Re: [Caml-list] Trying to define a functor combining polymorphic variants

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2014/05/02 05:58, Berke Durak wrote:
> So I naturally wanted to write a functor that does what the module AB does:
> 
>  module type S = sig
>    type message
>    val string_of_message : message -> string
>  end
> 
>  module PROD(A : S)(B : S) = struct
>    type message = [ A.message | B.message ]
>    let string_of_message = function
>    | #A.t as msg -> A.string_of_message msg
>    | #B.t as msg -> B.string_of_message msg
>  end
> 
> But we (me + people on #ocaml: mrvn, drup, ggole, whitequark...) couldn't find a
> way to specify, in the signature S, that message is a polymorphic variant so
> that [ A.message | B.message ] is legal.

There is a branch of the compiler which allows you to do that.

	http://caml.inria.fr/cgi-bin/viewvc.cgi/ocaml/branches/varunion/

Note that it contains only part of the directories. You can get the others
from a pristine 3.10.

Using that version, you would write:
  module type S = sig
     type message = private [> ]
     …
  end

  module PROD (A : S) (B : S with type message = private [> ] ~ A.message) = …

where the ~ is a compatibility annotation.

This was never merged because I couldn’t find a clean way to fix some gap between
theory and practice, but I may give it another try.
Keigo Imai used it in his async_session code.

https://github.com/keigoi/async_session/blob/master/varunion_session.ml

Jacques Garrigue