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Date: Fri, 5 Dec 2008 15:21:09 +0100
From: =?iso-8859-1?Q?J=E9r=E9mie?= Dimino <jeremie@dimino.org>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] New language feature in OCaml 3.11
Message-ID: <20081205142109.GB6587@aurora>
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X-Spam: no; 0.00; ocaml:01 0100,:01 subtyping:01 abbreviation:01 abbreviation:01 sig:01 val:01 struct:01 foo:01 ocaml:01 coerced:01 foo:01 coerced:01 polymorphic:01 polymorphic:01 

On Fri, Dec 05, 2008 at 12:57:42PM +0100, Paolo Donadeo wrote:
> > - Subtyping is now allowed between a private abbreviation and its definition,
> >  and between a polymorphic method and its monomorphic instance.
>
> Is there anybody who wants to elaborate this with an example,
> especially the second statement, regarding polymorphic methods?

Here is an example for private abbreviation:

,----
| module M : sig
|   type t = private int
|   val x : t
| end = struct
|   type t = int
|   let x = 1
| end
`----

[M.x] can be seen as a value of type [int] by using a coercion:

,----
| # M.x;;
| - : M.t = 1
| # (M.x :> int);;
| - : int = 1
`----

And one example for polymorphic methods:

,----
| class type foo = object
|   method f : int -> int
| end
|
| let x = object
|   method f : 'a. 'a -> 'a = fun x -> x
| end
`----

with ocaml < 3.11 [x] can not be coerced to type [foo]:

,----
| # (x :> foo);;
| Characters 1-2:
|   (x :> foo);;
|    ^
| This expression cannot be coerced to type foo = < f : int -> int >;
| it has type < f : 'a. 'a -> 'a > but is here used with type #foo
| Types for method f are incompatible
`----

but it can with ocaml 3.11:

,----
| # (x :> foo);;
| - : foo = <obj>
`----

Jérémie