Summary   

We looked at two important themes in this chapter. An immutable value is never changed once it is constructed. Immutability rules out, by design, any shared state issue as you cannot change the values. Immutable code makes for increased thread safety. Copy-on-write is used when a thread needs to modify an immutable data structure. We looked at persistent data structures, which are multiple versions (copies) of the same data structure. Structural sharing helps ensure algorithmic performance. 

Next, we looked at Scala's futures, an abstraction used to express asynchronous computations. We saw how futures map with threads, and how to avoid blocking the underlying thread. Futures allows for functional composition—a functional design pattern for creating processing pipelines.   

With all this know-how, let's now look at the...