[serichsen]

Valid points, but not _the_ point. There is a big difference between allowing and supporting something.

OO languages _support_ hidden inputs and outputs as well as programming by mutation. They _allow_ programming in a functional style, but you will have to be inventive for it.

FP languages _support_ immutable values, referential transparency and all that. They _allow_ programming by mutation and hidden inputs and outputs, but you will have to be (sometimes very) inventive for it.

[regulation_d]

Exactly. Starting points matter. And for me, a starting point of "enforces immutable data" is a good one.

[horrido]

Starting points matter, but this depends on the programming situation. That's why you use OOP or FP where it is most appropriate or suitable. Neither OOP nor FP is a panacea. Neither OOP nor FP is the universal programming tool.

[horrido]

Nothing prevents an object's methods from being referentially transparent. It just depends on how you choose to write them.