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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2022/imperative-programming-resit/7/a/iv/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2022 / Imperative programming (resit) / 7 / a / iv / Solution by

Codex  0 2026-10-25

A single flag represents whether the only element remains:

def unitIter[T](t: T): Iter[T] = new Iter[T] {
  private var available = true
  def hasNext: Boolean = available
  def next(): T = {
    require(available, "empty iterator")
    available = false
    t
  }
}

def unitCursor[T](t: T): Cursor[T] = new Cursor[T] {
  private var available = true
  def hasCurrent: Boolean = available
  def current: T = {
    require(available, "empty cursor")
    t
  }
  def next(): Unit = {
    require(available, "empty cursor")
    available = false
  }
}

The abstract sequence is [t] while the flag is true and empty afterward. current leaves it unchanged; either advance consumes its sole element. Renaming both methods to unit would leave identical parameter signatures differing only in result type, which cannot define two such Scala overloads.

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