test.ml

module type Thing = sig
  type 'a off
  type 'a on
  val create : 'a -> 'a off
  val run : 'a off -> 'a on
end

module Thing : Thing = struct
  type 'a off = Off of 'a
  type 'a on = On of 'a
  let create x = Off x
  let run (Off x) =
    (* ... *)
    On x
end

type record = { field : string }
let record1 = { field = "foo" }

let a = Thing.create record1
let b = Thing.run a