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From: Stephen Dolan <stephen.dolan@cl.cam.ac.uk>
To: Ocaml Mailing List <caml-list@inria.fr>
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Subject: [Caml-list] Private types and subtyping

My intuition is that exporting a private type from a module with "type
t = private foo" makes "t" a new subtype of "foo". So, any means of
eliminating "foo" can eliminate "t", but constructing my own "foo"
does not necessarily mean I've made a "t".

This intuition also says the following should be OK:

    module Sub : sig
      type t = private [`A | `B]
    end = struct
      type t = [`A]
    end

The compiler (4.02.1) rejects this. Is this just a missing feature, or
is there some deep reason I'm missing why the above is not OK?

Stephen