meta-lisp provides (define-struct) for single-constructor types and (define-enum) for multi-constructor types.

A first attempt

Say we want to model a user: an id, and possibly an email, possibly a phone. (The types email-address-t and phone-number-t are assumed to be defined elsewhere.)

(define-struct user-t
  (id int-t)
  (email (maybe-t email-address-t))
  (phone (maybe-t phone-number-t)))

This works, but the type allows a state that the domain forbids: a user with neither email nor phone.

Better: encode the cases in the type

A user must have at least one contact method. There are exactly three cases: email only, phone only, or both.

(define-struct user-t
  (id int-t)
  (contact user-contact-t))

(define-enum user-contact-t
  (email-contact (email email-address-t))
  (phone-contact (phone phone-number-t))
  (both-contact (email email-address-t) (phone phone-number-t)))

The illegal state is gone: it is impossible to construct a user without a contact method.

A new requirement

Now we need system users, who have no email or phone.

A tempting change: add a (maybe-t user-contact-t) field and an is-system-user flag.

(define-struct user-t
  (id int-t)
  (contact (maybe-t user-contact-t))
  (is-system-user bool-t))

The two fields combine into four states, one of which is illegal: a non-system user without contact. The type system cannot prevent it.

Better: add a constructor

The new requirement is not a new field — it is a new case of user contact.

(define-struct user-t
  (id int-t)
  (contact user-contact-t))

(define-enum user-contact-t
  (system-contact)
  (email-contact (email email-address-t))
  (phone-contact (phone phone-number-t))
  (both-contact (email email-address-t) (phone phone-number-t)))

The change is local: user-t stays untouched. All four states are legal, and all four are represented.