gistfile1.txt

class BankAccount {
    enum BankAccountError: Error {
        case insufficientFunds
    }
    
    private(set) var balance: Double = 0
    
    func deposit(_ amount: Double) {
        balance += amount
    }
    
    func withdraw(_ amount: Double) throws(BankAccountError) {
        guard balance >= amount else {
            throw .insufficientFunds
        }

        // multiple threads mutating this may cause a data race,
        // therefore crashing or data corruption is likely
        balance -= amount
    }
}