The XQuery formal semantics infers types for the result of a path expression
based on schema knowledge; if the inferred type is "empty" then the path
expression can't select anything. However, the inference rules aren't
perfect, in the sense that there are cases where a more precise type could
be inferred. It's a while since I looked at them in detail, but IIRC they
only deal with downwards axes.

Michael Kay
http://www.saxonica.com/ 

> -----Original Message-----
> From: Stefan Tilkov [mailto:stefan.tilkov@i...] 
> Sent: 01 June 2005 16:13
> To: xml-dev@l...
> Subject: [xml-dev] XPath and XSD/RNG
> 
> Is there a way to check whether an XPath expression could 
> possibly be  
> true for a document conforming to a particular W3C XML Schema (or  
> Relax NG schema) without having access to the document? Is this  
> something that toolkits support?
> 
> E.g. if the schema disallows /root/element/subelement, the XPath  
> expression /root/element/subelement[0] cannot yield a 
> non-null result  
> for any conforming document.
> 
> Thanks,
> Stefan
> 
> --
> Stefan Tilkov, stefan.tilkov@i..., http://www.innoq.com
> innoQ Deutschland GmbH, Halskestr. 17, D-40880 Ratingen, Germany
> Phone: +49 170 471 2624  Fax: +49 2102 77160-1
> ICQ: 177869128, AIM: stefantilkov, Weblog: 
> http://www.innoq.com/blog/st/
> 
> 
> 
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