cpp-optional-stream/include/stream/optional_stream.hpp

//
//  optional_stream.hpp
//  optional.stream
//
//  Created by Sam Jaffe on 1/28/17.
//

#pragma once

#if __cplusplus > 201402L
#include <optional>
#else
#include <experimental/optional>
namespace std {
  using experimental::optional;
  using experimental::nullopt;
}
#endif

namespace stream { namespace optional {
  namespace detail {
    template <typename T> struct flatmap_impl_t;
    template <typename T>
    struct flatmap_impl_t<std::optional<T>> { using type = T; };
    
    template <typename T>
    class optional_stream {
    private:
      template <typename F>
      using map_f = decltype(std::declval<F>()(std::declval<T>()));
      template <typename F>
      using flatmap_f = typename flatmap_impl_t<map_f<F>>::type;
    public:
      explicit optional_stream() : value(std::nullopt) {}
      explicit optional_stream(T const & v) : value(v) {}
      explicit optional_stream(T && v) : value(std::forward<T>(v)) {}
      explicit optional_stream(std::optional<T> const & v) : value(v) {}
      explicit optional_stream(std::optional<T> && v) : value(std::forward<std::optional<T>>(v)) {}
      
      operator std::optional<T>() const { return value; }
      
      template <typename F>
      optional_stream<map_f<F>> map(F && fun) const {
        using next_t = optional_stream<map_f<F>>;
        return !value ? next_t{} : next_t{fun(*value)};
      }
      
      template <typename F>
      optional_stream<flatmap_f<F>> flatmap(F && fun) const {
        using next_t = optional_stream<flatmap_f<F>>;
        return !value ? next_t{} : next_t{fun(*value)};
      }
    private:
      std::optional<T> value;
    };
  }
  
  template <typename T>
  auto make_stream(std::optional<T> const & opt) -> detail::optional_stream<T> {
    return detail::optional_stream<T>{opt};
  }
  
  template <typename T>
  auto make_stream(T const & val) -> detail::optional_stream<T> {
    return detail::optional_stream<T>{val};
  }
  
  template <typename T>
  auto make_stream() -> detail::optional_stream<T> {
    return detail::optional_stream<T>{};
  }
} }