file.ml

type record = {
  field: string;}
type on
type off
module type Thing  =
  sig
    type ('a,'p) t
    val create : 'a -> ('a,on) t
    val run : ('a,off) t -> ('a,on) t
  end
module Thing : Thing =
  struct
    type ('a,'p) t = record
    let run (r : ('a,'p) t) = let value = r.field in r
    let create r = r
  end 
let record1 = { field = "foo" }
let a = Thing.create record1
let b = Thing.run a

(*
Signature mismatch:
...
Values do not match:
  val create : 'a -> 'a
is not included in
  val create : 'a -> ('a, on) t
*)

file2.ml

type record = {
  field: string;}
type on
type off
module type Thing  =
  sig type 'a t val create : record -> off t val run : off t -> on t end
module Thing : Thing =
  struct
    type 'a t = record
    let run r = let value = r.field in r
    let create r = r
  end 
let record1 = { field = "foo" }
let a = Thing.create record1
let b = Thing.run a