"I could only find a couple tutorials/guides and both were imperative"

Aren't Quadtrees covered by almost all basic data-structure books? It is the most simple form of taking the binary tree into the next (2D) dimension.

The problem is rather, that most data structure tutorials and books don't even get the idea, to introduce a purely functional version, but merely state the imperative versions. Coming up with the functional versions of data structures can be difficult. Papers about it can be hard to understand and often require one to already know some niche language, that a researcher used for the paper. Even if you can find a functional implementation of a data structure, there is often not a good explanation and you need to, sort of, reverse engineer it and translate it to the language you are using.

In short, it seems relatively few people have the skills to implement them and even fewer have the skills to come up with functional versions of ordinary data structures. They are almost completely absent from university lectures as well, as far as I am aware. For example for AVL trees I could only find a document from ETH from a lecture, that no longer exists or is taught. The language is Isabel and I need to understand its syntax first, before being able to translate it to Scheme.

If anyone has an obscure source for implementations one can learn from and implement oneself in another language, please share.

You can even build them with basically one line of code by sorting points using Morton / Z-curve order. It's linear time if you use a counting/radix sort.

Edit: lol, downvoted for this post. Never change, HN.