A functional data structure is a data structure that is suitable for implementation in a functional programming language, or for coding in an ordinary language like C or Java using a functional style. Functional data structures are closely related to persistent data structures and immutable data structures—in fact, the three terms are often used interchangeably. Howevber, there are subtle differences. The term persistent data structures refers to the general class of data structures in which an update does not destroy the previous version of the data structure, but rather creates a new version that co-exists with the previous version. See Chapter 33 for more details about persistent data structures. The term immutable data structures emphasizes a particular implementation technique for achieving persistence, in which memory devoted to a particular version of the data structure, once initialized, is never altered. The term functional data structures emphasizes the language or coding style in which persistent data structures are implemented. Functional data structures are always immutable, except in a technical sense discussed in Section 41.4.