Functional programming was invented by Backus to solve the problem of interchangeable parts. Unfortunately, he did not finish his work, and as a result people are still trying to paper over the missing hole in the theory. State monads as described above make the process of storing and manipulating state a very obscure and mysterious process. This additional complexity and abstraction can only add to the lifecycle cost of software generated using this technique, with an attendant increase in maintenance cost. If you were to take a 10 year old, and give them basic training, would they be able to understand this? I doubt it, yet computers at their core are incredibly simple. So a better paradigm awaits.