That said, I don't see what circular dependencies have to do with reflection, much less a functional style? For a trivial example, Rust lacks reflection but the following (functional-ish) code works just fine:

    mod ModA {
        use crate::ModB::B;
    
        pub struct A {
            ref_to_b: Option<Box<B>>
        }
        
        pub fn into_b(a: A) -> Option<Box<B>> { a.ref_to_b }
    }
    
    mod ModB {
        use crate::ModA::A;
    
        pub struct B {
            ref_to_a: Option<Box<A>>
        }
        
        pub fn into_a(b: B) -> Option<Box<A>> { b.ref_to_a }
    }