You're right, I would absolutely be surprised. Persistent data structures intrinsically involve copying on updates, and no amount of compiler cleverness can av

You're right, I would absolutely be surprised. Persistent data structures intrinsically involve copying on updates, and no amount of compiler cleverness can avoid that. I can see the possibility for compiler magic with stream fusion, for example, but I'm not seeing it for data structures. The only win I can see is if you have multiple clients of your data structure holding references to multiple versions, but even there the compiler doesn't help you. Am I missing something?