Editing Persistent data structure (section)

{{Short description|Data structure that always preserves the previous version of itself when it is modified}}
{{Distinguish|Persistent storage}}

In [[computing]], a '''persistent data structure''' or '''not ephemeral data structure''' is a [[data structure]] that always preserves the previous version of itself when it is modified. Such data structures are effectively [[Immutable object|immutable]], as their operations do not (visibly) update the structure in-place, but instead always yield a new updated structure. The term was introduced in Driscoll, Sarnak, Sleator, and Tarjan's 1986 article.<ref name="Driscoll">{{Cite book |vauthors=Driscoll JR, Sarnak N, Sleator DD, Tarjan RE |title=Proceedings of the eighteenth annual ACM symposium on Theory of computing - STOC '86 |year=1986 |chapter=Making data structures persistent |isbn=978-0-89791-193-1 |doi=10.1145/12130.12142 |pages=109–121|citeseerx=10.1.1.133.4630 |s2cid=364871 }}</ref>

A data structure is '''partially persistent''' if all versions can be accessed but only the newest version can be modified. The data structure is '''fully persistent''' if every version can be both accessed and modified. If there is also a meld or merge operation that can create a new version from two previous versions, the data structure is called '''confluently persistent'''. Structures that are not persistent are called ''ephemeral''.<ref name="kaplan2">{{Cite journal|author=Kaplan, Haim|year=2001|title=Persistent data structures|url=http://www.math.tau.ac.il/~haimk/papers/persistent-survey.ps|journal=Handbook on Data Structures and Applications}}</ref>

These types of data structures are particularly common in [[Logic programming|logical]] and [[functional programming]],<ref name="kaplan2" /> as languages in those paradigms discourage (or fully forbid) the use of mutable data.