title: What is the best way to manage mutable state?
body: <p>I just finished Martin Odersky's scala class at Coursera. Scala being my first FP language, I was excited by the idea of limiting mutable state. This allows for much easier concurrency and also makes the code super maintainable. </p>
  
  <p>While learning all this, I realized you could guarantee the mutability of an object as long as it had no mutable variables and only referenced immutable objects. So now I can do everything by creating a new state instead of modifying an old one, use tail recursion when possible. </p>
  
  <p>Great. So I can only do this so far up the chain. At some point, my application needs to be able to modify some existing state. I know where put in concurrency control at this point, locks, blah blah. I'm still defaulting to my standard multi-threaded concurrency control I've always used. </p>
  
  <p>Oh scala community, is there a better way? Monads maybe?</p>
  
  <p>EDIT: this question is a bit general, so I wanted to give a use case:
  I have a machine learning algorithm that stores several collections of data. They have functions that return updated representations of the data (training, etc), all immutable. Ultimately I can keep this return-updated-state pattern going up the chain to the actual object running the simulation. This has a state that is mutable and holds references to the collections. I may want to distributed to multi-cores, or multi-system.</p>