W3C home > Mailing lists > Public > xmlschema-dev@w3.org > June 2001

Re: ASN.1 => XML Schema questions

From: <Noah_Mendelsohn@lotus.com>
Date: Tue, 26 Jun 2001 20:31:23 -0400
To: Dan Connolly <connolly@w3.org>
Cc: elgey@dstc.qut.edu.au, kohsukekawaguchi@yahoo.com, xmlschema-dev@w3.org
Message-ID: <OF2EE6D796.28963739-ON85256A78.000201A8@lotus.com>
Dan Connolly writes:

>> I think you can get at the PSVI via XSV

Yes, with the -r switch I believe.  It produces an extremely verbose dump, which is frankly mostly useful (and 
mostly intended) for debugging and testing.  It does demonstrate that 
maintaining and producing the PSVI is computationally tractable, and I 
expect that at least much of the PSVI will be showing up in widely 
deployed processors via APIs soon.  Certainly, anyone wishing to migrate 
from DTDs to schema will be interested in features such as attribute value 
defaults; as you point out, neither Relax nor Trex attempts to support 
such features of DTDs.  I suspect, but don't know for sure, that future 
versions of XSL and XPath may support templates such as "match on all 
integers", from which it would be possible when producing HTML from a 
stylesheet to "format all negative integers in red".  That would depend on 
the PSVI (in this case type information) being available.

As to the reason for limitations on <all>.  My recollection is that 
certain members of the workgroup claimed that experiences with SGML had 
shown that support for generalized <all>  would be complex and expensive. 
Perhaps this was due to the general wish to use DFA technology, perhaps 
for more subtle reasons.   The initial proposal was therefore to stick 
with the XML 1.0 decision and provide no such support for the so-called 
"AND connector".  Some of us pointed out that a limited form of <all>, 
allowing only elements, is a particularly good match to database columns, 
programming language structures, and other systems in which fields must be 
unique and named, but not ordered.  The limited version of <all> was 
introduced primarily as an 80/20 trade-off to meet an interesting fraction 
of this need.  Implementation of the limited <all> in schema is very 
simple.  Later, the argument was made that allowing minOccurs=0 (I.e. 
Optional elements in an <all> group) added only modest complexity and was 
a significant value.  It also has what I believe to be the appealing 
property that it is symmetric with the rules for attributes:  at most once 
each of the named elements, with some being required if you want.

So, that is my unofficial recollection of how the design came to be as it 
is.

------------------------------------------------------------------------
Noah Mendelsohn                                    Voice: 1-617-693-4036
Lotus Development Corp.                            Fax: 1-617-693-8676
One Rogers Street
Cambridge, MA 02142
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Received on Tuesday, 26 June 2001 20:36:41 GMT

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