Why namespaces?

Oren Ben-Kiki oren at capella.co.il
Wed Sep 1 17:04:12 BST 1999

David Megginson <david at megginson.com> wrote:
>...  The problem is that a DTD does (and XML Schemas will
> do) two entirely different things:
> a) supply default values and types; and
> b) specify a set of validation rules.
> (a) is an essential layer in the interpretation of a document
> (assuming that the document author takes advantage of it); (b) is
> simply one of many processes that can be applied to an XML document,
> and calling it a layer is rather confusing.

Is there's no practical way to specify them separately? For example, suppose
that it was possible to use some form of PIs embedded in the document to
specify (a), while keeping (b) in a separate DTD document.

> As far as I know, the three HTML 4.0 variants differ only in (b), not
> in (a), so the differences are not really an essential part of
> interpretation -- they affect only one specific type of process,
> structural validation.

<Stunned>?!%!%$#</Stunned> If that's true, then why on earth would one even
consider using three separate namespaces, instead of just three alternate
DTDs? I thought the problem was that all sort of defaults were different in
each variant (even if these defaults boil down to "which stylesheet to
use"). Otherwise, what's the point?

> This is something that SGML got wrong and XML got right -- SGML
> assumed that there was always a *single* DTD that applied to any
> existing document (though that never worked in practice, so we always
> had to invent kludges for non-trivial systems), so that a document
> instance could not exist independently of its schema and vice-versa
> (external DTD subsets are not independent objects in SGML, but simply
> part of the document that includes them).
> By allowing documents without explicit DOCTYPE declarations, XML (and,
> eventually, WebSGML) acknowledged that document instances can exist
> independently of schemas, and thus, that there can potentially be
> *many* schemas applied to any existing document.

Doesn't this contradict (a)? That is, must all these schemas agree on the
default values? Or is it intentional that you can replace the default values
as well?

Share & Enjoy,

    Oren Ben-Kiki

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