ceph/src/boost/libs/hana/example/lazy/monad.cpp

   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 }