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

[Berke Durak <berke.durak@gmail.com>]

Thanks Leo and Jacques for the responses.

Jacques, if the PROD functor has to have a compatibility tag, doesn't
it mean that you can't reuse it to generate large products?  i.e. how
would one write: A*B*C*D = PROD(A)(PROD(B)(PROD(C)(D))))?


On Fri, May 2, 2014 at 2:14 AM, Jacques Garrigue
<garrigue@math.nagoya-u.ac.jp> wrote:
> 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