fun badFactorial n =
    if n < 0 then
        0 (* pick something here *)
    else if n = 0 then
        1
    else
        n * (badFactorial (n-1));
        
exception NegativeNumberException;

fun factorial n =
    if n < 0 then
        raise NegativeNumberException
    else if n = 0 then
        1
    else
        n * (factorial (n-1));