mbank6.scala

   def eventToState(event: BankAccountEvent): AccountState = {
    event match {
      case DepositEvt(time, amount) =>
        AccountState(amount, None)
      case PurchaseEvt(time, amount) =>
        AccountState(-amount, None)
      case AssignAccountHolderEvt(time, accountHolder) =>
        AccountState(0, accountHolder.some)
    }
  }