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Date: Thu, 21 Nov 1996 18:10:27 +0100
Message-Id: <199611211710.SAA08291@arthur.u-strasbg.fr>
From: Christian Boos <boos@arthur.u-strasbg.fr>
To: Hendrik Tews <tews@tcs.inf.tu-dresden.de>
CC: caml-list@margaux.inria.fr
Subject: Questions about class types
In-Reply-To: <199611211357.OAA10366@ithif18.inf.tu-dresden.de>
References: <199611211357.OAA10366@ithif18.inf.tu-dresden.de>
Sender: weis


Hello,


Hendrik Tews writes:
 > ...
 > Unfortunately B does not match A, because (as it is written in
 > the documentation under 4.9.2) one can add additional instance
 > variables, but no additional methods. What is the reason for
 > this?

That's a good question. Maybe O'Caml will add private methods some day ?

 > The first idea is to implement f as an instance variable (of
 > functional type) as well, but this gets very ugly as soon as
 > there are some mutually recursive f's.

But this helps in some cases !

 > The next idea was to specify only a type and some
 > functionality, not a class ie:
 > 

This is a good idea. You're mistake was on using the ellipsis '..' which 
doesn't seem to be needed in your example.
Here is an example how to write it:

  module type A' = sig
    val newb : unit ->  < init : unit >
  end

  module B' : A' =
    struct
      class b () as self =
        val a = ()
        method init = self#f
        method f = a
      end
 
      let newb () = (new b () :>  < init : unit >)
    end


This compiles fine !

-- Christian