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

Yoshi Okamoto yoshiokamoto at yahoo.com
Mon Jan 21 03:45:56 PST 2013

Dear Michael Kay,

this was a quick and excellent reply, which answered all my open questions.

Best regards


----- Original Message -----
From: Michael Kay <mike at saxonica.com>
To: talk at x-query.com
Sent: Monday, January 21, 2013 12:24 PM
Subject: Re: [xquery-talk] Conditional use of fn:error() ?

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 

Michael Kay

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
> _______________________________________________
> talk at x-query.com
> http://x-query.com/mailman/listinfo/talk

talk at x-query.com

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