Hello and welcome to the forum,
It’s normal to design your business model with concept like User, Meetup, both in OOP and FP.
But You do not process them the same way. Design patterns in OOP and FP are totally different.
I would recommend this book (Examples are in F#).
Which was written by the author of this video
Please remember FP is just a mean to achieve Concurrency. Immutability allows a better isolation between processes. Erlang is described as CSP.
It is complicate to model concurrency in OOP, but it is a building block of Erlang/Elixir, with OTP. And supervision tree allows tolerance to failure.
