[mejutoco]

My preferred solution would be:

You have 2 types

UnvalidatedEmail

ValidatedEmail

Then ValidatedEmail is only created in the function that does the validation: a function that takes an UnvalidatedEmail and returns a ValidatedEmail or an error object.

[squeaky-clean]

That can work in some situations. One thing I won't like about it in some other situations is that you now have 2 nullable fields associated with your user, or whatever that email is associated with. It's annoying or even impossible in a lot of systems to have a guaranteed validation that user.UnvalidatedEmail or user.ValidatedEmail must exist but not both.

[mejutoco]

I see. In my example they would be just types and internally a newtype string.

So an object could have a field

email: UnvalidatedEmail | ValidatedEmail

Nothing would be nullable there in that case. You could match on the type and break if not all cases are handled.