[xquery-talk] tail recursive identity transformation

Kendall Shaw kshaw at kendallshaw.com
Fri Sep 15 03:55:24 PDT 2017

I was trying to do something like:

   def id(e: List[Int], acc: List[Int]): List[Int] =
     if (e.isEmpty)
       id(e.tail, e.head :: acc)


<xsl:template match="@*|node()">
     <xsl:apply-templates select="@*|node()"/>

But, I didn't figure how to pass anything analogous to "head" of an 
element, or analagous to <xsl:copy/>.

If you can give an example of a tail recursive identity transformation 
in XQuery that would be helpful to me.


On 09/15/2017 02:23 AM, Michael Kay wrote:
> I'm very confused by this. The function body contains two recursive calls. The one inside the typeswitch is certainly not tail-recursive, so you're going to get recursion depth equal to the maximum tree depth. The other call, as far as I can see, merely returns its second argument unchanged, which seems totally pointless.
> Michael Kay
> Saxonica
>> On 15 Sep 2017, at 09:30, Kendall Shaw <kshaw at kendallshaw.com> wrote:
>> I don't remember seeing an xquery function that tries to return it's input unchanged.
>> Can this be improved:
>> declare function local:id($e as element()?, $acc as element()?) as element() {
>>    if (empty($e)) then
>>      $acc
>>   else
>>      local:id(()
>>              , element {node-name($e)} {
>>                $e/@*
>>                , for $node in $e/node()
>>                  return typeswitch($node)
>>                           case element() return local:id($node, $acc)
>>                           default return $node
>>              })
>> };
>> The idea is that functions that do more than return their input could be based on the function, and benefit from tail call optimization.
>> Kendall
>> _______________________________________________
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