Last week's receipt changed

transcript

You tap Reorder on last week's dinner. Same pizza, same colas. You raise the colas to three. And last week's receipt changes. Nobody touched it.

The reorder says 420. That's right. Three colas, plus the pizza. But the past order says 420 as well. It should still say 380, with two colas. It's last week's receipt.

The one liner made a new cart. But its list arrow points at the same list as the old cart. Replace the note, and only the reorder's arrow moves. The old note is safe. Raise the cola, and the edit follows the arrow. Into the shared list, to the one cola. Last week's cart follows its own arrow to that same cola. So it says 420 too.

The past order was 380. After the edit it says 420. Because 2 of 2 items are shared between the carts.

A shallow copy duplicates only the outside. What the cart holds is still shared. That's where the bug lives.

Full breakdown in the lecture.