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- // Copyright Louis Dionne 2013-2017
- // Distributed under the Boost Software License, Version 1.0.
- // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- #include <boost/hana/assert.hpp>
- #include <boost/hana/chain.hpp>
- #include <boost/hana/eval.hpp>
- #include <boost/hana/lazy.hpp>
- #include <functional>
- #include <iostream>
- #include <sstream>
- namespace hana = boost::hana;
- template <typename T>
- T read_(std::istream& stream) {
- T x;
- stream >> x;
- std::cout << "read " << x << " from the stream\n";
- return x;
- }
- int main() {
- std::stringstream ss;
- int in = 123;
- std::cout << "creating the monadic chain...\n";
- auto out = hana::make_lazy(read_<int>)(std::ref(ss))
- | [](auto x) {
- std::cout << "performing x + 1...\n";
- return hana::make_lazy(x + 1);
- }
- | [](auto x) {
- std::cout << "performing x / 2...\n";
- return hana::make_lazy(x / 2);
- };
- std::cout << "putting " << in << " in the stream...\n";
- ss << in; // nothing is evaluated yet
- std::cout << "evaluating the monadic chain...\n";
- auto eout = hana::eval(out);
- std::cout << "the result of the monadic chain is " << eout << "\n";
- BOOST_HANA_RUNTIME_CHECK(eout == (in + 1) / 2);
- }
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