type_hackery.ml

type 'payload head =
  | Foo : int head
  | Bar : float head
  | App : t list head

and t = T : {
  mutable head : 'payload head;
  mutable payload : 'payload;
} -> t

let set_to_float (T t) =
  (* this does not typecheck ofc... *)
  t.head <- B;
  t.payload <- 1.0