Immutability is not enough

As I’ve gotten more experience working with immutable data structures, I’ve found that they can be beneficial in many ways. However, most programs that interact with the outside world must model state changes somehow. In these cases, I’ve noticed that I still need to be careful about state updates – even though I’m using immutable data structures. Surprisingly, many of the state update bugs that are rife in imperative programs can also occur in purely functional code.