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   Re: Property role and domain (was: Playing with XML Schema & XSLT)

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  • From: Rick JELLIFFE <ricko@geotempo.com>
  • To: xml-dev@xml.org
  • Date: Thu, 06 Jul 2000 19:56:52 +0800

Kay Michael wrote:
> > "About that, I regret that the first example in the
> > introduction to XML Schema has a tag <shippingDate> , and not
> <shipping><date>, > which is extendible, and allows machine understanding."
> For XML I struggled a little to identify best practice on this when writing
> Chapter 4 of Wrox Professional XML: the discussion of alternative approaches
> is on page 129-130. I came to the conclusion that <Billing><Address> was
> probably preferable in theory to <Billing.Address> or <Address
> role="Billing"> or <Billing type="Address">, but rather clumsy in practice;
> and in the end I dodged making a firm recommendation.
> Any other views on this? 

XML presents a serialized view of data. Every view embodies a theory of
type or form (ontology) traded-off against pragmatic concerns about data
processing and influenced by personal aesthetic considerations. So there
is no general answer, and there should not be one--XML should be general
enough to support many different methodologies.

Personally, I would prefer <Address role="Billing">. The question is
whether one expects the type or the role to push processing (or be the
primary way data is pulled in).  If you are lucky enough to have clear
usage patterns in mind, then that will influence which to use.  

> Does it become easier with XML Schema replacing DTDs?
I think XSL Schema can handle all these cases. (This should be
distinguished from the case Henry and I were discussing, where the
attribute is used to subtype.)

However, the big architectural issue is whether you are creating your
data to require an XSL Schema processor or not: for example, if you use
<Billing.Address> but not a DTD (using architectural forms) or XML
Schema how will your recipients know that it is an Address related to

At the initial stages, a developer has to consider whether the
recipients of the data have an XSL Schema processor available. If the
developer cannot be sure, then it will only be prudent to use create
schemas so that needed information about type or domain is explicitly
markedup in the document (or carried around by DTD). This is off course
what one would expect with deploying any technology. (I would expect XML
Schemas to be used initially for validation and creation of tools:
initially one should assume that general users will not have information
in the Post Schema-Validation (PSV) infoset available when processing
the document.  This consideration will, of course, change over time.) 

Rick Jelliffe

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