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Nice write up. For those who can't or won't read it the capsule summary is that it points out exactly how and why the RELAX NG type system is incompatible with that of W3C XML Schema. Or in XQuery parlance structural vs. named typing.
PS: Murali, you spelled my last name wrong at the end, it's "Obasanjo" not "Obsanjo" :)
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> -----Original Message-----
> From: Murali Mani [mailto:mani@CS.UCLA.EDU]
> Sent: Monday, July 08, 2002 10:24 AM
> To: Derek Denny-Brown
> Cc: Michael Fitzgerald; xml-dev@lists.xml.org
> Subject: Re: [xml-dev] RE: RELAX NG and type derivation
> [Was:] XQuery and DTD/Schema?
>
>
>
> Hi, I checked this message. I think my write-up is kind of
> preliminary, though itdiscusses exactly what Michael said. I
> think is is important to understand two different ways of
> doing subtyping --explicit subtyping and implicit subtyping.
> The issues in implicit subtyping are not fully solved. I will
> try to upgrade my report at my earliest, but I will encourage
> you to read the preliminary version, and please give any
> comments you may have.
>
> you may please find the report at
> http://www.cs.ucla.edu/~mani/xmlresearch/datamodels/subtyping.doc
> (or) http://www.cs.ucla.edu/~mani/xmlresearch/datamodels/subtyping.pdf
>
> I conjecture this will be an important topic to be discussed
> and probably solved in the next few months.
>
> thanks and regards - murali.
>
> On Mon, 8 Jul 2002, Derek Denny-Brown wrote:
>
> > -----Original Message-----
> > >From: Michael Fitzgerald [mailto:mike@wyeast.net]
> > >
> > > RELAX NG can do type derivation by restriction, union,
> and list. In
> > > some ways, RELAX NG's list type derivation is more
> powerful that XML
> > > Schema's derivation.[1] All the facets that are available in XML
> > > Schema are available in implementations of RELAX NG. In what
> > > specific ways does RELAX NG fall short in type derivation?
> >
> > > As for named typing/subtyping in XML Schema, I think
> Murali Mani's
> > > analysis is worthy of consideration.[2] There he
> discusses implicit
> > > and explicit assignment, subsumption, and even (briefly) an
> > > instanceof method a la XQuery. RELAX NG does not offer
> named typing
> > > yet, but a proposal is on the table to provide it in
> 2.0.[3] As such
> > > it will most probably be presented in a module rather
> than added to
> > > the core specification, in keeping with RELAX NG's
> layered approach.
> > > I predict it will be a well-reasoned spec that won't later invoke
> > > pangs of regret and won't be unduly difficult to implement or use.
> >
> > I was not talking about simple-type (as WXS uses the term)
> > derivation. I was talking about complex type derivation. The
> > ability to define a type 'book' <xs:complexType name="book">
> > <xs:sequence> <xs:element name="title"
> type="xs:string"/>
> > <xs:element name="author" type="properName"/> <xs:any
> > minOccurs="0" maxOccurs="unbounded" namespace="##any"/>
> > <xs:element ref="chapter" maxOccurs="unbounded"/> <xs:any
> > minOccurs="0" maxOccurs="unbounded" namespace="##any"/>
> > </xs:sequence> </xs:complexType>
> >
> > And then differentiate the sub-types via subtyping.
> > <xs:element name="referenceBook">
> > <xs:complexType>
> > <xs:complexContent mixed="0">
> > <xs:restriction base="book">
> > <xs:sequence>
> > <xs:element name="title" type="xs:string"/>
> > <xs:element name="author" type="properName"/>
> > <xs:element ref="chapter" maxOccurs="unbounded"/>
> > <xs:element name="references"
> type="sourceReference" minOccurs="0" maxOccurs="unbounded"/>
> > </xs:sequence>
> > </xs:restriction>
> > </xs:complexContent>
> > </xs:complexType>
> > </xs:complexType>
> >
> > An application can now unambiguously query whether a given
> element is of type 'book'. With RELAX-NG, the closest you
> can come is to ask if an instance matches 'book', but that in
> now way unambiguously determines whether it was intended to
> be considered a type of book or not.
> >
> > This ability to extend the type model of one schema is one
> of the core principles behind OOP, and as such is something
> that people want in a schema language.
> >
> > Also note, that I am not even remotely defending WXS as a
> complete, well-designed, nor elegant spec. It just is, and
> it achieves certain things which people wanted. I am simply
> arguing that it is not an utter failure, and that there
> exists nothing which could fully replace it currently.
> There is no doubt in my mind that RELAX-NG is a more elegant
> creation, nor that it could be extended to fill the same role
> as WXS. The problem is that WXS is already out there, and a
> large number of vendors are already committed to it. If
> that is not an issue for you, use RELAX-NG. I would.
> >
> > -derek
> >
> >
>
>
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