[Caml-list] avoiding boilerplate with phantom types

["Milan Stanojević" <milanst@gmail.com>]

This is my use case.
I have a module A with type t and bunch of functions. I want to create
module B which is just a wrapper around A with a phantom type
Roughly
module A : sig
  type t
  val foo : t -> int
  val bar : t -> string
end

module B : sig
   type phantom1
   type phantom2
   type 'a t   (* or maybe even type 'a t = private A.t *)
   val foo : _ t -> int
   val bar : _ t -> string

   val validate : phantom1 t -> phantom2 t
end

I was hoping I can avoid copy-pasting declarations and that I can have
some easy way to have A and B in sync as I add or remove functions
from A.
I was able to do something for implementation B but not for interface.

B.ml
type this_name_is_not_in_scope = A.t
include (A : module type of A with type t = this_name_is_not_in_scope)

type 'a t = A.t
type phantom1
type phantom2
let validate = ....

But for interface I can't do anything unless I expose type equality
between 'a B.t and A.t (which renders phantom types useless) so I had
to list all the functions by hand.
Does anyone have a better way to do this?

The problem is that I can't say include module type of A with type t := 'a t
Why is this disallowed?

Thanks,
   Milan