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   RE: 'document types' was ...Re: [xml-dev] Re: Flexible Schemas

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I think that we now agree at 99% :).

I just think we should not put an emphasis on the DTDs, and try to
dissociate the concept of document type and the concept of DTDs. For me, a
DTD is a schema, expressed in a particular language. DTDs should not have a
special status compared to XML Schema, RELAX NG, Schematron, etc. So to me,
the PUBLIC identifier is more interesting than the SYSTEM identifier, which
is a URI that can be resolved into a DTD.

And yes, an OASIS catalog could be used as the meta-data resource directory
that we dream of, provided that we define standard URIs for a schema, a
stylesheet etc. This means that we would need a way to reproduce the
purpose/nature model of RDDL links, but in a single URI. Suppose that I need
a RELAX NG schema, I could obtain it by resolving a URI like
'metadata://schema//RELAXNG'. URNs would be better, because they do not need
to be directly resolvable. Can OASIS catalogs help resolving URNs ?

Best regards,

>-----Message d'origine-----
>De : Jonathan Borden [mailto:jborden@mediaone.net]
>Envoyé : lundi 21 janvier 2002 18:19
>À : Nicolas LEHUEN; xml-dev@lists.xml.org; Elliotte Rusty Harold
>Objet : 'document types' was ...Re: [xml-dev] Re: Flexible Schemas
>Nicolas Lehuen wrote:
>>... You summed it up perfectly : RDDL is for namespaces only. It
>> won't work for document types. So let's try to solve the problem of
>> types together.
>To the best I can tell an XML 'document type' would refer to 
>that which is
>defined by a 'document type definition' i.e. a DTD. A 
>'document type', then,
>is refered to by the <!DOCTYPE declaration, i.e. a PublicID/SystemID.
>Catalogs are a terrific way to associate document types with 'stuff'.
>This, of course, is mostly orthogonal to the namespace of the 
>root element
>of a document.
>It is not completely orthogonal because XML 1.0 defines an 
>'element type'
>simply as the name of an element (no hierarchy, inheritance etc.). The
>namespace name of the root, or document, element of a document 
>is of course
>_part_ of the element's QName. So one can correctly call the 'document
>_element_ type' the QName of the document element.
>What is the relationship between the namespace name of the 
>document element
>and its XML type? To the extent that an element is said to be 
>'contained' in
>its namespace, a namespace can be thought of as a container 
>for such types.
>To be perfectly clear, however, a namespace should not be 
>equated with a
>type. In my mind one of the most important reasons to propose 
>RDDL in the
>first place was the fear that namespaces would be incorrectly 
>equated with
>types. A DTD calls itself a type definition, and by extension 
>a schema can
>be thought of as a type definition (both are properly 
>collections of type
>definitions but nonetheless). If a namespace name were to 
>directly reference
>a schema definition, this improper equation might become a de facto
>standard, and this is exactly what we wanted to avoid.
>Regarding the 'problem of document types' either it is 
>completely specified
>by a DTD, or else we are extending 'document type' into the 
>schema realm
>(fair enough). Personally I think the issues related to schema 
>especially across schema languages is an interesting one. Some 
>time ago I
>sketched some thoughts regarding a simple 'schema algebra'
>http://www.openhealth.org/RDDL/SchemaAlgebra , ( this is not 
>part of the
>RDDL spec), precisely as an attempt to write down some principles in a
>logical fashion, and in a way that does not depend on the 
>intracasies of
>DTDs, XML Schema, RELAXNG etc. I would say, however, that the 
>'problem', if
>you define it as: how do we validate an XML document instance 
>whose type
>definitions can be _derived_ from their namespaces, but 
>without respect to a
>particular single schema. Firstly I don't know what the term 
>'valid' means
>in such situatuions (that would be up to the specifics of the schema
>language). So in general, the answer to many questions 
>regarding 'document
>type' fall outside the scope of RDDL. On the other hand, for 
>the reasons I
>have outlined above, at the very least it is important NOT to directly
>relate a namespace to a single schema, and this is what RDDL prevents.
>> I hope you have understood that I'm OK with RDDL for 
>namespace description
>> (apart a few critics). But it is only provided that RDDL is 
>not used for
>> what is not meant, i.e. a substitute for a resource 
>directory for document
>> types.
>That would be an OASIS Catalog. Designed to locate a Document Type
>Definition given a Public Identifier.


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