You often lose performance in traditional imperative languages when aiming for persistence. When you have immutability guarantees (like in many functional prog

You often lose performance in traditional imperative languages when aiming for persistence. When you have immutability guarantees (like in many functional programming languages like ML or Haskell) you can avoid making copies by sharing the parts of the data structure that don't change. If this kind of thing interests you, you should check out Chris Okasaki's book "Purely Functional Data Structures".