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.
[1] are the exercises with solutions (!) of a lecture [2] at ETH Zurich, which include AVL trees in Isabel. The whole list of exercise PDFs is at [3]. I started porting the AVL trees to Scheme [4], but lately have not worked more on this. I also try to make it easy to understand the implementation by providing explanatory comments.
Okasaki's is basically the Bible for this stuff. Anyone writing data structure libraries in a functional language will have read this or have it on their to-read list.
I have the book, but it also doesn't contain that many data structures. Some of the maybe most used (AVL tree?) are not in there.
Not all exercises have solutions. I get stuck on some exercise and have a hard time finding solutions to them, that I can compare with my implementations in Scheme. Not being a Haskell user (yet), I also have some issues translating Haskell to Scheme. In languages like Haskell one often controls flow by pattern matching on type. In Scheme one needs to make structures or records explicitly and use their predicates to explicitly check the type. It's all possible, but not exactly great for learning. Sort of the road has many sticks and stones to stumble.
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.