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Re: ocaml, inheritance vs. subtyping?
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Date: 1996-06-03 (18:40)
From: Didier.Remy@i...
Subject: Re: ocaml, inheritance vs. subtyping?

> I am somewhat perplexed by the passage in the manual on virtual
> methods (pp. 29-30).  Let's start with the first paragraph on p.30: it
> says that int_comparable2 is not a subtype of int_comparable, even
> though the former inherits from the latter!  Now for a person with an
> experience in a non-functional OO language (C++ or Eiffel or Sather)

> it seems _very strange_ that the subtype and inheritance (derivation)
> relations aren't the same.  

No, the two relations are not the same.  This is one of the main
problem with object orientation: subtyping and subclassing should not
be identified.  The are different notions.

Inheritance is the ability to build a class from some older ones, and in
particular to share its source and compiled code. The subclassing relation
is defined between classes, and is purely syntactic. 

On the opposite, the subtyping relation is defined between types, and the
coercions are defined between values, usually objects.  The subtyping
relation tells wether an object of some type can safely be considered as an
object of another type.  This is a more semantic concept. 

There is *no* relation between sub-classing and sub-typing as illustrated by
the two following examples (You may read [1] for a longer discussion of
this problem):

Let first see an example of subclassing were subtyping fails:           ;;

    class point x as self : 'mytype = 
      val x = (x:int)
      method getx = x
      method equal (q:'mytype) = (self#getx = q#getx)

    class color_point x c as self = 
      inherit point x as super
      val c = (c : string)
      method getc = c
      method equal q = super#equal q &&  (self#getc = q#getc)

    let p = new point 1;;
    let cp = new color_point 1 "color";;

But,                                                                    ;;

    (cp : color_point :> point);;

complains with the error message:                                       

    color_point is not a subtype of point

Indeed, point is not a subtype of point_color. Otherwise, you could send the
message equal to cp seen as a point and then pass it a point as argument, as
in the following program:
    (cp : color_point :> point)#equal p;;

where the execution of method equal of cp would attempt to access the c
field of the point p.

Conversely, you can easily define two classes that are not in a subclass
relation, while the types of objets of those classes are always
in a subtype relation: For example, define
    class very_efficient_point x as self : 'mytype = 
      val x = string_of_int x
      method getx = int_of_string x
      method equal (q:'mytype) = (self#getx = q#getx)

    let q = new very_efficient_point 1;;

Then the two classes point and very_efficient_point are unrelated. 
However, objects of those classes always have the very same type, and 
a fortiori have types in a subtype relation! Indeed, you can safely test
them for equality:

     p # equal q;;

> It even seems that not having such an
> overlap defeats the purpose of having OO features in the first place.

I would rather say that identifying the two notions fools the user.  In the
past, the identification of the two notions resulted in big holes in some
type systems of languages that were widely used (in the industry), generally by
accepting the above incorrect example.

More recently, other languages choose to weaken their type system, thus
forcing the user touse dynamic type cases (i.e. allowing a runtime failure)
on examples that can be statically typed in O'caml.

> How is the programmer to predict where a subtype relation actually
> exists?

The subtyping relation in ocaml is quite standard [2]. 
A simple and efficient algorithm can be found in [3]. 

> Now the manual goes on to say that this occurs because `the self type
> appears in contravariant position in the type of method leq'.  First,
> am I right in decoding this as `the types of the argument of
> int_comparable#leq and int_comparable2#leq are in covariant relation,
> but they would have to be in contravariant relation for a subtyping
> relation to exist between int_comparable and int_comparable2'?

You should rather read:

In order for the type of int_comparable2 to be a subtype of int_comparable,

    1/ int_comparable2#leq must be a subtype of int_comparable#leq, i.e. 
    (int_comparable2 -> bool) must be a subtype of (int_comparable -> bool)

    2/ by contra-variant propagation on the left side of the arrow, 
    int_comparable must be a subtype of int_comparable2

    3/ since set_x is a method of int_comparable2, 
    set_x must be a method of int_comparable

    4/ which is not the case.
Therefore, int_comparable2 is not a subtype of int_comparable. 

> Second, and this may be the heart of the issue, _WHY_ is type 'a ->
> bool inferred for leq, and not < x: unit -> int; .. > -> bool , as
> it surely would for a global function?

the type 'a -> bool was not inferred. It was a constraint put in the source

        #class virtual comparable () : 'a =
        #  virtual leq : 'a -> bool
        class virtual comparable (unit) : 'a =
          virtual leq : 'a -> bool

In this particular case, if you define, as you suggest:
    class virtual 'a comparable () =
      constraint 'a = < x : int; .. > 
      virtual leq : 'a -> bool

and leave int_comparable and int_comprable2 classes unchanged, then 
int_comparable2 would be a subtype of int_comparable. 

> that the OO features need more explanation in the manual, though.

Presently, the chapter on objects in the "Introduction in Objective Caml"
part of the manual is more a safety kit than an extended course on
objects. Still, it will probably be extended in a future release.


[1] @incollection  (
  author       =   "William R. Cook and Walter L. Hill and Peter S. Canning",
  title        =   "Inheritance is not Subtyping",
  booktitle =    "Theoretical Aspects Of Object-Oriented Programming. 
                  Types, Semantics and Language Design",
  publisher =    "MIT Press",
  year =         1993,
  editor =    "Carl A. Gunter and John C. Mitchell")

[2] @inproceedings   (
author       =   "Roberto M. Amadio and Luca Cardelli",
title        =   "Subtyping Recursive Types",
booktitle    =   "Proceedings of the Eighteenth ACM Symposium on
                  Principles of Programming Languages", 
address      =   "Orlando, FL",
month        =   "January",
year         =   "1991",
pages        =   "104--118",
note         =   "Also available as DEC Systems Research Center
                  Research Report number 62, August 1990.  
                  To appear in TOPLAS")

        author =        "Dexter Kozen and Jens Palsberg",
        title =         "Efficient Recursive Subtyping",
        booktitle =     "Proc. 20th symp. Principles of Programming Languages",
        year =          "1993",
        pages =         "419--428",
        publisher =     "ACM press"}