boost::optionalにモナドのbindを

C++のboost::optionalを、HaskellのMaybeモナドみたいに使い隊。

#include <iostream>
#include <type_traits>
#include <boost/optional.hpp> 

template <
  typename Input,
  typename Functor,
  typename OptionalOutput = typename std::result_of<Functor(Input)>::type
>
OptionalOutput operator >>= (
  const boost::optional<Input>& a,
  Functor f
) {
  if (a) return f(*a);
  return boost::none;
} 

template<typename T>
std::ostream& operator<<(std::ostream& os, const boost::optional<T>& a) {
  if (a) return os << *a;
  else   return os << "none";
}

int main() {
  auto f = [](bool b){ return b ? boost::optional<int>(42) : boost::none; };;
  std::cout << (boost::optional<bool>(true)  >>= f) << std::endl; // 42
  std::cout << (boost::optional<bool>(false) >>= f) << std::endl; // none
  std::cout << (boost::optional<bool>()      >>= f) << std::endl; // none
//std::cout << (boost::none                  >>= f) << std::endl; // boost::none使うと型推論できなくて残念

  return 0;
}

実行結果：[Wandbox]三へ( へ՞ਊ ՞)へ ﾊｯﾊｯ