cpp-either-stream/include/stream/either_stream.hpp

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

#pragma once

#include <type_traits>
#include <variant>

template <typename T, typename E> using either = std::variant<T, E>;

namespace stream {
  template <typename T> class either_stream;

  template <typename T> struct either_left;
  template <typename T, typename E> struct either_left<::either<T, E>> {
    using type = T;
  };

  template <typename T> using either_left_t = typename either_left<T>::type;

  template <typename T, typename E> class either_stream<::either<T, E>> {
  private:
    template <typename F, typename T2 = T>
    using map_f = decltype(std::declval<F>()(std::declval<T2>()));

    template <typename F, typename T2 = T>
    using flatmap_f = either_left_t<map_f<F, T2>>;

  private:
    ::either<T, E> value_;

  public:
    either_stream(E const & v) : value_(v) {}
    either_stream(E && v) : value_(std::forward<E>(v)) {}

    either_stream(T const & v) : value_(v) {}
    either_stream(T && v) : value_(std::forward<T>(v)) {}
    either_stream(::either<T, E> const & v) : value_(v) {}
    either_stream(::either<T, E> && v)
        : value_(std::forward<::either<T, E>>(v)) {}

    template <typename F>
    either_stream<::either<map_f<F>, E>> map(F && fun) const {
      using next_t = either_stream<::either<map_f<F>, E>>;
      return value_.index() == 0 ? next_t{fun(std::get<0>(value_))}
                                 : next_t{std::get<1>(value_)};
    }

    template <typename F>
    either_stream<::either<flatmap_f<F>, E>> flatmap(F && fun) const {
      using next_t = either_stream<::either<flatmap_f<F>, E>>;
      return value_.index() == 0 ? next_t{fun(std::get<0>(value_))}
                                 : next_t{std::get<1>(value_)};
    }

    template <typename FT, typename FE,
              typename = typename std::enable_if<
                  !(std::is_void<map_f<FT>>::value &&
                    std::is_void<map_f<FE, E>>::value)>::type>
    typename std::common_type<map_f<FT>, map_f<FE, E>>::type
    match(FT && left, FE && right) {
      return value_.index() == 0 ? left(std::get<0>(value_))
                                 : right(std::get<1>(value_));
    }

    template <typename FT, typename FE,
              typename = typename std::enable_if<
                  std::is_void<map_f<FT>>::value &&
                  std::is_void<map_f<FE, E>>::value>::type>
    void match(FT && left, FE && right) {
      value_.index() == 0 ? left(std::get<0>(value_))
                          : right(std::get<1>(value_));
    }

    ::either<T, E> const & value() const { return value_; }
  };
}

namespace stream { namespace either {
  /**
   * Construct a stream from the given type while explicitly providing both the
   * type and the error class
   */
  template <typename T, typename E>
  auto make_stream(T const & opt) -> either_stream<::either<T, E>> {
    return {{opt}};
  }

  /**
   * Construct a stream from the given type where the real type is an inferred
   * property
   */
  template <typename E, typename T>
  auto make_stream(T const & opt) -> either_stream<::either<T, E>> {
    return {{opt}};
  }

  /**
   * Construct a stream with the given either-type as a template parameter.
   */
  template <typename E>
  auto make_stream(either_left_t<E> const & opt) -> either_stream<E> {
    return {{opt}};
  }
}}