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Date: Thu, 25 Sep 2008 23:40:49 -0700 (PDT)
Message-Id: <200809260640.m8Q6enK4019826@alumnus.caltech.edu>
From: Tim Rentsch <txr@alumni.caltech.edu>
To: Sebastien.Hinderer@ens-lyon.org
Cc: caml-list@yquem.inria.fr
In-reply-to: <20080925222439.GA7968@galois> (message from
	=?iso-8859-1?Q?S=E9bastien?= Hinderer on Fri, 26 Sep 2008 00:24:39
	+0200)
Subject: Re: [Caml-list] mutually dependent class and type
Reply-To: txr@alumni.caltech.edu
References:  <20080925222439.GA7968@galois>
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X-Spam: no; 0.00; ens-lyon:01 recursive:01 ocaml:01 functors:01 abstract:01 caml-list:01 data:02 data:02 define:02 define:02 modules:02 defined:02 defined:02 parameter:02 string:02 

>  Date: Fri, 26 Sep 2008 00:24:39 +0200
>  From: =?iso-8859-1?Q?S=E9bastien?= Hinderer <Sebastien.Hinderer@ens-lyon.org>
>  
>  Is there a (clean) way to define simultaneously a class and a type that
>  are mutually recursive ?
>  Something like this :
>  class element (c : content) =
>  object
>    ...
>  end and type content =
>    | Data of string
>    | Elements of element list;;
>  
>  This is of course not a valid OCaml definition, but is there a way to
>  express it ?

Frequently a good way around such problems is to abstract the type
definition with respect to the mutually dependent defined item.
Then, supply the appropriate parameter when defining the class:

    type 'a generic_content =
     | Data of string
     | Elements of 'a list

    class element (c : element generic_content) =
       object
	  method c_value = c
       end

Voila!  The class is defined in terms of a type that depends on
the class.

For classes, this technique produces a nicer result if applied to
a class type rather than a class directly.  Then we can define
the desired type 'content' appropriately and use it in the
class definition:

    type 'a generic_content =
     | Data of string
     | Elements of 'a list

    class type element_t =
       object
	  method c_value : element_t generic_content
       end

    type content = element_t generic_content

    class element (c : content) =
       object
	  method c_value = c
       end

This technique of parameterizing a type definition with
respect to a dependent type is a good one to know;  it
also is sometimes useful with modules/functors.