Function map

extern/boost/boost/optional/optional.hpp:1406–1412

    template <typename F>
    optional<typename boost::result_of<F(reference_type)>::type> map(F f) &
      {
        if (this->has_value())
          return f(get());
        else
          return none;
      }

    template <typename F>
    optional<typename boost::result_of<F(reference_const_type)>::type> map(F f) const&