RE: [xml-dev] XQuery and DTD/Schema?

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

You miss my point which is understandable since most people don't have
to deal with the aggravations of XML Query on a daily basis. Here's an
example of where type inferencing breaks down. Given the following
schema 

<xs:schema  targetNamespace="urn:xmlns:example-com" 
         xmlns:xs="http://www.w3.org/2001/XMLSchema";
elementFormDefault="qualified">   
 <xs:element name="root">
  <xs:complexType>
   <xs:sequence>
    <xs:element name="child" maxOccurs="3" />
   </xs:sequence>
  </xs:complexType>
 </xs:element>
</xs:schema>

And the following XQuery expression 

  schema "urn:xmlns:example-com" at "http://www.example.com/schema.xsd"; 
  namespace ex = "urn:xmlns:example-com"

  define function get_children(element of type ex:root $r) 
     returns element of type ex:child{

	  $r/child
  }


  get_children(<ex:root>{
	
document("http://www.25hoursaday.com/root.xml";)//child
	          }</ex:root>)

How do you statically infer the type passed to the get_children()
function call? The correct answer is that you can't. So what is the best
workaround? Push it to a dynamic evaluation like so 

 
get_children(validate(<ex:root>{document("http://www.25hoursaday.com/roo
t.xml")//child}</ex:root>))

	
This example is relatively simple and could be made hairier by adding
substitution groups or derived types.

-- 
PITHY WORDS OF WISDOM 
Lynch's Law: When the going gets tough, everyone leaves.

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



> -----Original Message-----
> From: Rick Jelliffe [mailto:ricko@allette.com.au] 
> Sent: Wednesday, July 03, 2002 12:38 PM
> To: xml-dev@lists.xml.org
> Subject: Re: [xml-dev] XQuery and DTD/Schema?
> 
> 
> From: "Dare Obasanjo" <dareo@microsoft.com>
> 
> > Type inference can be ridiculously expensive and in some cases 
> > undecidable for complex types. Named typing and enforcing explicit 
> > casts by users via operations such as validate() are the 
> best option 
> > in creating something reasonably implementable and 
> understandable. Of 
> > course, these things are only issues because XQuery is 
> supposed to be 
> > "statically typed"
> 
> Is inferencing always that difficult? 
> 
> If we assume DTDs, then we have both a kind of Unique 
> Particle Attribute rule and no local elements or equivalents. 
> If the DTDs were 
> namespace aware and allowed declarations of the WXS builtin 
> datatypes, it seems pretty trivial to annotate a query with 
> the correct casts. 
> 
> If we have local elements, then the path in a query would 
> have to also be augmented with enough of a path to ensure 
> locality, and other definitions of the same element name 
> would have to be similarly augmented so they exclude the 
> local element.
> 
> Presumably XQuery expects the Unique Particle Attribute rule, 
> then presumably it just means that the intersection of 
> ambiguous elements in RELAX would have to be used. 
> 
> Cheers
> Rick Jelliffe
> 
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