bad_stm_example4.scala

import scala.concurrent.stm._ 

class Account(val accountNo: String, beginningBalance: Int){
  private var _balance = beginningBalance
  def balance = this.synchronized{ _balance }
  def credit(amount: Int) = this.synchronized{ _balance = _balance + amount }
  def debit(amount: Int) = this.synchronized { _balance = _balance - amount }
}

object Bank{
        
  val accountsRef = Ref(Map("from" -> new Account("from", 500), "to" -> new Account("to", 500) ))

  def transfer(fromNo: String, toNo: String, amount: Int): Unit = {
    atomic{ implicit t => 
      var accounts = accountsRef()
      val (from, to) = (accounts(fromNo), accounts(toNo))
      if(from.balance > amount){
        to.credit(amount)
        from.debit(amount)
        accounts = accounts + (to.accountNo -> to)
        accounts = accounts + (from.accountNo -> from)
        accountsRef() = accounts
      }
    }
  }
}