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

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

On 2014/05/03 00:50, Berke Durak wrote:
> 
> 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))))?

The compatibility annotation is on the functor. If the variant types
in the modules you pass to the functor are concrete, they don’t need
annotations.
If you want to define a 4-ary functor that calls PROD, then its arguments
will need annotations too, of course.

For the details, you can see the internship report by Romain Bardou (in French):
Unions de variants polymorphes abstraits
http://www.math.nagoya-u.ac.jp/~garrigue/papers/index.html

Jacques Garrigue

> 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