RE: XQuery

It's true that a helpful implementation might not barf on this query. But
that's not much help to the user, who has a requirement that no conformant
implementation will barf on it.

Michael Kay

> 
> As I understand it, this query (like almost all queries that 
> raise dynamic errors) is nondeterministic -- XQuery allows an 
> implementation to return the name of the single matching 
> person without error (for example, if the implementation has 
> indexed on age and thus compares only the one person).
> 
> As section 2.5.3 says, "the detection and reporting of 
> dynamic errors is implementation dependent.  When an 
> implementation is able to evaluate an expression without 
> evaluating some subexpression, the implementation is never 
> required to evaluate that subexpression solely to determine 
> whether it raises a dynamic error."
> 
> Because an implementation does not need to evaluate the where 
> clause for every node just to exclude the ones that cannot 
> match (and yet still produce results consistent with the 
> formal semantics), the type error need not be raised.
> 
> 
> Cheers,
> michael brundage
> 
> -----Original Message-----
> From: public-qt-comments-request@w3.org 
> [mailto:public-qt-comments-request@w3.org] On > Behalf Of Kay, 
> Michael
> Sent: Wednesday, October 15, 2003 8:02 AM
> To: Guido Moerkotte; Kay, Michael; public-qt-comments@w3.org
> Cc: mrys@microsoft.com; moer@pi3.informatik.uni-mannheim.de
> Subject: RE: XQuery
> 
> 
> > The remaining question is when is data correct or incorrect.
> Clearly there are different ways of defining this. One way is 
> that data is correct if it matches the state of the real 
> world (if the data says my date of birth is 13 Nov 1980, it 
> is incorrect). I was using the term "bad data" to mean "data 
> that violates the assumptions that my query/stylesheet makes 
> about what constitutes good data". In general this is a 
> stricter requirement than "valid according to a schema/DTD" 
> since there are many constraints that cannot be expressed in 
> a schema or DTD - not only cross-document constraints, but 
> also contextual constraints (a date must be in the future) 
> and constraints that are too complex to express in a given 
> schema language (e.g. if @x=1 then @y must be present).
> > If there is a schema (XSchema,DTD) against which it is to be
> > validated, then I would tend to define data as correct if and 
> > only if it validates correctly. If there is no schema, data 
> > is correct if it is well-formed. 
> > 
> > The above document is well-formed and (if you agree) correct data.
> > 
> > I don't see anything wrong with the following query:
> > 
> > for $p in document("p.xml")//person
> > where $p/@age = 3 
> > return $p/@name 
> > 
> > Why should it fail?
> I think you have found a bug. I don't think this query should 
> fail simply because the untyped attribute @age contains a 
> value that cannot be converted to a double. I think the 
> comparison should return false. My colleagues may disagree; 
> but certainly, if this fails, it is a nasty incompatibility 
> with XPath 1.0. Michael Kay 
>