Where does the "100% purely functional" requirement come from? By some strict definition, this is impossible, because you'd not have any means of communicating

Where does the "100% purely functional" requirement come from? By some strict definition, this is impossible, because you'd not have any means of communicating the result of your computation. In pure OOP speak: A "final static" method is easier to reason about than some non-static, non-final method. (But when a method is defined as "final" and "static", the providing class only serves as a namespace - where is the OOP in that?)