Boost.Monads

This is a small library to show how monads could be implemented in C++11 and what possibilities they offer.

It serves as an addition to my GSoC 2014 proposal.

Examples

The examples directory contains some examples on how to use this library. Some excerpts:

// unique_ptr as Maybe monad
std::unique_ptr<int> maybe(new int(4));
assert(*((boost::monads::monad_pipe(maybe) >>= inc) >>= inc).unpipe() == 6);

// Continuations with call/cc
mon::run_cont(
  (mon::monad_pipe(mon::call_cc(call_cc_test{3})) >>= loud_sqr).unpipe(),
  printer{});

// Faster iteration over std::deque with continuations
// continuation: 2.62ms, accumulate: 6.19ms
boost::monads::run_cont(deque_foreacher<int>{d},
                        [&](int i){sum += i;});
int sum=std::accumulate(d.begin(), d.end(), 0);

An application of the library is found at example/pipelines.cpp, where pipelines are implemented using monads.

const char* test[] =
    {"Error:1","all right","Error... not really an error",
     "Error", "Error:          trim   me      ", "Error: end of pipe"};
(pipeline<segment_monad>(segment_monad::from_range(begin(test), end(test)))
 >> [](std::string const& s) { return boost::starts_with(s, "Error:")
                                   ? segment_monad::mreturn(s)
                                   : segment_monad::mempty<std::string>(); }
 | [](std::string const& s) { return s.substr(std::string("Error:").size()); }
 | [](std::string s) { boost::trim(s); return s; })
 << [](shared_blocking_queue<std::string> q)
        { for (std::string s; q->pop(s);) std::cout << s << '\n'; };
// output:
//   1
//   trim   me
//   end of pipe

Library

It's best to have a look at the include directory to see how monads are defined in this demo.