demo · v146

Pull trace

Visualises every next() call as it travels through a concatenated chain. Source A holds 6 values, source B holds 6, source C holds infinitely many. As you slide the take(n) dial, watch which cells light up — blue is "pulled", green is "delivered to caller", grey is "never touched". Eager spread would pull every finite source dry; lazy stops the instant the consumer is satisfied.

take(5): include .drop(1) include .filter(x => x > 0) show eager spread comparison

pulled delivered never touched eager finite pull

Eager spread preview: [...a, ...b, ...c()] would pull every finite A and B cell, then keep pulling the infinite C source forever. This demo marks the finite work but never runs the hanging spread.

source A (size 6)

source B (size 6)

source C (∞)

0total pulls

0delivered

0cells untouched

const a = [1, 2, 3, 4, 5, 6];
const b = [-1, 7, 8, 9, 10, 11];
function* c() { let i = 100; while (true) yield i++; }

const result = Iterator.concat(a, b, c())
  .drop(1)
  .filter(x => x > 0)
  .take(N)
  .toArray();
// concat is lazy → c() is only entered if a + b ran dry first

// Eager comparison, shown as an overlay only:
const eager = [...a, ...b, ...c()];
// c() is infinite, so the spread would never finish.

see also