alt Hacker NewsThe computer-science answer to this problem are called "refinement types", where you can attach arbitrary predicates to a type, e.g. (pseudo-code):
fn send_birthday_mail(user: {u: User, u.birthday != null})
The difference between this and an assert is that it gets checked at compile time (it can get quite expensive to do the check though).
What can you do in mainstream languages? As much as is worth and no more than that. String -> User is worth it, User -> UserWithBirthday is not.
this looks cool, but you are doing validation when accepting the object, you probably can't do it excessively, for example, if you are dealing with objects with heights, you might have a HumanLikeHeight where height range is between 40cm and 250cm, and you want to send email to that human, would you keep adding these conditions to the predicates?