Re: [Fwd: Re: [XPath] Consistency Constraints]

Martin Duerst wrote:

> If an implementation must ensure that the constraints are met, then
> I don't understand why the spec then goes on to say "Enforcement of
> these consistency constraints is beyond the scope of this specification."
> I think it should rather say something like "If the constraints are
> not met, then an implementation has to stop immediately and produce
> an appropriate error message.", or whatever the usual wording for
> draconic errors is in the XQuery spec.
> 
> I couldn't immagine the XML spec reading something like:
> "Well-formedness is a prerequisite for the correct functioning
> of XML. Enforcement of well-formedness is beyond the scope of
> the XML specification.", and if it's not appropriate for XML,
> then I can't see this being appropriate for XML Query.

I think there are at least two reasons for not doing that.

1. It may be extremely expensive to check some of these constraints. For 
instance, some of our constraints might require every single node to be 
examined, and most queries on the given data model instance may be 
perfectly well defined, because the annotation of one particular node is 
never needed by the query. When should the implementation stop and 
produce the error if it never happens to look at that type annotation?

2. Some implementations may have models that provide consistent behavior 
in a less constrained environment than the one we describe. These may be 
instructive for the future development of XQuery.

Some environments can assure a priori that they never encounter a data 
model instance that does not meet these constraints, but some can not.

>> Is that clear now? Can we close this issue, or is there an issue that 
>> I am not aware of?
> 
> I may be misunderstanding what you want to say with the above text,
> but in any case, I think the text needs to be fixed.

I don't like the fix you suggest. Do you have a different suggestion for 
fixing it?

Jonathan

Received on Monday, 16 August 2004 13:50:58 UTC