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Date: Sat, 19 Mar 2011 18:48:03 +0100
From: Guillaume Yziquel <guillaume.yziquel@citycable.ch>
To: Raphael Proust <raphlalou@gmail.com>
Cc: caml-list@yquem.inria.fr
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Subject: Re: [Caml-list] Recursive Parametric class type Typing

Le Saturday 19 Mar 2011 à 16:48:40 (+0100), Raphael Proust a écrit :
> Hi list,
> 
> Trying to bind a javascript library for js_of_ocaml, I encountered the following
> pattern (here drastically simplified):
> 
> class type ['t] c =
>   object
>     method plus: 't -> unit
>     method minus: unit -> 't
>     method container: unit -> container
>   end
> and container =
>   object
>     method int: unit -> int c
>     method string: unit -> string c
>   end
> 
> The following error is raised at compile time:
> Error: This type string should be an instance of type int
> 
> In the use case:
> - instead of [int] and [string] there are six different [class type]s;
> - [classe type]s have more methods; and
> - there are additional recursive [class type]s
> 
> Why is there such a limitation on the types? Is there a work around that
> wouldn't induce (too  much) code duplication?

The reason why there is this behaviour is unification within the 'and'
declaration. method int says that 't in ['t] c is of type int,
therefore it should also be an int c in method string instead of a
string c.

Workaround: Use recursive modules. They are a solution for keeping
recursion while breaking type unification. Something in the taste of:

yziquel@seldon:~$ ocaml
        Objective Caml version 3.12.0

# module rec Q : sig
    class type ['w] e = object method r : 'w end
  end = struct
    class type t = object                   
      method z : int Q.e
      method u : string Q.e
    end
    class type ['w] e = object method r : 'w end
  end;;
module rec Q : sig class type ['a] e = object method r : 'a end end

Not sure if this fits your bill when it comes to avoiding code
duplication.

-- 
     Guillaume Yziquel