[captaincrisp]

> You do not need FP to use pure functions

I think this is a bit more subtle, and a bit less interesting to say, than you're thinking it is and responding to. You need to be impure in OOP world but not in every function you write. I write many pure functions in my OOP work; they make the whole thing easier to reason about.

[dwohnitmok]

Sure, but it's precisely those places where you're using pure functions that aren't object-oriented. That is it is possible in some nominally OO languages to write code that is not object-oriented and that's where pure functions come in. In a language that has only classic OO-style objects pure functions (and their counterpart in data space, immutable data structures) are generally not used or are not very idiomatic, e.g. as in Smalltalk (you will have things that are kind of pure, but the purity can almost always be broken in some way).