> In Haskell (and Erlang etc), all data structures 'want' to be copy-on-write by default. That means you don't need to do anything extra to act as if your code is running a against a duplicate: it's behaving like that anyway by default.

Right, but still, that's an implementation detail. It could have been implicit, or it can (like in the authors case be explicit), how is that important?

I'm well aware of languages that does this implicitly, as I'm mostly writing Clojure code which is all about immutability and persistent data structures. I still don't understand why you want to highlight that this could be implicit instead of explicit, what the author had to do wouldn't change really.