du.cs

    public abstract class Result<TValue, TError>
    {
        public bool IsOk => this is Ok;
        public sealed class Ok : Result<TValue, TError>
        {
            public TValue Value { get; }
            internal Ok(TValue value) => Value = value;
        }

        public sealed class Err : Result<TValue, TError>
        {
            public TError Error { get; }
            internal Err(TError error) => Error = error;
        }

        public static Result<TValue, TError> NewErr(TError error) => new Err(error);
        public static Result<TValue, TError> NewOk(TValue value) => new Ok(value);
    }