>Because you can't make any guarantees about state in a language that wasn't intended for FP because how do you know what side effects the built-in functions are having

This is your original argument. I said that FP programs offer none of the said guarantees because they are at it's core compiled into imperative programs... You responded with a statement about a stack machine being an abstraction over a "register machine." How does the stack machine statement move your original argument forward...? We're getting side tracked here...