Theory: Persistent Data Structures
Clojure has 4 built in data structures:
- list - simple linked list, the first element is assumed to be a function call
- map - a key value pair with keys often defined using Clojure keywords
- vector - an indexed array-like structure
- set - a unique set of elements, not ordered by default
Each of these data structures are immutable, so once they are created they cannot be changed.
When you run a function over these data structures then a new data structure is returned. This keeps your code very simple as you can guarantee there are no side effects due to other parts of your code changing state.
It may seem inefficient to create a new copy of a data structure each time, especially when working on very large data structure. Clojure creates very efficient copies, by sharing elements from the original data structure.