Re: [Caml-list] Private types and subtyping

[Jeremy Yallop <yallop@gmail.com>]

On 11 May 2015 at 17:31, Stephen Dolan <stephen.dolan@cl.cam.ac.uk> wrote:
> 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?

This isn't quite an answer to your question, but private rows might do
what you need:

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

Jeremy.