[Spendar89]

> The world is very much mutable

This depends on your definition of "the world". If time and memory are taken into account, you could easily make the case that the world is very much immutable. For example, if your friend changes his email address, its not as if his old address no longer exists. Even if it only exists in memory, it still exists somewhere, and you're still able to reference it.

If you're interested in this topic, I highly recommend Rich Hickey's talk "The Value of Values"[1].

[1] http://www.infoq.com/presentations/Value-Values.

[seanmcdirmid]

In that case, the reference is still mutable even if the email address isn't. One may also delete an account; i.e. you can change the state of something. A language should set clear boundaries on what is mutable or not, and I see both FP and IP languages going with extreme boundaries in either direction.