Lean Architecture for Agile Software Development

Appendix A. Scala Implementation of the DCI Account Example

import java.util.Date

// Account is just a base class for the domain
// objects, that are different kinds of accounts
// (e.g. SavingsAccount, CheckingAccount...)

trait Account {
  private var balance: Long = 0
  def availableBalance: Long = balance
  def decreaseBalance(amount: Long) {
    if (amount < 0)
      throw new InsufficientFundsException
    balance -= amount
  }
  def increaseBalance(amount: Long) {
     balance += amount
  }
  def updateLog(msg: String, date: Date,
                amount: Long) {
    println("Account: " + toString + ", " + msg + ", "
      + date.toString + ", " + amount)
  }
}

// MoneySource is a methodless role type that captures // the form (interface) of part of the // Transfer behavior trait MoneySource { def transferTo(amount: Long, recipient: MoneySink) } // MoneySink is a methodless role type that captures // the form (interface) of the other part of the // Transfer behavior trait MoneySink { def increaseBalance(amount: Long) def updateLog(msg: String, date: Date, amount: Long) } // TransferMoneySink is the methodful role for the // recipient in a money transfer trait TransferMoneySink extends MoneySink { this: Account => def transferFrom(amount: Long, src: MoneySource) { increaseBalance(amount) updateLog("Transfer in", new Date, amount) } } class InsufficientFundsException extends RuntimeException // This is the methodful role for the source account // for the money transfer trait TransferMoneySource extends MoneySource { this: Account => // ...