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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2017/imperative-programming-resit/6/d/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Imperative programming (resit) / 6 / d / Solution by

Codex  0 2026-10-31

Use a covariant element type and a lower type bound when adding potentially more general elements:

class VendingMachine[+A](val currentItem: Option[A], items: List[A]) {
  def this(items: List[A]) = this(None,items)
  def dispenseNext(): VendingMachine[A] = items match {
    case Nil =>
      if (currentItem.isDefined) new VendingMachine[A](None,Nil)
      else this
    case t :: ts => new VendingMachine[A](Some(t),ts)
  }
  def addAll[B >: A](newItems: List[B]): VendingMachine[B] =
    new VendingMachine[B](items ++ newItems)
}

The lower type bound permits the old pending elements of type A to be used as type B, so concatenation is type-correct. The resulting machine is immutable and can return the wider type without changing the existing machine. With the given client expressions, VM1 has type VendingMachine[Juice] and VM2 can have type VendingMachine[Drink]. The supplied addAll body constructs a new pending-item machine; its constructor sets currentItem to None.

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