6 Purely functional state

Working with program state is tricky, and even more so in functional programming, where we value principles such as immutability and eradicating side effects. Mutating state comes at a considerable cost, making programs difficult to reason about and maintain. Fortunately, we have a design pattern at hand to deal with program state in a purely functional way. Applying this pattern allows us to deal with state in a deterministic fashion and subsequently lets us reason about and test our programs more easily.

By viewing the program state as a transition or action that is passed along as context during a series of transformations, we can contain and localize the complexity associated with state machines. We can even take this a step further by hiding these state transitions altogether through the use of higher-order combinator functions that pass along state actions implicitly in the background. A pattern begins to emerge as we combine these concepts of passing and hiding state transitions in the background.

6.1 Generating random numbers using side effects

6.2 Purely functional random number generation

6.3 Making stateful APIs pure

6.4 An implicit approach to passing state actions

6.4.1 More power by combining state actions

6.4.2 Recursive retries through nested state actions

6.4.3 Applying the combinator API to the initial example

6.5 A general state action data type

6.6 Purely functional imperative programming

6.7 Conclusion

Summary