public abstract class Result
{
    public sealed class Ok : Result
    {
        public string AccessToken { get; }
        public string RefreshToken { get; }

        public Ok(string accessToken, string refreshToken)
        {
            AccessToken = accessToken;
            RefreshToken = refreshToken;
        }
    }

    public sealed class Unauthorized : Result
    {
        public static Unauthorized Instance = new Unauthorized();

        private Unauthorized()
        {
        }
    }

    public sealed class HttpError : Result
    {
        public int StatusCode { get; }
        public string Reason { get; }

        public HttpError(int statusCode, string reason)
        {
            StatusCode = statusCode;
            Reason = reason;
        }
    }

    public static Result Return(string accessToken, string refreshToken) =>
        new Ok(accessToken, refreshToken);

    public static Result FlatMap(Result x, Func<Ok, Result> f) => x switch
    {
        Ok ok => f(ok),
        HttpError httpError => httpError,
        Unauthorized unauthorized => unauthorized,
        _ => throw new ArgumentOutOfRangeException(nameof(x))
    };
}

public static class ResultExtensions
{
    public static Result FlatMap(this Result x, Func<Result.Ok, Result> f) =>
        Result.FlatMap(x, f);
}

public abstract class Result
{
    public sealed class Ok : Result
    {
        public string AccessToken { get; }
        public string RefreshToken { get; }

        public Ok(string accessToken, string refreshToken)
        {
            AccessToken = accessToken;
            RefreshToken = refreshToken;
        }
    }

    public sealed class Unauthorized : Result
    {
        public static Unauthorized Instance = new Unauthorized();

        private Unauthorized()
        {
        }
    }

    public sealed class HttpError : Result
    {
        public int StatusCode { get; }
        public string Reason { get; }

        public HttpError(int statusCode, string reason)
        {
            StatusCode = statusCode;
            Reason = reason;
        }
    }

    public static Result Return(string accessToken, string refreshToken) =>
        new Ok(accessToken, refreshToken);

    public static Result FlatMap(Result x, Func<Ok, Result> f) => x switch
    {
        Ok ok => f(ok),
        HttpError httpError => httpError,
        Unauthorized unauthorized => unauthorized,
        _ => throw new ArgumentOutOfRangeException(nameof(x))
    };
}