I could be wrong but I’m pretty sure the call to second is required because when we enqueue we recursively shift along the unfulfilled promises with the call to (swap! promises next).

If we handed the reader the very first item of promises then the reader would block forever because the delivery of a new list (made of the enqueued values cons’d with a lazy seq of the remaining promises) happens in the next cell.

This does appear to mean that the very first promise is unfulfilled (and gc’d because nothing references it), but it means we need less logic in our enqueueing.

I personally find this code to be incredibly elegant and concise… (Though it did take a while to figure out how it worked)