Either<T,U> in action

gistfile1.cs

public abstract partial class Either<T, U> {...}
//here are some business rules
public Either<LoanDeclinedReason, LoanRequest> CheckCustomerCredit(LoanRequest request) {
    if (request.Customer.CreditScore > 8) return Either.Right(request);
    return Either.Left(LoanDeclinedReason.InsufficientCreditScore);
}

public Either<LoanDeclinedReason, LoanRequest> CheckGoldPrerequisiteCustomer(LoanRequest request) {
    if (request.Value < 10000) return Either.Right(request); //dont need gold status for less than 10,000
    if (request.Customer.GoldCustomer) return Either.Right(request);
    return Either.Left(LoanDeclinedReason.RequiresGoldCustomer);
}

public Either<LoanDeclinedReason, LoanRequest> CheckSillyAmount(LoanRequest request) {
    if (request.Value > 1000000)
        return Either.Left(LoanDeclinedReason.LoanRequestTooHigh);
    return Either.Right(request);
}
    
public void LoanApproval(){
    LoanRequest request = new LoanRequest {
    Customer = new Customer { CreditScore = 10, GoldCustomer = true },
    Value = 9000};

    Either<LoanDeclinedReason, LoanRequest> checkOne = CheckCustomerCredit(request);
    Either<LoanDeclinedReason, LoanRequest> checkTwo = checkOne.FlatMap(CheckGoldPrerequisiteCustomer);
    Either<LoanDeclinedReason, LoanRequest> checkThree = checkTwo.FlatMap(CheckSillyAmount);
    bool loanApprovalStatus = checkThree.Fold(failReason=>false,passedLoan=>true);
    loanApprovalStatus.Should().BeTrue();
}