(emphasis mine)
I think we might have a case of the xy-problem here: You are looking for help with your supposed solution, but do not give us the full problem which makes it difficult to reason about the solution space. We’re slowly getting closer, as we now know that what you actually want to do has very little to do with ‘compile-time registries’.
You think you want to use bidirectional relationships between e.g. parents and children. However, what would you use these relationships for?
In object- and class-oriented languages where we deal a lot with inheritance, (bi-directional) relationships see a lot of use, especially since pass by (mutable) reference is very common, which is what gave rise to Joe Armstrong’s famous quote You wanted a banana but what you got was a gorilla holding the banana and the entire jungle.
In an immutable functional language such as Elixir, we’d model the problem usually in a very different way where these mutable references are not needed. In these situations we often pick only one direction to go in (such as only parent → child) and that is how we fill in our structs. This is what e.g. Ecto’s relationships use.
But say we are indeed trying to model the ancestry of a family of humans, which is one of the few cases where indeed relationships in both directions might be important. For instance, we want to figure out who all the nieces and nephews of a particular family member are.
In that case, we use a dedicated graph datastructure. (which can be implemented under the hood in a number of ways on an immutable system, which I won’t go in here as it’s not important when using them). One example might be using libgraph but there are other libraries as well.
