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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2015/imperative-programming-resit/7/c/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2015 / Imperative programming (resit) / 7 / c / Solution by Codex 0 2026-10-31
If f is a pure function on immutable values, a simple implementation stores the next value and copies it into a fresh FunIter. Part (d)'s suggested mutable programming closure, however, would be shared by such copies and violate independence. A memoized iterator snapshot avoids that contradiction: all cursors share already determined sequence values, but each has its own position.
class FunIter[T](v: T, f: T => T) extends Iter[T] {
  private class Cell(val head: T) {
    lazy val tail: Cell = new Cell(f(head))
  }

  private class Cursor(private var cell: Cell) extends Iter[T] {
    def hasNext: Boolean = true

    def next(): T = {
      val answer = cell.head
      cell = cell.tail
      answer
    }

    def copy(): Iter[T] = new Cursor(cell)
  }

  private val cursor = new Cursor(new Cell(v))

  def hasNext: Boolean = true
  def next(): T = cursor.next()
  def copy(): Iter[T] = cursor.copy()
}
The first cell stores ; its lazily memoized successor stores ; the next stores , and so on. A cell's successor is evaluated at most once. The representation invariant is that each cursor points to the cell holding its next result. next returns that result and advances only this cursor. copy shares the cell, whose future is memoized, rather than the mutable cursor. Advancing either copy therefore cannot skip entries in the other.
(1)
Assume each demanded application of f terminates and generated values are immutable for purposes of observation. A stateful generator should be privately owned by this family of cursors; arbitrary external changes to it cannot be made independent by the supplied interface. Unlike the pure-function implementation, memoization can retain a long prefix while one copy lags far behind another. The question imposes no constant-storage requirement.
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