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Comment by microtonal | Hacker News Reader

You can't change the chair because it's all immutable, so you create a new chair along with a new universe and build a new car with the new chair.

No, if you read Okasaki, you knew you only have to reconstruct the part of the universe (up to its origin) that refers (directly or indirectly) to the seat ;).

In fact, you can safely have two universes with different seats, where most of the universes is shared.

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Oh that makes for a funny part of the ending too then.

While going about your work, you suddenly disappear along with your universe and everything in it. Someone somewhere in the world drank a cup of coffee, which means that he now has an empty cup instead of a full cup. A new universe had to be created with the full cup replaced, and unfortunately your universe had to be thrown away. You continue life as a clone in a new universe with an empty coffee cup somewhere in Nebraska.

Why did this happen? Someone picked a copy-on-write map implementation because it had a slightly faster O(logN) delete behavior and unfortunately the re-use function of the map wasn't implemented yet because the author has gone on to do a series on the use of monads in baking the ultimate croissant.

"Someone somewhere in the world drank a cup of coffee, which means that he now has an empty cup instead of a full cup"

That cannot happen in an immutable world. To fill an empty cup, you cons a cup and its contents. To get an empty cup again, you take the car of the cons. The full cup will only get garbage collected if you do not reference it anymore.

> To fill an empty cup, you cons a cup and its contents.

That simply has the wrong type in mathematically civilized languages like Standard ML or Haskell.

> The full cup will only get garbage collected if you do not reference it anymore.

This is orthogonal to functional programming. You can avoid the need for a garbage collector with substructural types.

Hence, Many World Interpretation of quantum mechanics.

Sorting in O(n) time:

1. Apply a quantum permutation to your list.

2. Check if it's sorted.

3. If not, destroy the universe.

Related (amazing) paper by Scott Aaronson: "NP-complete Problems and Physical Reality" (http://arxiv.org/abs/quant-ph/0502072). He proposes "anthropic computing" as a way of solving NP-complete problems in polynomial time: guess an answer, then kill yourself if the answer is wrong.

It would be very eerie if suddenly all the guesses started being right.

I am worried that someone will actually try this sometime and it will turn out not to work, either because MWI is wrong or because the destruction of the universe was incomplete — for example, collapsing the false vacuum would merely change the state of the things in the universe, not destroy the universe per se.

Fortunately, we don't have any currently promising avenues for nucleating a true vacuum or anything like that, so this algorithm is purely theoretical.

For now.

That made me think of Greg Egan's Quarantine:

https://en.wikipedia.org/wiki/Quarantine_%28Greg_Egan_novel%...

Best-case runtime = 1. Worst-case = infinity.

I think the joke is that in all cases but the best-case, you won't be around to observe it, this becomes practical once you can create a universe for every permutation and destroy all but the universe which notifies you that it's the best-case.

In all good text books this is left as an exercise for the reader.

In a really good text book, this is left as an exercise that the reader won't do.

No, checking whether a list is sorted is O(n) ;).

Bogosort is O(n) in the best case and O(inf) in the worst case).

Optimize out the check, so it will be O(1) in some cases. ;)

The convergence of this method depends on the underlying distribution for your random process. It could be O(never).

As Scott Aaronson points out, the easy solution to this is that you add a slight quantum chance that the algorithm will yield "Reject". As long as that chance is non-zero, but greatly smaller than the probability of any particular successful permutation, you will survive in the universe that says "Reject".

This sounds like a joke but this is exactly the proof that sorting is in NP.

What is a quantum permutation?

A new chair isn't created - the legs, seat, and back of the old chair are re-used with new screws.

If you want to replace a leg, you still need to disassemble the chair and reassemble it with new screws.

I've never considered the many-worlds interpretation of quantum physics under the light of persistent data structures.. very interesting!

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