Hello there,

I've been thinking on different approaches to constraint particular functions to a particular constructor of an ADT in order to reduce representation of invalid states. Say for example I've this types:

data Address = Address { ... }

newtype Email = Email String

data Package = Package { ... }

data EmailMsg = EmailMsg { ... }

data User

= RealUser Address

| VirtualUser Email

And I would like to implement two functions:

deliverPackage :: User -> Package -> IO Bool

sendEmail :: User -> EmailMsg -> IO ()

I would like to constraint both deliverPackage and sendEmail to receive only the semantically correct constructor of User.

I know of an approach I could use, that is wrapping each constructor in it's own newtype, and create some smart constructor that way, that approach works, but I find it rather verbose.

Is there any other well known approach to deal with this scenarios?