[kagakuninja]

I don't know exactly what you are trying model with this hypothetical circular dependency.

However, circular dependencies can be represented with lazy (aka non-strict) references and deferred function calls (aka thunks / call-by-name), and are IMO easier to reason about than mutable imperative techniques for representing such relationships. They also have the advantage of being totally thread-safe.

The OP (Lihaoyi) is the author of an important set of Scala libraries, and Scala is an example of what you are describing. Scala is a hybrid OO / FP language that is not dogmatic about purity. You can be totally imperative if you want.

It is common in the Scala ecosystem to implement performance-critical library code using local mutability and null references. Internally the function is imperative; to the caller, it is functionally pure.