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1 | // Copyright Louis Dionne 2013-2016 |
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/chain.hpp> | |
7 | #include <boost/hana/eval.hpp> | |
8 | #include <boost/hana/lazy.hpp> | |
9 | ||
10 | #include <functional> | |
11 | #include <iostream> | |
12 | #include <sstream> | |
13 | namespace hana = boost::hana; | |
14 | ||
15 | ||
16 | template <typename T> | |
17 | T read_(std::istream& stream) { | |
18 | T x; | |
19 | stream >> x; | |
20 | std::cout << "read " << x << " from the stream\n"; | |
21 | return x; | |
22 | } | |
23 | ||
24 | int main() { | |
25 | std::stringstream ss; | |
26 | int in = 123; | |
27 | ||
28 | std::cout << "creating the monadic chain...\n"; | |
29 | auto out = hana::make_lazy(read_<int>)(std::ref(ss)) | |
30 | | [](auto x) { | |
31 | std::cout << "performing x + 1...\n"; | |
32 | return hana::make_lazy(x + 1); | |
33 | } | |
34 | | [](auto x) { | |
35 | std::cout << "performing x / 2...\n"; | |
36 | return hana::make_lazy(x / 2); | |
37 | }; | |
38 | ||
39 | std::cout << "putting " << in << " in the stream...\n"; | |
40 | ss << in; // nothing is evaluated yet | |
41 | ||
42 | std::cout << "evaluating the monadic chain...\n"; | |
43 | auto eout = hana::eval(out); | |
44 | ||
45 | std::cout << "the result of the monadic chain is " << eout << "\n"; | |
46 | BOOST_HANA_RUNTIME_CHECK(eout == (in + 1) / 2); | |
47 | } |