
11 #include <boost/hana/optional.hpp>
12 #include <boost/hana/tuple.hpp>
13 #include <boost/hana/type.hpp>
14 namespace hana = boost::hana;
15 
16 
main()17 int main() {
18     BOOST_HANA_CONSTEXPR_LAMBDA auto block = [](auto ...types) {
19         return [=](auto x) {
20             return hana::if_(hana::contains(hana::make_tuple(types...), hana::typeid_(x)),
21                 hana::nothing,
22                 hana::just(x)
23             );
24         };
25     };
26 
27     BOOST_HANA_CONSTEXPR_LAMBDA auto f = block(hana::type_c<double>);
28     BOOST_HANA_CONSTEXPR_LAMBDA auto g = block(hana::type_c<int>);
29     BOOST_HANA_CONSTEXPR_LAMBDA auto h = hana::monadic_compose(g, f);
30     BOOST_HANA_CONSTANT_CHECK(h(1)    == hana::nothing); // fails inside g; 1 has type int
31     BOOST_HANA_CONSTANT_CHECK(h(1.2)  == hana::nothing); // fails inside f; 1.2 has type double
32     BOOST_HANA_CONSTEXPR_CHECK(h('x') == hana::just('x')); // ok; 'x' has type char
33 }
34