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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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