I beg to somewhat differ: As far as purely functional languages go, it is about reification of side effects (eg via monads or uniqueness types) in such a way th

I beg to somewhat differ: As far as purely functional languages go, it is about reification of side effects (eg via monads or uniqueness types) in such a way that you do get to use pure functions. My personal definition of 'functional' is based on this lack of side-effects (or, more abstractly, on modelling computation as a declaration of relations between sets, which is what mathematical functions are), but it gets fuzzy once you leave the realm of purely functional languages. If you prefer another definition for pragmatic reasons, I'm fine with it, it's just not my choice.