Lab

Iterator / generator

The marshal pulls; the iterator fetches

The lesson asked what the log shows if the generator closed the session after its loop instead of in finally, and the marshal broke out once the board was full. Lines 25 to 37 become the generator below, the lesson’s snippet; run it, and the log line answers the prediction. Restore code puts it back.

function* pairings(schedule: Schedule): Generator<Pairing> {
  schedule.open();
  let page: number | null = 1;
  while (page !== null) {
    const fetched = schedule.fetch(page);
    for (const pairing of fetched.pairings) yield pairing;
    page = fetched.next;
  }
  schedule.close();
}

What to look forWho closes the session when the marshal stops early

open, fetch 1, fetch 2, and no close. A for-of loop that breaks calls the generator’s return(), which resumes it at the paused yield as if that yield were a return statement: the loop inside stops, and only finally blocks run on the way out. The close after the loop never runs, so the session stays open. In the program as written close sits in finally, so a break, the end of the schedule and an exception all reach it.

type Pairing = { white: string; black: string };
type Page = { pairings: Pairing[]; next: number | null };

// The schedule service: pairings in start-time order, a page at a time.
class Schedule {
  log: string[] = [];
  pages: Pairing[][];
  constructor(pages: Pairing[][]) {
    this.pages = pages;
  }
  open(): void {
    this.log.push('open');
  }
  fetch(page: number): Page {
    this.log.push(`fetch ${page}`);
    const next = page < this.pages.length ? page + 1 : null;
    return { pairings: this.pages[page - 1], next };
  }
  close(): void {
    this.log.push('close');
  }
}

// The iterator: it keeps the page, its place and the session.
function* pairings(schedule: Schedule): Generator<Pairing> {
  schedule.open();
  try {
    let page: number | null = 1;
    while (page !== null) {
      const fetched = schedule.fetch(page);
      for (const pairing of fetched.pairings) yield pairing;
      page = fetched.next;
    }
  } finally {
    schedule.close();
  }
}

// The floor marshal posts pairings until the board's three slots are full.
const schedule = new Schedule([
  [{ white: 'Ayla', black: 'Boris' }, { white: 'Camila', black: 'Dinesh' }],
  [{ white: 'Edda', black: 'Finn' }, { white: 'Gita', black: 'Hugo' }],
  [{ white: 'Ines', black: 'Jamal' }],
]);
const board: string[] = [];
for (const pairing of pairings(schedule)) {
  board.push(`${pairing.white} vs ${pairing.black}`);
  if (board.length === 3) break;
}
const log = schedule.log.join(', ');
console.log(board.join(' | '));
console.log(log);

TODO

Visual to come