Programmer Humor

What I described isn't necessarily functional. This is just a principle for ensuring objects represent clear and well-defined contracts. The idea is that to mutate something, you should own it; that means interfaces / public APIs, which can be called externally, should take immutable arguments. You can still mutate instance members internally because those are owned by the instance. If mutation is really necessary between two objects then it should be coordinated by an object owning them both.