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>

#if __cplusplus > 201402L
#include <variant>
template <typename T, typename E>
using either = std::variant<T, E>;
#else
#include "variant/variant.hpp"
template <typename T, typename E>
using either = variant<T, E>;
#endif

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

    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, T>, E>> map(F && fun) const {
        using next_t = either_stream<::either<map_f<F, T>, 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, T>, E>> flatmap(F && fun) const {
        using next_t = either_stream<::either<flatmap_f<F, T>, 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, T>>::value && std::is_void<map_f<FE, E>>::value)>::type>
      typename std::common_type<map_f<FT, T>, 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, T>>::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));
      }
    private:
      ::either<T, E> value;
    };
  }
  
  template <typename T, typename E>
  auto make_stream(T const & opt) -> detail::either_stream<::either<T, E>> {
    return {{opt}};
  }

  template <typename E, typename T>
  auto make_stream(T const & opt) -> detail::either_stream<::either<T, E>> {
    return {{opt}};
  }

  template <typename E>
  auto make_stream(typename detail::either_left<E>::type const & opt) -> detail::either_stream<E> {
    return {{opt}};
  }
} }