Am kind of using this at work. All IO is isolated in an imperative shell layer, that communicates with an core via event. The core is kind of a state transformer, but actually does in place mutations. All state (of the business logic) is kept at one place, which has some nice benefits.

Functional programming is not too important in the core, a lot of benefits arise without it, e.g.: I can replay a prod session from the events generated and get the exact same behaviour. Makes debugging a breeze.

One advantage of having the state in one big place is that I can serialize it, send it to another thread that serves a UI from it.