When One Shell Isn’t Enough: Scaling the Functional Core–Imperative Shell Pattern with Actors in C++

How to structure growing C++ systems into multiple actor-driven core–shell pairs The previous post showed how the functional core–imperative shell pattern separates code into a pure part where the business logic is implemented and an effectful part that calls the pure one and handles its side effects. In consequence, the shell must handle all side-effects of the application. But as the application grows, handling different kinds of side effects (IO, state, timers) for the entire system in a single place quickly becomes problematic. Too many unrelated concerns end up in one central hub, making the code harder to reason about and maintain. Which raises the crucial question: how do we scale the functional core–imperative shell pattern? The Design Idea: Actors as Shell In order to keep things clean, we need to decouple unrelated shell responsibilities. Thus, a single shell isn’t enough—we need multiple ones, each focusing on a subset of side effects to be handled. To fully understand what