I also have trouble defining "functional". My ex husband, had 2 or more drinks per day, and he NEEDED them. Sometimes he had more and he'd get argumentative or pass out, but if he didn't get the 2 he would go looking for them.
He never got a DUI even though there were plenty of times he would have been arrested if he had been stopped, he never lost a job (that I know of), and he only embarrassed me in public a few times. The group we hung out with were social drinkers mixed with a couple of heavier drinkers. He fit right in.
He was such a good actor that I didn't realize the extent of the problem. The infidelity was my big wake-up call.