Consider a function that takes an integer and adds two, but only if it is positive. If the input is zero or less, then that is an error. Write this function as a Haskell function, handling the error in the functional way.

f x $=$ if x $>0$ then $2+$x else Null

f x $=$ if x $>0$ then $2+$x else error "invalid input"

f x $=$ if x $>0$ then Just $(2+$x$)$ else Nothing