RE: [xml-dev] XPath 2.0 Best Practice Issue: Graceful Degradation

["Michael Kay" <mike@saxonica.com>]

> 
> The if-statement is testing to see if the airplane element 
> conforms to an XML Schema declaration for the airplane 
> element.  Suppose the test fails.  Is failure to be interpreted as:
> 
> (a) The airplane element did not conform to the airplane 
> element declaration in the XML Schema.
> 
> (b) There was no XML Schema against which to validate the 
> airplane element.
> 
> (c) Both (a) and (b).
> 

Your (c) doesn't make sense. If there's no schema declaration then it's
meaningless to ask whether the instance conforms to it.

However, there is a third option: (c) There is an XML Schema declaration of
the airplane element, but no attempt has been made to validate the instance
against that declaration.

You can't distinguish these three cases in XPath, rather your application is
expected to be in control of the validation. You should deal with validation
failures and analyze their cause at the time you do the validation, the
subsequent "if-statement" (or template match rule) is needed only as a check
that validation has been successfully performed.