In practice, you don't actually need to return a new universe at all, because of course, that would be quite costly. Instead, the runtime can modify the existin

In practice, you don't actually need to return a new universe at all, because of course, that would be quite costly. Instead, the runtime can modify the existing universe and return it with a new unique identity - the reason we can do IO and remain referentially transparent is because every time we perform a side-effect, we do so on a unique universe - there is no way to duplicate a universe. Haskell actually models the real universe better than any other languages - in the real world, we have the concept of now - some state of the current universe until an event elapses, when we have a new now, and we have a concept of the past, but in fact, this past does not exist anywhere - it was transient - we can't go back and modify this old universe to cause a change in the future, unless we introduce theories of parallel universes. the only thing that ever exists is the now, as you might learn from a Buddhist. A Haskell function which does IO is just an event which causes a change from an old now in the universe to a current now, and the old one, the past, no longer exists.