To add, there is another angle on immutability / purely functional definitions, which is compatibility of APIs. Rich Hickey (the creator of Clojure) has made a

To add, there is another angle on immutability / purely functional definitions, which is compatibility of APIs. Rich Hickey (the creator of Clojure) has made a talk titled "Spec-ulations", in which he pointed out that there are certain operations on an API, like adding functions, adding symbols (like enumeration values), loosening preconditions (like, for example, adding keyword arguments to a function), tightening post-conditions (like, for example, removing possible error exit codes), and so on, that keep an API compatible, while others, such as removing functions, tightening preconditions, widening post-conditions (such as adding error codes or exception types), break compatibility. And then he points out that the API itself can be seen like a persistent data structure, like a dictionary where you can add new things but not remove old things, because that would break client code. And I think this is a very important idea.