Re: [xquery-talk] tail recursive identity transformation

[John Snelson]

I don't think you can do a tail recursive tree walk of an XDM tree - you 
at least need a stack of the path taken to a particular node in order to 
process first it's children, and then it's following siblings. You could 
track the stack as an accumulating argument to the recursive call, but 
it may just be easier and more readable to use the function call stack.

Additionally when constructing a result tree, I think most XQuery 
implementations will need to generate an "end tag" after the recursive 
call - which will mean the recursive call won't be at the "tail" at all. 
It's probably possible to optimize this case specifically - but again it 
will entail the implementation keeping track of constructed elements 
that still need to be closed in some kind of stack.

John

On 15/09/17 11:55, Kendall Shaw wrote:
> I was trying to do something like:
>
>   def id(e: List[Int], acc: List[Int]): List[Int] =
> if (e.isEmpty)
>   acc.reverse
> else
>   id(e.tail, e.head :: acc)
>
> or
>
> 
>   
> 
>   
> 
>
> But, I didn't figure how to pass anything analogous to "head" of an 
> element, or analagous to .
>
> If you can give an example of a tail recursive identity transformation 
> in XQuery that would be helpful to me.
>
> Kendall
>
>
> 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  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
>>>
>>>
>>> ___
>>> talk@x-query.com
>>> http://x-query.com/mailman/listinfo/talk
>>
>> ___
>> talk@x-query.com
>> http://x-query.com/mailman/listinfo/talk
>>
>>
>
> ___
> talk@x-query.com
> http://x-query.com/mailman/listinfo/talk


-- 
John Snelson, Principal Engineer  http://twitter.com/jpcs
MarkLogic Corporation http://www.marklogic.com

___
talk@x-query.com
http://x-query.com/mailman/listinfo/talk

Re: [xquery-talk] tail recursive identity transformation

[Michael Kay]

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


I don't see how it can be possible. It's a task that naturally requires a 
stack. One could possibly do something very contrived that uses an 
application-level data structure in place of the stack, but it's not clear what 
benefits this would bring.

Michael Kay
Saxonica


___
talk@x-query.com
http://x-query.com/mailman/listinfo/talk

Re: [xquery-talk] tail recursive identity transformation

[Kendall Shaw]

I was trying to do something like:

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

or


  
    
  


But, I didn't figure how to pass anything analogous to "head" of an 
element, or analagous to .


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


Kendall


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  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


___
talk@x-query.com
http://x-query.com/mailman/listinfo/talk


___
talk@x-query.com
http://x-query.com/mailman/listinfo/talk




___
talk@x-query.com
http://x-query.com/mailman/listinfo/talk

Re: [xquery-talk] tail recursive identity transformation

[Michael Kay]

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  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
> 
> 
> ___
> talk@x-query.com
> http://x-query.com/mailman/listinfo/talk


___
talk@x-query.com
http://x-query.com/mailman/listinfo/talk