a simple event sourcing example to help myself comprehend the idea.

EventSourcing.scala

import scala.collection.mutable.ListBuffer
import scala.collection.mutable.HashMap

case class Account(val user:String,val money:Double)

sealed abstract class Op

case class Despoit(val money:Double) extends Op

case class Withdrawal(val money:Double) extends Op

object Bank {
  self =>
  var editLog  = List.empty[Op]

  val account = Account("cc",100)
  val map = new HashMap[String,List[Account]]
  private val KEY = "history"
    
  def push(op:Op):Unit = {
    editLog = (op :: editLog.reverse).reverse
    map.remove(KEY)
  }
  
  def pop():Unit = {
    if(!editLog.isEmpty) {
      editLog = editLog.tail
      map.remove(KEY)
    }
  }
  
  def now:Account = {
    history.last
  }

  def history: List[Account] = {
    val list = map.getOrElse(KEY, {
      var list = List.empty[Account]
      var account = self.account
      for (op <- editLog) {
        list = (account :: list.reverse).reverse
        op match {
          case Despoit(money) => account = Account(account.user, account.money + money)
          case Withdrawal(money) => account = Account(account.user, account.money - money)
        }
      }
      list = (account :: list.reverse).reverse
      map.put(KEY, list)
      list
    })
    list
  }
}

object Main {
  def main(args: Array[String]) = {
	  println(Bank.history)
	  Bank.push(Despoit(100))
	  println(Bank.now)
	  Bank.push(Withdrawal(50))
	  println(Bank.now)
	  println(Bank.history)
  }
}