class User{phone: ?PhoneNumber}
class User{phone: ?string}.It's more about writing
struct User {phone: MaybePhoneNumber} // give or take, it's a monoid
struct User {phone: Option<String>}> monoid
nullables with `??` and `?.` are also give-or-take monoids. is it common though to `or` two MaybePhoneNumbers together or to apply a PhoneNumber->MaybePhoneNumber function to it? if not then why mention it?
let's see something meaningfully different like a database schema.
[1] https://esolangs.org/wiki/Trivial_brainfuck_substitution