Depends on how hard and the context.

I'm generally against corporal punishment but it can be effective in some instances. A parent slapping a kid's wrist if they are about to do something stupid or dangerous is alright.

Punches are always bad. Hitting hard enough to leave an obvious bruise is bad. Anything that makes them bleed is bad. Anything done out of anger rather than discipline is bad. A parent needs to be level headed to punish their kid effectively.

I think other forms of punishment like being grounded and/or restricting use of computer and video games and television should be prioritised. Also its incredibly important to tell your child why they are being punished. I would also reduce punishment if the child is apologetic, and increase it if they lie or are otherwise resentful. Educating the child as to why what they did was wrong should be paramount.

Of course this is all hypothetical, as I have no first hand knowledge of raising a child, but if I were to become a parent, this is what I would try to do.