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RE: Feature incompatiblity in XML Schema 1.0

From: Matthew Fuchs <matt@westbridgetech.com>
Date: Wed, 11 Dec 2002 14:17:05 -0800
To: "Henry S. Thompson" <ht@cogsci.ed.ac.uk>
Cc: "Don Box" <dbox@microsoft.com>, <www-xml-schema-comments@w3.org>, "Ashok Malhotra" <ashokma@microsoft.com>, "Martin Gudgin" <mgudgin@microsoft.com>, "Allen Brown" <allenbr@microsoft.com>
Message-ID: <MNELKPFAIFJKHNJLJDOIKELPCPAA.matt@westbridgetech.com>

I am confused.  The only thing that made me suspect it was OK was that you'd
claimed it was.  Don asked a question, and you responded by posting the
schema (the one below, minus my insertions).  I stated I thought it was
illegal and explained why.  Why do you think the original schema should have
been legal?

I've tried to express the upside of static type checking.  Can you please
try to express the downside, or at least why the benefits are irrelevant?


-----Original Message-----
From: ht@inf.ed.ac.uk [mailto:ht@inf.ed.ac.uk]On Behalf Of Henry S.
Sent: Wednesday, December 11, 2002 12:49 AM
To: Matthew Fuchs
Cc: Don Box; www-xml-schema-comments@w3.org; Ashok Malhotra; Martin
Gudgin; Allen Brown
Subject: Re: Feature incompatiblity in XML Schema 1.0

"Matthew Fuchs" <matt@westbridgetech.com> writes:

> I believe this is (or ought to be) illegal because the restriction may
> content not legal in the parent.

I think under both the current REC and the proposed move to
extensional checking, the restriction in the final schema doc. is
broken, because the substitution group heads are mandated to be
treated as implicit <choice>s over their group membership, so you end
up trying to 'restrict' <y> with (<y>|<y-prime>), which is not

What made you suspect it was OK?

> However, if it is legal, you win the grand
> prize - an example where static type checking, as I've defined it
> is broken.  Note my insertions below.

Doesn't obtain, since it isn't legal, but I'm still not happy about
your definition of static type checking, or any suggestion we should
enforce it.

> -----Original Message-----
> From: www-xml-schema-comments-request@w3.org
> [mailto:www-xml-schema-comments-request@w3.org]On Behalf Of Henry S.
> Thompson
> Sent: Thursday, December 05, 2002 1:52 AM
> To: Don Box
> Cc: www-xml-schema-comments@w3.org; Ashok Malhotra; Martin Gudgin; Allen
> Brown
> Subject: Re: Feature incompatiblity in XML Schema 1.0
> You analysis is correct, but I don't understand the implicit
> evaluative component of your message - why is this a problem that
> needs to be fixed?  By making a declaration both local and qualified,
> the author has essentially staked a permanent claim on the association
> between those qualifided names and their types in that context.  You can
> override this, but to do so you have to usurp the author's namespace
> explicitly, either using <redefine>, or as follows:
> a.xsd:
> <xs:schema targetNamespace="urn:a:aaa"
>   xmlns:xs="http://www.w3.org/2001/XMLSchema"
>   xmlns:a="urn:a:aaa" >
>   <xs:complexType name="Base" >
>     <xs:sequence>
>       <xs:element name="x" type="xs:int" form="qualified" />
>       <xs:element name="y" type="xs:int" minOccurs="0" form="qualified"/>
>     </xs:sequence>
>   </xs:complexType>
>   <xs:element name="BE" type="a:Base" />
> </xs:schema>
> aprime.xsd:
> <xs:schema targetNamespace="urn:a:aaa"
>   xmlns:xs="http://www.w3.org/2001/XMLSchema"
>   xmlns:a="urn:a:aaa" >
>  <xs:include schemaLocation="a.xsd"/>
>  <xs:element name="y" type="xs:int"/>
>  <xs:element name="x" type="xs:int"/>
> </xs:schema>
> <xs:schema targetNamespace="urn:b:bbb"
>   xmlns:a="urn:a:aaa"
>   xmlns:xs="http://www.w3.org/2001/XMLSchema">
>  <xs:import namespace="urn:a:aaa" schemaLocation="aprime.xsd"/>
>   <!-- MDF - note use of substitution groups -->
>   <xs:element name="y-prime" type="xs:int" substitutionGroup="a:y"/>
>   <xs:element name="x-prime" type="xs:int" substitutionGroup="a:x"/>
>   <!-- MDF -->
>   <xs:complexType name="Derived">
>     <xs:complexContent>
>       <xs:restriction base="a:Base" >
>         <xs:sequence>
>           <xs:element ref="a:x" />
>           <xs:element ref="a:y" minOccurs="1" />
>         </xs:sequence>
>       </xs:restriction>
>     </xs:complexContent>
>   </xs:complexType>
> </xs:schema>
> As long as the local element decls you want to appropriate don't have
> anonymous type definitions, such a move is always possible.
> MDF>  Because of the presence of substitution groups, the restriction
> allow
> <a:BE
> which clearly is not valid for <a:BE>.  Once again, very nasty non-local
> effects become possible - the element subtypes could be in a third schema
> completely.  It's also very nasty for implementation - since there could
> any number of layers, restrictions with qualified elements would need to
> matched against all the ancestor types, not just the most local one - I'll
> be no one does this.  Therefore, if this is not illegal, it should be made
> so.
> Matthew

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2002, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
 [mail really from me _always_ has this .sig -- mail without it is forged
Received on Wednesday, 11 December 2002 17:17:44 UTC

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