[xquery-talk] Conditional use of fn:error() ?

[Michael Kay]

For all these queries, implementations have the option whether to 
evaluate the base expression (error()) before or after evaluating the 
predicate. If the value of a predicate doesn't depend on the focus, 
rewriting A[B] as "if (B) then A else ()" is a good thing to do, so many 
processors will do it.

The only reliable way to generate an error conditionally is to use 
if/then/else (or typeswitch):

if (contains('http:...', 'details')) then error() else ...

There are specific rules for conditional expressions that prevent 
optimizers evaluating the then/else branch unless the condition is 
true/false.

Michael Kay
Saxonica

On 21/01/2013 11:07, Yoshi Okamoto wrote:
> Dear list subscribers,
>
> I noticed that different XQuery processors (BaseX,
> Saxon, XMLPrime, Zorba) return different results for the following queries:
>
> Query A:
>    fn:error()[false()]
>
>
> Query B:
>    error(())[contains('http://....', 'details')]
> Query C:
>    let $url := <url>...</url>
>    where contains($url, 'details')
>    return error(())
>
>
> All except for XMLPrime return an empty sequence.
>
>
> Query D:
>    let $url := <url>...</url>
>    return error(())[contains($url, 'details')]
>
> All except for Zorba return an error.
>
>
> My original intention was to raise an error whenever a certain condition is true. My questions would be:
>
> - what are the correct results for Query A-D?
>
> - XMLPrime is the implementation that always raises an error. If this behavior should be correct, how would I proceed to only raise errors under a certain condition?
>
>
> Thank you for your attention and patience.
>
> Best regards
> Yoshi OKAMOTO
>
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