"There is two ways basically to deal with concurrency. Mutable state, you must have shared memory, you must have threads, you must have mutexes when you access critical sections. If you are dealing with immutable state, what you do is processes will not share memory. The only way for them to share data is through a message passing that needs to copy the data from one process to another. They'll each have their own copy. So, two completely different paradigms, two completely different results."