From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: weis
Received: (from weis@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id JAA28213 for caml-redistribution@pauillac.inria.fr; Tue, 25 Jan 2000 09:39:59 +0100 (MET)
Resent-Message-Id: <200001250839.JAA28213@pauillac.inria.fr>
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id SAA00802 for <caml-list@pauillac.inria.fr>; Mon, 24 Jan 2000 18:06:14 +0100 (MET)
Received: from uni-sb.de (uni-sb.de [134.96.252.33])
	by nez-perce.inria.fr (8.8.7/8.8.7) with ESMTP id SAA20864
	for <caml-list@inria.fr>; Mon, 24 Jan 2000 18:06:12 +0100 (MET)
Received: from cs.uni-sb.de (cs.uni-sb.de [134.96.252.31])
	by uni-sb.de (8.9.3/2000011400) with ESMTP id SAA23691
	for <caml-list@inria.fr>; Mon, 24 Jan 2000 18:06:11 +0100 (CET)
Received: from grizzly.ps.uni-sb.de (grizzly.ps.uni-sb.de [134.96.186.68])
	by cs.uni-sb.de (8.9.3/1999031900) with ESMTP id SAA10911
	for <caml-list@inria.fr>; Mon, 24 Jan 2000 18:06:11 +0100 (CET)
Received: from ps.uni-sb.de (IDENT:rossberg@grieg.ps.uni-sb.de [134.96.186.139])
	by grizzly.ps.uni-sb.de (8.9.1a/8.9.1) with ESMTP id SAA25096;
	Mon, 24 Jan 2000 18:06:10 +0100
Sender: rossberg@ps.uni-sb.de
Message-ID: <388C8682.6E3A8E4@ps.uni-sb.de>
Date: Mon, 24 Jan 2000 18:06:10 +0100
From: Andreas Rossberg <rossberg@ps.uni-sb.de>
Organization: =?iso-8859-1?Q?Universit=E4t?= des Saarlandes
X-Mailer: Mozilla 4.7 [en] (X11; I; Linux 2.2.12-20 i686)
X-Accept-Language: German, de, en
MIME-Version: 1.0
To: caml-list@inria.fr
Subject: Re: Compiler translation of array indexing
References: <200001192250.XAA25766@pauillac.inria.fr>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Resent-From: weis@pauillac.inria.fr
Resent-Date: Tue, 25 Jan 2000 09:39:59 +0100
Resent-To: caml-redistribution@pauillac.inria.fr

Pierre Weis wrote:
> 
> To summerize, I really would like to write
>  v.(1) instead of Array.get v 1
>  s.(1) instead of String.get s 1
> 
> I would consider as an additional benefit of the new type system, if I
> could use:
>  t.("ok") instead of Hashtbl.find t "ok",
>  l.("ok") instead of List.assoc "ok" l.

Recent work by Mark Jones tries to make this work in Haskell/Hugs by
extending multi parameter type classes with dependency annotations:

	http://www.cse.ogi.edu/~mpj/fds.html

Using dependency annotations, one can declare a class like (in some
mixed Haskell/Caml pseudo syntax):

        class Index 'c 'i 'e | 'c -> 'i 'e where
                .() : 'c -> 'i -> 'e

(note that all parameters have kind *) and instances

        instance Index ('a array) int 'a
        instance Index string int char
        instance Index (('a,'b) Hashtbl.t) 'a 'b
        instance Index (('a*'b) list) 'a 'b

These should provide the desired type-driven behaviour for indexing. The
dependency annotation ('c -> 'i 'e) in the class ensures that index and
element type are determined by the collection type. For example, the
compiler could infer the following:

	[|1;2;3|].(0)         : int
	"hello".(3)           : char
	[(2,39);(4,0)].(2)    : int
	fun i -> "hello".(i)  : int -> char

Note however, that overloading can quickly become ambiguous, even with
dependencies. While

	let f x = x.(0)

would be possible and have qualified type like

	(Index 'c int 'e) => 'c -> 'e,

the only slightly more involved

	let g x = x.(0).(0)

would not be allowed without additional type annotations since

	(Index 'c1 int 'c2, Index 'c2 int 'e) => 'c1 -> 'e

is an ambiguous type.

Best regards,

        - Andreas