From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (from majordomo@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id DAA19424; Fri, 25 Jul 2003 03:52:09 +0200 (MET DST)
X-Authentication-Warning: pauillac.inria.fr: majordomo set sender to owner-caml-list@pauillac.inria.fr using -f
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id DAA19942 for <caml-list@pauillac.inria.fr>; Fri, 25 Jul 2003 03:52:07 +0200 (MET DST)
Received: from wetware.com (wetware.wetware.com [199.108.16.1])
	by concorde.inria.fr (8.11.1/8.11.1) with ESMTP id h6P1q6f15721
	for <caml-list@inria.fr>; Fri, 25 Jul 2003 03:52:06 +0200 (MET DST)
Received: from ra07 ([199.108.16.87] helo=wetware.com)
	by wetware.com with esmtp (Exim 4.20)
	id 19frkb-0004WE-D4
	for caml-list@inria.fr; Thu, 24 Jul 2003 18:52:05 -0700
Date: Thu, 24 Jul 2003 18:52:07 -0700
Subject: Re: [Caml-list] Re: findlib-0.8.1 and ocaml-3.07+beta1
Content-Type: text/plain; charset=US-ASCII; format=flowed
Mime-Version: 1.0 (Apple Message framework v552)
From: james woodyatt <jhw@wetware.com>
To: The Trade <caml-list@inria.fr>
Content-Transfer-Encoding: 7bit
In-Reply-To: <3F2087BF.5080903@1969.ws>
Message-Id: <98E41980-BE42-11D7-AF49-000393BA7EBA@wetware.com>
X-Mailer: Apple Mail (2.552)
X-Loop: caml-list@inria.fr
X-Spam: no; 0.00; woodyatt:01 jhw:01 wetware:01 caml-list:01 findlib-:01 3.07:01 covariant:01 expansive:01 forall:01 functors:01 val:01 functor:01 struct:01 3.06:01 3.06,:01 
Sender: owner-caml-list@pauillac.inria.fr
Precedence: bulk

On Thursday, Jul 24, 2003, at 18:28 US/Pacific, Karl Zilles wrote:
> james woodyatt wrote:
>> I'm specifically excited about the polymorphic generalization of 
>> covariant parts of expansive expressions.
>
> I read about this in the changes document, but I'm unable to wrap my 
> head around it.
>
> > For instance, if f: unit -> 'a list, "let x = f ()"
> > gives "x" the generalized type forall 'a. 'a list, instead of '_a 
> list
> > as before.
>
> Can someone simplify this for me?  Under what circumstances would this 
> be useful?

Sure.  I see it with functors.  Consider the following example:

	# module type X = sig
	    type +'a t
	    val f: unit -> 'a t
	  end;;

This is a module type I plan to use as the input to a functor.  The top 
responds with this following:

	module type X = sig type +'a t val f : unit -> 'a t end

So, imagine you want to write a functor like so:

	# module Y(X: X) = struct
	    let x = X.f ()
	  end;;

Here's what the 3.06 top-level says is the type of that module:

	module Y : functor (X : X) -> sig val x : '_a X.t end

Here's what the 3.07+beta1 version says:

	module Y : functor (X : X) -> sig val x : 'a X.t end

This is so much better.  In the 3.06 case, I get a module whose type 
isn't complete yet.  In the 3.07+beta1 case, I get a module whose type 
is complete.  With 3.06, I have to write 'x' as a function that returns 
a constructed object every time I call it.  With 3.07+beta1, I can 
write 'x' as a value that gets constructed once when the functor is 
called.


-- 
j h woodyatt <jhw@wetware.com>
that's my village calling... no doubt, they want their idiot back.

-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners