Agreeing with @pwnstigator I think the important thing is to separate the pure functional code from the impure code with side effects. And be very explicit that

Agreeing with @pwnstigator I think the important thing is to separate the pure functional code from the impure code with side effects. And be very explicit that this code is impure and does change state so when you use it you must take this into account. If this is done properly you generally find that the impure code need only be a small portion of the whoe program.