[grogs]

I think Scala is doing a decent job at bringing these concepts together. Scala ticks all the OOP points you made - even more than Java does (Java has special primitives for int/booleans etc, in Scala that's more hidden, really everything in an object). FP also likes Encapsulation as you decided it - pure functions, that don't do side effects (even logging) without it being explicit in the type signature.

FP can model relationships/interactions OK. They just have new tools/techniques to do it. For example, instead of mutating things, you create a copy with the change. You then push the actual state/mutation to the edges of your system (e.g. the database), and thus hopefully make it easier to reason about. But it can be slower or hard to change things deep in an object graph. For slowness - we have persistent collections which re-use unchanged parts of the collection and implement performant "copying". For deep structures, Haskell & Scala have lenses.

It's still an ongoing exploration, with people borrowing/discovering new techniques, but I think it's working quite well and I'm happy to keep exploring.

In the other direction, I've been following Rust. I find it very interesting that it's got a very good type system, but still has mutation. Instead of avoiding mutation, they track ownership in the type system, and thus keep it easier to reason about. By doing this, they make the the performance/memory management easier to reason about, without making it too hard to reason about where effects are happening.