Part 4. Stateful and reactive applications

This part examines the realms of managing state in functional programming and the dynamics of reactive application design. We begin by demystifying the concept of state in functional programming, emphasizing that it not only accommodates state but also introduces innovative approaches for creating safe, well-designed stateful applications. Furthermore, we study the nuances of pure and impure states, the singular approach of argument-passing state, and how state monads facilitate the handling of the immutable state in a manner that mimics imperative programming while remaining functional and combinatorial. The discussion extends to the internal workings of state monads, their role in structuring monadic applications, and their evolution into state monad transformers, highlighting strategies for managing multiple state contexts and dealing with impure mutable states in industrial functional languages.