Obviously the pseudo code leaves to the imagination, but what benefits does this give you? Are you checking that it is 10-digits? Are you allowing for + symbols for the international codes?

▲ xboxnolifes an hour ago | parent | next [-]

You have functions

    void callNumber(string phoneNumber);
    void associatePhoneNumber(string phoneNumber, Person person);
    Person lookupPerson(string phoneNumber);
    Provider getProvider(string phoneNumber);

I pass in "555;324+289G". Are you putting validation logic into all of those functions? You could have a validation function you write once and call in all of those functions, but why? Why not just parse the phone number into an already validated type and pass that around?

    PhoneNumber PhoneNumber(string phoneNumber);
    void callNumber(PhoneNumber phoneNumber);
    void associatePhoneNumber(PhoneNumber phoneNumber, Person person);
    Person lookupPerson(PhoneNumber phoneNumber);
    Provider getProvider(PhoneNumber phoneNumber);

Put all of the validation logic into the type conversion function. Now you only need to validate once from string to PhoneNumber, and you can safely assume it's valid everywhere else.

▲ flqn 6 hours ago | parent | prev | next [-]

Can't pass a PhoneNumber to a function expecting an EmailAddress, for one, or mix up the order of arguments in a function that may otherwise just take two or more strings

▲ munk-a 6 hours ago | parent | prev | next [-]

That's going to be up to the business building the logic. Ideally those assumptions are clearly encoded in an easily readable manner but at the very least they should be captured somewhere code adjacent (even if it's just a comment and the block of logic to enforce those restraints).

	▲	bjghknggkk 6 hours ago \| parent [-]
		How to make a crap system that users will hate: Let some architecture astronaut decide what characters should be valid or not.

▲ JambalayaJimbo 6 hours ago | parent | prev | next [-]

If you are not checking that the phone number is 10 digits (or whatever the rules are for the phone number for your use case), it is absolutely pointless. But why would you not?

	▲	jghn 6 hours ago \| parent [-]
		I would argue it's the other way around. If I take a string I believe to be a phone number and wrap it in a `PhoneNumber` type, and then later I try to pass it in as the wrong argument to a function like say I get order of name & phone number reversed, it'll complain. Whereas if both name & phone number are strings, it won't complain. That's what I see as the primary value to this sort of typing. Enforcing the invariants is a separate matter.

▲ bjghknggkk 6 hours ago | parent | prev [-]

And parentheses. And spaces (that may, or may not, be trimmed). And all kind of unicode equivalent characters, that might have to be canonicalized. Why not treat it as a byte buffer anyway.