Since HVM is linear, adding some mutability would be completely fine and would NOT break referential transparency (because values only exist in one place!). So, we could actually have "pure mutable arrays". In order to make that mix up well with lazy cloning, though, we'd need to limit the size of array to about 16. But larger arrays could be made as trees of mutable 16-tuples. There was a post about it, but I can't find it right now.