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, typename> class either_stream;
}

namespace stream::traits {
  template <typename Either> struct either_traits {
    static constexpr bool const is_either = false;
  };

  template <typename T, typename E> struct either_traits<::either<T, E>> {
    static constexpr bool const is_either = true;
    using stream_type = either_stream<T, E>;
    using value_type = T;
    using error_type = E;
  };

  template <typename Either>
  using either_stream_t = typename either_traits<Either>::stream_type;
  template <typename Either>
  using either_value_t = typename either_traits<Either>::value_type;
  template <typename Either>
  inline constexpr bool is_either_v = either_traits<Either>::is_either;

}

namespace stream {

  /**
   * An either stream is an object that allows functional compositions on a type
   * that is either a concrete value or an error-type. This can be used as an
   * alternative to the "status and out-param" pattern or exception-throwing
   * code. It combines the benefits of both types - gaining the immediate
   * knowledge of error-information that is provided by the first, while also
   * allowing you to avoid out-params.
   * The weakness of this design is that it often requires remodeling your
   * entire codebase to operate around the either_stream. In order to terminate
   * the stream, a fail-fast option is provided with operator bool(). However,
   * the primary intent is to chain together functions with map and flatmap
   * until you either have the desired result, or an error. At that point, you
   * invoke match and handle the error with a default return value, an
   * error-handler callback, or a thrown exception.
   */
  template <typename T, typename E> class either_stream {
  private:
    template <typename F, typename T2 = T>
    using result_of = decltype(std::declval<F>()(std::declval<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)) {}

    // This probably should be for internal use only
    operator bool() const { return value_.index() == 0; }

    template <typename F>
    auto map(F && fun) const -> either_stream<result_of<F>, E> {
      static_assert(!traits::is_either_v<result_of<F>>, "Cannot nest eithers");
      if (*this) {
        return fun(std::get<0>(value_));
      } else {
        return std::get<1>(value_);
      }
    }

    template <typename F>
    auto flatmap(F && fun) const
        -> either_stream<traits::either_value_t<result_of<F>>, E> {
      if (*this) {
        return fun(std::get<0>(value_));
      } else {
        return std::get<1>(value_);
      }
    }

    template <typename FT, typename FE>
    auto match(FT && left, FE && right)
        -> std::common_type_t<result_of<FT, T>, result_of<FE, E>> {
      if (*this) {
        return left(std::get<0>(value_));
      } else {
        return right(std::get<1>(value_));
      }
    }
  };

}

namespace stream::either {

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

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

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

}