| 1 | // Copyright Louis Dionne 2013-2022 |
|---|---|
| 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(t&: 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 | } |
| 48 |