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 NAA01035 for caml-redistribution@pauillac.inria.fr; Fri, 25 Feb 2000 13:57:35 +0100 (MET)
Resent-Message-Id: <200002251257.NAA01035@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 VAA09368 for <caml-list@pauillac.inria.fr>; Thu, 24 Feb 2000 21:51:20 +0100 (MET)
Received: from mail5.microsoft.com (mail5.microsoft.com [131.107.3.121])
	by nez-perce.inria.fr (8.8.7/8.8.7) with SMTP id VAA03300
	for <caml-list@inria.fr>; Thu, 24 Feb 2000 21:51:18 +0100 (MET)
Received: from 157.54.9.108 by mail5.microsoft.com (InterScan E-Mail VirusWall NT); Thu, 24 Feb 2000 12:51:16 -0800 (Pacific Standard Time)
Received: by INET-IMC-05 with Internet Mail Service (5.5.2651.58)
	id <FSWDB2W0>; Thu, 24 Feb 2000 12:51:15 -0800
Message-ID: <783D93998201D311B0CF00805FEAA07B7E9115@RED-MSG-42>
From: Manuel Fahndrich <maf@microsoft.com>
To: "'caml-list@inria.fr'" <caml-list@inria.fr>
Subject: # types and polymorphic variants
Date: Thu, 24 Feb 2000 12:51:11 -0800
X-Mailer: Internet Mail Service (5.5.2651.58)
Resent-From: weis@pauillac.inria.fr
Resent-Date: Fri, 25 Feb 2000 13:57:35 +0100
Resent-To: caml-redistribution@pauillac.inria.fr


What is the meaning of #foo, when foo is a polymorphic variant set?

type foo = [`A | `B | `C]

I assumed that it was the following:

	#foo = [<`A | `B | `C ]

that is a set containing at most `A | `B | `C. In other words a subtype of
foo.

But consider the following example.

type foo = [`A | `B | `C]

let foo1 (x : #foo as 'a) = x

let y1 = foo1 (`A : [`A])


let foo2 (x : [< `A | `B | `C] as 'a) = x

let y2 = foo2 (`A : [`A])


The complier tells me:

ocamlc -c -i ex1.ml

type foo = [`A|`B|`C]
val foo1 : (#foo as 'a) -> 'a
val y1 : foo
val foo2 : ([<`A|`B|`C] as 'a) -> 'a
val y2 : [`A]

The first function returns me only a foo, the second function returns me
[`A] as expected. Thus my question, what exactly does #foo stand for. I do
understand row variables, but the issue with the lower and upper bounds > <
in variant types seems new. Is there a paper describing these?

I seem to write types of the form [< `A | `B | `C] a lot. Shouldn't there be
an abbreviation for those as well? Maybe #<foo?


Thanks in advance for such a wonderfully expressive type system!

	Manuel