declare variable $a external;
let $default := "<b>I am default</b>"
let $vals := ( try { $a } catch * {()}, $default )[1]
return parse-xml($vals)

the try clause tries to assign $a to $vals, and if catched assigns the empty sequence to $vals.

The [1] predicate selects the first element of this sequence, as empty subsequences are flattened, the value for $default is returned if $a has been set to the empty sequence.

I chose parse-xml instead instead of util:eval; might be both a little faster and definitely more secure :-)

Hope this helps (and works ;-)).

Kind Regards

Michael

Am 13.03.2012 um 17:20 schrieb Jan Vlčinský (TamTam Research):

let $xml := util:eval($xmlString)