On this friendship example, I think already FP is showing you something useful about the way you are designing your model.
A friendship is really an edge in a graph. if you manage friendships by recording a collection of friend PersonId's on each person, you are modelling a graph by recording the same edge in two different places, and then hoping the programmer is diligent enough to never make a mistake and update the edge in one place but not the other. If you model the friendship graph as a collection of id pairs, not encapsulated inside a specific person structure, this invalid state is no longer possible.
I'm not sure what you are on about regarding making transitive updates - that idea is obviously not going to work on any moderately sized network. Imagine if Facebook tried to record all your transitive friendships on your account!
Sure immutability is non-intuitive, but it's really helpful! Like borrow checking in Rust, immutability helps eliminate an entire class of bugs. And immutable code is often much easier to reason about, you need to keep significantly less of the context in your head.
A friendship is really an edge in a graph. if you manage friendships by recording a collection of friend PersonId's on each person, you are modelling a graph by recording the same edge in two different places, and then hoping the programmer is diligent enough to never make a mistake and update the edge in one place but not the other. If you model the friendship graph as a collection of id pairs, not encapsulated inside a specific person structure, this invalid state is no longer possible.
I'm not sure what you are on about regarding making transitive updates - that idea is obviously not going to work on any moderately sized network. Imagine if Facebook tried to record all your transitive friendships on your account!
Sure immutability is non-intuitive, but it's really helpful! Like borrow checking in Rust, immutability helps eliminate an entire class of bugs. And immutable code is often much easier to reason about, you need to keep significantly less of the context in your head.