A traditional OO mutation code example showing methods that hide side effects

TraditionalMutation.scala

class TraditionalMutation{

  var total : List[Int] = Nil   // Mutable List

  /*
     Add i to total, return it's sum
     This method hides the effect of updating total
   */
  def sum(i:Int):Int = {
    total = i :: total  // SIDE EFFECT
    if(total.length > 1) 99 else  total.sum // this is ancontrived bug based on internal state
  }
}

object TraditionalTest {

      val traditional = new TraditionalMutation

      // The type of this function is Int => Int, but it should be (Int,TraditionalMutation) => (Int,TraditionalMutation)
      val one = traditional.sum(1)

      // I haven't explicitly tracked the fact that traditional.total has changed
      val two = traditional.sum(2)

      // nothing tells me that traditional.total is different, I just better know "in my head"
      // I have to worry about how  traditional evolves over time
      val three = traditional.sum(3)

    }