We all know about the immutable vectors of clojure (tries, basically). I always preferred the RRB-tree which is the same thing, but relaxes the leftwise dense requirement. This means you have a bit-partitioned trie until you do a split, merge or insert, after which you get a slightly slower operations, but still very competitive.
It is actually a quite trivial change until you come to the merge algorithm which is finicky in all the wrong ways. The benefits are (in clojure terminology) O(1)ish splits, inserts and concatenations instead of O(n).
I don't know if it can be called obscure anymore, since Scala's vectors are RRB-trees and clojure actually has an implementation of them, but I often see people talk about ropes in situations where an RRB tree should be a no-brainer.
It is actually a quite trivial change until you come to the merge algorithm which is finicky in all the wrong ways. The benefits are (in clojure terminology) O(1)ish splits, inserts and concatenations instead of O(n).
I don't know if it can be called obscure anymore, since Scala's vectors are RRB-trees and clojure actually has an implementation of them, but I often see people talk about ropes in situations where an RRB tree should be a no-brainer.