Actor.swift

actor BankAccount {
  let id = UUID()
  private var balance: Double = 0.0
  
  func send(_ amount: Double, to destination: BankAccount) async throws {
      guard amount >= 0 else {
        throw Error.negativeAmountTransfer
      }
    
      if (amount > balance) {
        throw Error.insufficientFunds
      }
      else {
        self.balance -= amount
        await destination.deposit(amount) // if we try to do destination.balance += amount, the compiler will throw an error cause the `balance` variable can only be accessed via `self` reference
      }
  }
  
  func deposit(_ amount: Double) {
      guard amount >= 0 else {
        throw Error.negativeAmountTransfer
      }
      self.balance += amount
  }
}