[xsl] Error recovery from casting in XSLT 2.0

Hi, there,

I'm new to XSLT 2.0 and have a question about recovering from casting 
errors, say for instance I'm trying to cast to a date, I'd do something 
like -
  <xsl:function name="f:castToDate" as="xs:date">
       <xsl:param name="date-string" as="xs:string"  />
                     
       <xsl:sequence select="if ($date-string castable as xs:date)
       then xs:date($date-string)
       else if ($date-string castable as xs:dateTime)
       then xs:date(substring-before($date-string,'T'))
       else()" />
   </xsl:function>   

What I'm wondering is how do I raise some kind of error in the final 
else() if the function gets passed something it can't cast? e.g.
  <xsl:function name="f:castToDate" as="xs:date">
       <xsl:param name="date-string" as="xs:string"  />
                     
       <xsl:sequence select="if ($date-string castable as xs:date)
       then xs:date($date-string)
       else if ($date-string castable as xs:dateTime)
       then xs:date(substring-before($date-string,'T'))
       else("NASTY ERROR CAN'T BE RETURNED BECAUSE I'M A DATE")" />
   </xsl:function>   

Or do I have to write another wrapper function? I'm only a few hundred 
pages into the first of Mr Kay's books so forgive me if this is a bit basic.
Cheers,

Fraser






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