As we've noticed in the previous section, Clojure's data structures are not only immutable, but can produce new values without impacting the old version. Operations produce these new values in such a way that old values remain accessible; the new version is produced in compliance with the complexity guarantees of that data structure, and both the old and new versions continue to meet the complexity guarantees. The operations can be recursively applied and can still meet the complexity guarantees. Such immutable data structures as the ones provided by Clojure are called persistent data structures. They are "persistent", as in, when a new version is created, both the old and new versions "persist" in terms of both the value and complexity guarantee. They have nothing to do with storage or durability of data. Making changes to the old version doesn't impede working with the new version and vice versa. Both versions persist in a similar way.

Among the publications that...