Keep your Reducers dumb

The idea is that Reducer should contain only resolution to how an Effect modifies the State directly.

class ReducerImpl : Reducer<State, Effect> {
    override fun invoke(state: State, effect: Effect): State = when (effect) {
        // This is fine:
        is Effect1 -> state.copy(someFlag = true)
        is Effect2 -> state.copy(someData = effect.data)
        is Effect3 -> state.copy(counter = state.counter + 1)

        // Don't do this:
        is Effect4 -> if (someCondition) (someFlag = true) else state.copy(counter = state.counter + 1)
    }
}

If you find yourself adding conditionals, it's a smell that probably business logic is creeping from your Actor to your Reducer.

Resolution: Actor is the intended place for business logic:

1. Create Effects with meaningful names to describe what can happen to your State
2. Decide what happens inside your Actor, based on any conditional logic or async execution, and emit the corresponding Effects
3. Use your Reducer only to implement how it modifies the State