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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@a...]
> Sent: Wednesday, July 03, 2002 12:38 PM
> To: xml-dev@l...
> Subject: Re: XQuery and DTD/Schema?
>
>
> From: "Dare Obasanjo" <dareo@m...>
>
> > 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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