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 JAA18878; Tue, 30 Jul 2002 09:27:55 +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 JAA18875 for <caml-list@pauillac.inria.fr>; Tue, 30 Jul 2002 09:27:54 +0200 (MET DST)
Received: from laurelin.dementia.org ([208.167.88.73])
	by concorde.inria.fr (8.11.1/8.11.1) with ESMTP id g6U7Rhj20351;
	Tue, 30 Jul 2002 09:27:43 +0200 (MET DST)
Received: by laurelin.dementia.org (Postfix, from userid 1001)
	id BEB3670CF; Tue, 30 Jul 2002 03:34:56 -0400 (EDT)
To: "Scott J." <jscott@planetinternet.be>
Cc: "Xavier Leroy" <xavier.leroy@inria.fr>, "Caml-list" <caml-list@inria.fr>
Subject: Re: [Caml-list] OCaml 3.05 released
References: <20020729144527.A30919@pauillac.inria.fr>
	<000f01c2377c$36263060$2097fea9@janxp>
From: John Prevost <j.prevost@cs.cmu.edu>
Date: 30 Jul 2002 03:34:56 -0400 (31.593 UMT)
In-Reply-To: <000f01c2377c$36263060$2097fea9@janxp>
Message-ID: <861y9lmzxb.fsf@laurelin.dementia.org>
User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Sender: owner-caml-list@pauillac.inria.fr
Precedence: bulk

>>>>> "sj" == Scott J <jscott@planetinternet.be> writes:

    sj> What is meant by first first-class polymorphism?

First class polymorphism in this case lets you define values with
polymorphic types at levels other than the top level, like so:

type foo = { test : 'a 'b . ('a -> 'b) -> 'a list -> 'b list }
let a = { test = List.map }

# a.test;;
- : ('a -> 'b) -> 'a list -> 'b list = <fun>
# a.test ((+) 1) [3; 5; 7];;
- : int list = [4; 6; 8]
# a.test ((^) "a") ["a"; "b"; "c"];;
- : string list = ["aa"; "ab"; "ac"]

For objects, this allows you to define a list type, complete with map
method, as an object.  For example:

class ['a] clist x =
  object
    method map : 'b . ('a -> 'b) -> 'b list = fun f -> List.map f x
  end
let a = new clist [1;2;3]
let b = new clist ["a";"b";"c"]

# a #map (fun x -> x + 1);;
- : int list = [2; 3; 4]
# b #map (fun x -> x ^ "-");;
- : string list = ["a-"; "b-"; "c-"]

(Note: this is a very uninteresting definition if a list class, but it
should show you the kind of thing you can now do.)

John.
-------------------
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