Read about the Free Monad here: http://degoes.net/articles/modern-fp

free_monad.cc

#include <string>
#include <vector>
#include <iterator>
#include <iostream>

#include <boost/variant.hpp>

// http://degoes.net/articles/modern-fp


struct CloudFilesF final {

  struct SaveFile final {
    std::string const path;
    unsigned char const* const bytes;
  };

  struct ListFiles final {
    std::string const path;
  };

  boost::variant<SaveFile, ListFiles> const constructor;
};

auto saveFile(const std::string& path, unsigned char const* const bytes) { //
  return CloudFilesF{CloudFilesF::SaveFile{path, bytes}};
}

auto listFiles(const std::string& path) { //
  return CloudFilesF{CloudFilesF::ListFiles{path}};
}


struct HttpF final {

  struct Get final {
    std::string const path;
  };

  struct Post final {
    std::string const path;
    unsigned char const* const bytes;
  };

  boost::variant<Get, Post> const constructor;
};

auto httpGet(const std::string& path) { //
  return HttpF{HttpF::Get{path}};
}

auto httpPost(const std::string& path, unsigned char const* const bytes) { //
  return HttpF{HttpF::Post{path, bytes}};
}


auto cloudFilesI(const CloudFilesF& f) {
  struct final {
    auto operator()(const CloudFilesF::SaveFile& sf) const { //
      return std::vector<HttpF>{httpPost(sf.path, sf.bytes)};
    }
    auto operator()(const CloudFilesF::ListFiles& lf) const { //
      return std::vector<HttpF>{httpGet(lf.path)};
    }
  } const match{};

  return apply_visitor(match, f.constructor);
}


auto executor(const std::vector<HttpF>& fs) {
  struct final {
    auto operator()(const HttpF::Get& get) const { //
      std::cout << "GET " << get.path << std::endl;
    }
    auto operator()(const HttpF::Post post) const { //
      std::cout << "Post " << post.path << std::endl;
    }
  } const match{};

  for (auto const& each : fs)
    apply_visitor(match, each.constructor);
}


int main() {
  auto const save = saveFile("/to", nullptr);
  auto const list = listFiles("/from");

  auto const saveHttp = cloudFilesI(save);
  auto const listHttp = cloudFilesI(list);

  executor(saveHttp);
  executor(listHttp);
}