This stuff is quite new to me as I’ve been learning F#, so take this with a pinch of salt. Some of the things you’d want are: - a function to produce a list of customers

- a function or two to retrieve the data, which would be passed into the customer list function. This allows the customer list function to be independent of the data retrieval. This is essentially functional dependency injection

- a function to take a list of customers and return a list of effects: things that should happen

- this is where I wave my hands as I’m not sure of the plumbing. But the final part is something that takes the list of effects and does something with them

With the above you have a core that is ignorant of where its inputs come from and how its effects are achieved - it’s very much a pure domain model, with the messy interfaces with the outside world kept at the edges