Question

Is there any difference in a persistent and immutable data structure? Wikipedia refers to immutable data structure when discussing persistence but I have a feeling there might be a subtle difference between the two.

Answer 1

Immutability is an implementation technique. Among other things, it provides persistence, which is an interface. The persistence API is something like:

• version update(operation o, version v) performs operation o on version v, returning a new version. If the data structure is immutable, the new version is a new structure (that may share immutable parts of the old structure). If the data structure isn’t immutable, the returned version might just be a version number. The version v remains a valid version, and it shouldn’t change in any observe-able way because of this update – the update is only visible in the returned version, not in v.
• data observe(query q, version v) observes a data structure at version v without changing it or creating a new version.

For more about these differences, see:

• The chapter “Persistent data structures” by Haim Kaplan in Handbook on Data Structures and Applications
• “Making Data Structures Persistent” by Driscoll et al.
• The first lecture of the Spring 2012 incarnation of MIT’s 6.851: Advanced Data Structures