[bluGill]

Interesting, it makes more sense now. Though I'm not clear if you want/need that in a video game. In general you want two data structures, one current state of the world and one next state - since they have a lot of similarity it at first makes sense to do what you say, except the you lose the advantage of vector: it is very cache friendly to iterate on a vector, or even two vectors, while it appears (without knowing internal details) that your HAMT is less cache friendly as the next element is not in the next memory location.

Of course I'd need benchmarks to make a real judgement. The above is my gut reaction.