[Caml-list] How do I declare a value of 'a t instead of '_a t in a module?

[Goswin von Brederlow <goswin-v-b@web.de>]

Hi,

I'm writing a module that reimplements the option type without
redirection. For that I'm mapping None to a value with all bits set to
0, the NULL pointer in C. For that I have a little helper function:

external make_null : unit -> 'a t = "caml_shallow_null"

But now I want to also have a polymorphic value for NULL:

module Make : sig
  type 'a t
  val make_list : unit -> 'a list
  val make_null : unit -> 'a t
end = struct
  type 'a t
  let make_list () = []
  external make_null : unit -> 'a t = "caml_shallow_null"
end

module Shallow : sig
  type 'a t
  val list : 'a list
  val null : 'a t
end = struct
  type 'a t = 'a Make.t
  let list = Make.make_list ()
  let null = Make.make_null ()
end

File "shallow.ml", line 15, characters 6-102:
Error: Signature mismatch:
       Modules do not match:
         sig
           type 'a t = 'a Make.t
           val list : 'a list
           val null : '_a Make.t
         end
       is not included in
         sig type 'a t val list : 'a list val null : 'a t end
       Values do not match:
         val null : '_a Make.t
       is not included in
         val null : 'a t


What makes the Make.make_list differ from Make.make_null?

Is there a way that I can specify that null is still polymorphic or does
that only work for constructors like None and compiler magic like []?

----------------------------------------------------------------------

And here something odd:

module Make : sig
  type 'a t = 'a list
  val make_list : unit -> 'a list
  val make_null : unit -> 'a t
end = struct
  type 'a t = 'a list
  let make_list () = []
  external make_null : unit -> 'a t = "caml_shallow_null"
end

module Shallow : sig
  type 'a t
  val list : 'a list
  val null : 'a t
end = struct
  type 'a t = 'a Make.t
  let list = Make.make_list ()
  let null = Make.make_null ()
end

compiles fine. But

module Make : sig
  type 'a t = private 'a list
  val make_list : unit -> 'a list
  val make_null : unit -> 'a t
end = struct
  type 'a t = 'a list
  let make_list () = []
  external make_null : unit -> 'a t = "caml_shallow_null"
end

module Shallow : sig
  type 'a t
  val list : 'a list
  val null : 'a t
end = struct
  type 'a t = 'a Make.t
  let list = Make.make_list ()
  let null = Make.make_null ()
end

fails again. Just making the Make.t private makes it fail.

MfG
        Goswin