Re: XQuery/XPath 3.1: Node List to Node Set ("distinct

On Wed, Dec 29, 2021 at 10:27 AM Martin Honnen martin.honnen@xxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

>  I don't have to know one, I just pointed out that the spec doesn't
>   guarantee the order. Thus I don't see why, given the spec, one should
>   expect any implementation to preserve the order.

I am happy to state that the provided solution produces the correct result
on all currently-existing XPath engines :)

If one is about to be starved to death and there is a banana hanging inches
from his mouth, but the documentation says that there is no guarantee that
there is food nearby, what would be the best thing to do:

 - don't pick the banana and die, as per specification

 - acknowledge the reality and **live** in this reality

Cheers,
Dimitre

P.S.
If we were more pedantic, we could sort the results of distinct-values()
before using them.



On 29.12.2021 17:36, Dimitre Novatchev dnovatchev@xxxxxxxxx wrote:
> >
> >
> > On Wed, Dec 29, 2021 at 12:21 AM Martin Honnen martin.honnen@xxxxxx
> > <mailto:martin.honnen@xxxxxx> <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx
> > <mailto:xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>> wrote:
> >
> >
> >     Am 29.12.2021 um 00:32 schrieb Dimitre Novatchev
> >     dnovatchev@xxxxxxxxx <mailto:dnovatchev@xxxxxxxxx>:
> >>
> >>
> >>
> >>
> >>     Hit Send too early:
> >>
> >>     Do notice: this seems the only solution of all presented so far,
> >>     that preserves the original sequence order (not document order) of
> >>     the nodes.
> >
> >     Why is the original sequence order preserved?
> >     https://www.w3.org/TR/xpath-functions/#func-distinct-values
> >     <https://www.w3.org/TR/xpath-functions/#func-distinct-values>
> >     clearly says
> >
> >     "The function returns the sequence that results from removing
> >     from|$arg|all but one of a set of values that are considered equal
> >     to one another. [...]
> >
> >     The order in which the sequence of values is returned
> >     isB7implementation-dependentB7
> >     <https://www.w3.org/TR/xpath-functions/#implementation-dependent>.
> >
> >     Which value of a set of values that compare equal is returned
> >     isB7implementation-dependentB7
> >     <https://www.w3.org/TR/xpath-functions/#implementation-dependent>."
> >
> >
> >     So while
> >
> >         $nodes ! generate-id(.)
> >
> >     gives you the generated ids in the order of the nodes in $nodes
> >     after the call to distinct-values there is no order defined, it is
> >     implementation dependent.
> >
> >
> > @Martin Honnen <mailto:Martin.Honnen@xxxxxx> Could you, please, give us
> > an example of an existing XPath engine whose implementation of
> > `distinct-values()` produces its results in any other order than their
> > original order in the input sequence?
>
> I don't have to know one, I just pointed out that the spec doesn't
> guarantee the order. Thus I don't see why, given the spec, one should
> expect any implementation to preserve the order.
>
> Imagine you implement distinct-values in .NET with e.g.
>
>
https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.distinct?v
iew=net-6.0
> and it would probably pass all tests but also only give a "result
> sequence" that " is unordered".
>
> Aren't there also implementations of XQuery or XPath that exploit
> parallel processing? I could imagine such an implementation to easily
> not care about ordering if the spec allows it for distinct-values.
>
> It seems, on the other hand, eXide of eXist-db in the online version
> doesn't even grok some of the generate-id based attempts:
>
> let $nodes := (1 to 10) ! parse-xml-fragment('<node>' || . ||
> '</node>')/node(),
>      $nodes := (1 to 5) ! $nodes,
>      $ids := distinct-values($nodes ! generate-id(.))
> return  $ids ! (function($id) {$nodes[generate-id(.) eq $id][1]})(.)
>
> gives <node>1</node>
>
>
>
> Any eXist-db users reading here? Is there a known issue with generate-id?
>
>
>

--
Cheers,
Dimitre Novatchev
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