Either.cs

using System;

namespace LSD
{
	public interface IEither<T1, T2>
	{
		U Use<U>(Func<T1, U> ofLeft, Func<T2, U> ofRight);
		bool IsRight{
			get;
		}
		bool IsLeft{
			get;
		}
	}

	public static class Either{

		public static IEither<T1, T2> Left<T1, T2>(T1 value){
			return new Either.LeftEither<T1, T2>(value);
		}

		public static IEither<T1, T2> Right<T1, T2>(T2 value){
			return new Either.RightEither<T1, T2>(value);
		}

		sealed class LeftEither<T1, T2>: IEither<T1, T2>{
			private readonly T1 value;

			public LeftEither(T1 value)
			{
				this.value = value;
			}

			public U Use<U>(Func<T1, U> ofLeft, Func<T2, U> ofRight){
				if (ofLeft == null)
					throw new ArgumentNullException();
				return ofLeft(value);
			}

			public bool IsRight{
				get{
					return false;
				}
			}
			public bool IsLeft{
				get{
					return true;
				}
			}
		}

		sealed class RightEither<T1, T2>: IEither<T1, T2>{
			private readonly T2 value;

			public RightEither(T2 value)
			{
				this.value = value;
			}

			public U Use<U>(Func<T1, U> ofLeft, Func<T2, U> ofRight){
				if (ofRight == null)
					throw new ArgumentNullException();
				return ofRight(value);
			}

			public bool IsRight{
				get{
					return true;
				}
			}
			public bool IsLeft{
				get{
					return false;
				}
			}
		}
	}
}