howto in formula field

I would like to have a formula field return (A + ", " + B). But if B is empty, only return (A). So in other words, have some kind of if/else in there. Is this possible?

Example:
A = "This"
B = "That"
return: "This, That"

A = "This"
B = ""
return: "This"

I would like this to be dynamic - so using a formula. And not a field that gets filled through a script in the OnSucces of the form.