I wouldn't really agree that it doesn't work out in practice. For the systems that I work on, the concept of privacy is extremely useful, perhaps we're doing things 'the wrong way' to believe that but I don't think that's all that likely.
What would you suggest as a better alternative, outside of a purely functional approach? Just as an example, suppose that Java's LinkedList class didn't have a cached size and that instead it went and iterated its elements to calculate its size each time someone asked for it. I might write a wrapper around LinkedList with two private variables, the LinkedList and an int to cache its size in. Then I would update the int each time an operation was performed on my member list. If I can't make those two members private, how can I ensure that no one accidentally updates the list without updating the size, or vice-versa. It mightn't be an unreasonable mistake for someone to think calling mylist.size = 0 would empty the list. What is the 'safe' way of constructing this concept?
What would you suggest as a better alternative, outside of a purely functional approach? Just as an example, suppose that Java's LinkedList class didn't have a cached size and that instead it went and iterated its elements to calculate its size each time someone asked for it. I might write a wrapper around LinkedList with two private variables, the LinkedList and an int to cache its size in. Then I would update the int each time an operation was performed on my member list. If I can't make those two members private, how can I ensure that no one accidentally updates the list without updating the size, or vice-versa. It mightn't be an unreasonable mistake for someone to think calling mylist.size = 0 would empty the list. What is the 'safe' way of constructing this concept?