Breastfeeding and marijuana?

Ok so this is prolly gonna make me sound terrible but I need to know. I’m due in January and I really want to breastfeed. But I want to be able to smoke again after I deliver. My question is would it be safe to smoke and breastfeed? And if not, would I be able to switch between formula and breastfeeding? And when would I be able to start switching between milk and formula?