Transaction.scala

import java.time.Instant

trait Transaction {
  
  def label: String

  /** The amount of money in the transaction */
  def value: Double

  /** When this transaction occurs */
  def date: Instant

  /** 
    * Most events reoccur or repeat. This returns Some if there is another event, 
    * None if it's the last even in the sequence 
    */
  def next: Option[Transaction]

  /** The accumlated value of an event as it progresses in time */
  def accumulatedValue: Double

  def stream: Stream[Transaction] = {
    def loop(b0: Transaction): Stream[Transaction] = {
      b0.next match {
        case Some(b1) => b0 #:: loop(b1)
        case None => b0 #:: Stream.empty[Transaction]
      }
    }
    loop(this)
  }
}