regression/opaque2.sml

   1 structure S :> sig type ('a,'b) t
   2                    val f : ('a,'b) t -> ('b,'a) t
   3                    val mk : 'a * 'b -> ('a,'b) t
   4                end
   5              =
   6            struct
   7              type ('a,'b) t = 'b * 'a
   8              fun f (x,y) = (y,x)
   9              fun mk (a,b) = (b,a)
  10            end
  11
  12 val a = S.mk (5, "hello")
  13
  14 val b = S.f a