Contravariance

gistfile1.sml

datatype foo = A | B | C | D
(*[ datasort ab = A | B ]*)
(*[ datasort bc = B | C ]*)
(*[ datasort abc = A | B | C ]*)

(* This works *)
(*[ val x: ab & bc ]*)
val x = B

(* This, however, does not...
(*[ val y: ab & bc ]*)
val y = A
*)

(* Intuition: ab \/ bc = abc *)

(*[ val f: (ab -> ab) & (bc -> bc) ]*)
fun f A = A
  | f B = B
  | f C = C