I didn't think it was all that passive. It's frustrating to read a comment that's nitpicking the article by making the exact same point as the author, but using big-O notation as if that were the issue. Hint: it's not. The point is that whatever purely functional data structure is being used, either the whole thing is being copied on every single recursive call or some tricky behind-the-scenes magic is going on to make destructive-looking actions not actually destructive. Either way, you're nuts if you think its approaching C speed.