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From: Hendrik Tews <tews@irritatie.cs.kun.nl>
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To: caml-list@inria.fr
Subject: Re: subtyping and inheritance
In-Reply-To: <199901182118.WAA09639@miss.wu-wien.ac.at>
References: <19990118205542.06048@pauillac.inria.fr>
	<199901182118.WAA09639@miss.wu-wien.ac.at>
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Hi,

Markus Mottl writes (in reply to Jerome Vouillon):
   From: Markus Mottl <mottl@miss.wu-wien.ac.at>
   Date: Mon, 18 Jan 1999 22:18:40 +0100 (MET)
   Subject: Re: subtyping and inheritance
   
   Hello,
   
   > So, you want to be able to select a method depending on two
   > objets. 

   [...]
   
   To solve this problem I've done something similar (defined an extra
   comparison method for symbols in class symbol) - I don't know of any
   better means. But as is obvious: this workaround unfortunately does not
   really exhibit the virtues of code reuse...
   
This is true. But what you really need to solve your problem is a
feature called sometimes "(covariant) specialization of code".
Ocaml does not have this feature. Therefore you can not hope for
a solution which is typesafe and supports code reuse. Ocaml does
provide the feature "contravariant subtyping", which gives you a
save subtype relation. There is nice paper of Guiseppe Castagna
"Covariance and Contravariance: Conflict without a Cause" [1]
which discusses those features and their relation. 

This paper and the references therein contain also proposals how
to integrate specialization of code into a record based object
oriented language. Is their any hope, that ocaml adopts this (or
any other) solution in the future, such that Markus can implement
comparison of terminals and symbols in a clean way?

There was already a proposal how to solve the problem by Jerome
Vouillon. Just for demonstration I append another solution, which
uses a self implemented type case. It supports code reuse better,
but loses type security by using the Obj module. Maybe one of the
people who "understand the whole source code for the O'Caml
compiler, runtime and libraries" and by that can use the Obj
module, can comment on its use here.
;-)

[1] G. Castagna. Covariance and contravariance: conflict without
a cause. ACM Transactions on Programming Languages and Systems
17(3):431-447, March 1995. Also available at
http://www.ens.fr/~castagna/pub.html.

Gruss,

Hendrik


exception No_subclass

(* always use unique numbers for this_dynamic_type for every class,
   such that 
     this_dynamic_type(class1) mod this_dynamic_type(class2) = 0
              if and only if
     class1 inherits from class2
*)

class top = let this_dynamic_type = 1 in
object (self : 'self)
  method dynamic_type = this_dynamic_type
  method subclass (super : int) (sub : top) =
		(* sub belongs to a subclass of super *)
    sub#dynamic_type mod super = 0

  method eq (x : top) = print_string "top = top ? "; true
end 

class left (l1 : int) = let this_dynamic_type = 2 in
  object (self : 'self)
  inherit top as super
  method dynamic_type = this_dynamic_type
  method private cast_left other =
    if self#subclass this_dynamic_type other 
    then (Obj.magic(other) : left)
    else raise No_subclass

  method eq (x : top) = 
    try let x' = self#cast_left(x) in 
    begin
      print_string "left = left ? "; 
      self#m_left = x'#m_left
    end
    with No_subclass -> super#eq(x)

  method m_left = l1
end

class right (r1 : int) = let this_dynamic_type = 3 in
  object (self : 'self)
  inherit top as super
  method dynamic_type = this_dynamic_type
  method private cast_right other =
    if self#subclass this_dynamic_type other 
    then (Obj.magic(other) : right)
    else raise No_subclass

  method eq (x : top) = 
    try let x' = self#cast_right(x) in 
    begin
      print_string "right = right ? "; 
      self#m_right = x'#m_right
    end
    with No_subclass -> super#eq(x)

  method m_right = r1
  end

let l1 = new left 1
let l2 = new left 2
let r1 = new right 1

let x = l1#eq(l2:>top)
let y = r1#eq(l1:>top)