I could have one function that pulls the wallet balance for all users, and then passes it to a pure function that returns an object with flags for each user indicating what action to take. Then another function would execute the effects based on the returned flags (kind of like the example you gave of processing a pending charges table).
The value of that level of abstraction is less clear though. Maybe better testability? But it's hard to justify what would essentially be tripling the lines of code (one function to pull the data, one pure function to compute actions, one function to execute actions).
Additionally, there's a performance cost to pulling all relevant data, instead of being able to progressively filter the data in different ways depending on partial results (example: computing charges for all users at once and then passing it to a pure function that only bills customers whose billing date is today).
Would be great to see some more complex examples of "functional core imperative shell" to see what it looks like in real-world applications, since I'm guessing the refactoring I have in my head is a naive way to do it.
