Immutable Data Patterns

Returning New State

Compute the next state instead of mutating.

new-state-label Rather than mutating a variable, immutable code computes a new value and a label describing the change. The original value stays available for comparison.

Returning New State

deposit
NewStateLabel.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val deposit = 50
    val balance = 100
    val newBalance = balance + deposit
    val state = if (newBalance > balance) "grew" else "same"

    println("balance=" + balance)
    println("newBalance=" + newBalance)
    println("state=" + state)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val deposit = 0
    val balance = 100
    val newBalance = balance + deposit
    val state = if (newBalance > balance) "grew" else "same"

    println("balance=" + balance)
    println("newBalance=" + newBalance)
    println("state=" + state)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val deposit = 200
    val balance = 100
    val newBalance = balance + deposit
    val state = if (newBalance > balance) "grew" else "same"

    println("balance=" + balance)
    println("newBalance=" + newBalance)
    println("state=" + state)
  }
}

  1. deposit ← 50, balance ← 100, newBalance ← 150, state ← grew

    1object Main {2  def main(args: Array[String]): Unit = {3    val deposit→ 50 = 50 //@deposit=0, 2004    val balance→ 100 = 1005    val newBalance→ 150 = balance100 + deposit506    val state→ grew = if (newBalance150 > balance100) "grew" else "same"78    println("balance=" + balance100)9    println("newBalance=" + newBalance150)10    println("state=" + stategrew)11  }12}
    outputbalance=100
newBalance=150
state=grew

  1. deposit ← 0, balance ← 100, newBalance ← 100, state ← same

    1object Main {2  def main(args: Array[String]): Unit = {3    val deposit→ 0 = 04    val balance→ 100 = 1005    val newBalance→ 100 = balance100 + deposit06    val state→ same = if (newBalance100 > balance100) "grew" else "same"78    println("balance=" + balance100)9    println("newBalance=" + newBalance100)10    println("state=" + statesame)11  }12}
    outputbalance=100
newBalance=100
state=same

  1. deposit ← 200, balance ← 100, newBalance ← 300, state ← grew

    1object Main {2  def main(args: Array[String]): Unit = {3    val deposit→ 200 = 2004    val balance→ 100 = 1005    val newBalance→ 300 = balance100 + deposit2006    val state→ grew = if (newBalance300 > balance100) "grew" else "same"78    println("balance=" + balance100)9    println("newBalance=" + newBalance300)10    println("state=" + stategrew)11  }12}
    outputbalance=100
newBalance=300
state=grew