RE: [xml-dev] What are the characteristics of a good type system for XML?

["Dare Obasanjo" <dareo@microsoft.com>]

Defining a type system via the narrow lens of validation is partly the
cause of what many term the "brokenness" of W3C XML Schema and why some
of our devs and testers (of which I used to be one) have had some issues
with the XQuery type system. Watching the same mistake repeat itself on
XML-DEV even with the hindsight provided by W3C XML Schema is oddly like
watching a train wreck in slo mo. 

Pass the popcorn. 

-- 
PITHY WORDS OF WISDOM 
Any simple theory will be worded in the most complicated way.


This posting is provided "AS IS" with no warranties, and confers no
rights. 

>  
>  
> -----Original Message-----
> From: Thomas B. Passin [mailto:tpassin@comcast.net] 
> Sent: Tuesday, May 13, 2003 8:20 PM
> To: xml-dev@lists.xml.org
> 
> [Amelia A.Lewis
> 
> > It seems to me that the core XML 1.0 spec provides a definition of 
> > base-xml-string validation, in its well-formedness constraints for 
> > text and attribute nodes.  That is, base type validation is 
> equivalent 
> > to well-formedness for text and attribute nodes.
> >
> > My sense is that when the spec authors begin talking about "value 
> > space", then the discussion may already strayed out of 
> XML's yard and 
> > onto a busy street.  Is it important to be able to manipulate types?
> > Sure.  Is it something that XML can do?  No.  But as Bob 
> Foster points 
> > out elsewhere in this thread, it is something that 
> transformation and 
> > query languages can do.
> >
> 
> At first glance, this suggests that it is not __xml__ that 
> needs a type system, but the infoset (maybe), schema, 
> transformation, and query languages.  But there is a 
> confounding factor, and probably that is what has lead to so 
> much confusion and complexity.
> 
> It is not enough to say "let this thing be an amy:duration". 
> We also need to know "this thing in such and such a location, 
> but not in that other location, must be an amy:duration".  So 
> the value types easily get mixed in with the element types, 
> and bingo, the type system has become mixed up again with the 
> xml, which we hoped to avoid.
> 
> I think it will take some cleverness to see keep the type 
> system from getting mixed up with the structure, but I think 
> it will be worth it.
> 
> Maybe a step towards keeping the type system out of the xml 
> is to keep it out of the infoset too.  Then the types get 
> applied only after infoset creation, not during it.  We have 
> heard (I remember reading a post to this effect but I forget 
> who sent it) that this would be inefficient, but maybe it 
> would turn out to be not so bad after all.
> 
> In turn this notion implies the possibility of a validity 
> state of "well formed and correctly structured but with the 
> wrong simple types somewhere".
> That would be interesting because you could still picture 
> doing useful processing, and perhaps even casting the wrong 
> types to the ones you were interested in.
> 
> Cheers,
> 
> Tom P
> 
> 
> 
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