Purely functional data structure

A purely functional data structure is a type of data structure that stores information and processes data in a way that doesn't rely on changing any of the information that's already been stored. This means that when new data is added or existing data is changed, it won't affect data that was already there. Think of it like a computer game where nothing you do affects what's happened in the game before - you couldn't change the score of the game before you even started playing. A purely functional data structure works the same way - the data stays the same no matter what you do.