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RE: creation order vs. document order

From: Peter Fankhauser <fankhaus@ipsi.fraunhofer.de>
Date: Wed, 24 Sep 2003 14:52:00 +0200
To: "'Jan Hidders'" <jan.hidders@ua.ac.be>, <www-ql@w3.org>
Cc: <lehti@ipsi.fhg.de>
Message-ID: <006001c3829a$a5afeb50$fd1c0c8d@pcspelt>

Here's my try about what the formal semantics (and IPSI-XQ
as one of the implementations of XQuery that are closely aligned
with the FS - at least for static reasoning).

Guided by the normalization rules in the FS,
IPSI-XQ rewrites/normalizes

(<x><y id="1"/><y id="2"/></x>)/y

as follows:

      http://www.w3.org/TR/query-semantics:distinct-doc-order(let
$fs:sequence := (element x{
        element y{
          attribute id{
            "1"          }
        }
, element y{
          attribute id{
            "2"          }
        }
      }
)
        return
          let $fs:last := count($fs:sequence)
            return
              for $fs:dot at $fs:position in $fs:sequence
                return
                  child::y)

The interesting part happens in the expression

  element y{ attribute id{"1"}},
  element y{ attribute id{"2"}}          }

This is a "core sequence" expression;
see: http://www.w3.org/TR/xquery-semantics/#sec_constructing_sequences

The dynamic semantics is given by:

"The dynamic semantics of the sequence expression follows. Each
expression
in the sequence is evaluated and the resulting values are concatenated
into one sequence."

dynEnv |- Expr1 => Value1     dynEnv |- Expr2 => Value2  
-------------------------------------------------------
dynEnv |- Expr1, Expr2 => Value1, Value2

applied to our sequence expression we thus have:

dynEnv |- Expr1 => element y{ attribute id{"1"}}
dynEnv |- Expr2 => element y{ attribute id{"2"}}  
------------------------------------------------------------------------
-------------
dynEnv |- Expr1, Expr2 => element y{ attribute id{"1"}},element y{
attribute id{"2"}}   
 
The actual order (not the document order) appears to be well-defined
(maybe too well;).

Next, we construct the element:

http://www.w3.org/TR/xquery-semantics/#doc-xquery-ElementConstructor

Element construction is a rather complex beast, because a lot of special
cases to
deal with mixed content, typed vs. untyped values etc. needs to be taken
into account,
and a lot of implict processing for namespaces, validation etc. needs to
take place.
In your example, IPSI-XQ simplifies a bit, because the
fs:item-sequence-to-node-sequence
is an identity transformation in this case. Anyway, the input order to
the constructed
x-element is not changed, and by becoming the content of an element, the
y-elements
are now in document order within x.

Hope this clarifies,

Peter

> -----Original Message-----
> From: www-ql-request@w3.org [mailto:www-ql-request@w3.org] On 
> Behalf Of Jan Hidders
> Sent: Mittwoch, 24. September 2003 13:04
> To: www-ql@w3.org
> Subject: Re: creation order vs. document order
> 
> 
> 
> Bas de Bakker wrote:
> >>But now I'm still left with the following question. Is the
> >>semantics of 
> >>the following expression well defined in the formal semantics?
> >>
> >>    (<x><y id="1"/><y id="2"/></x>)/y
> > 
> > I don't know about the formal semantics,
> 
> Any information is welcome, although I really would like to know what 
> the formal semantics exactly say. Why else do they exist?
> 
> > but will refer you again to
> > section 3.2 of the data model:
> > 
> > "The relative order of siblings is determined by their order in the 
> > XML representation. A node N1 occurs before a node N2 in document 
> > order if and only if the start of N1 occurs before the 
> start of N2 in 
> > the XML document."
> 
> Since the fragment is constructed there is no original 
> representation we 
> can refer to.
> 
> > And also:
> > 
> > "Informally, document order is the order returned by an in-order, 
> > depth-first, left-to-right traversal of the data model."
> 
> Shouldn't that be a *pre*order traversal? Also this cannot 
> tell us what 
> the relative order of the children is if we don't know that 
> order already.
> 
> -- 
>     Jan Hidders
> 
>   
> .-------------------------------------------------------------
> --------.
>   | Post-doctoral researcher               e-mail: 
> jan.hidders@ua.ac.be |
>   | Dept. Math. & Computer Science         tel: (+32) 3 218 
> 08 73       |
>   | University of Antwerp                  fax: (+32) 3 218 
> 07 77       |
>   | Middelheimlaan 1, BE-2020 Antwerpen, BELGIUM     room: G 
> 3.21       |
>   
> `-------------------------------------------------------------
> --------'
> 
> 
Received on Wednesday, 24 September 2003 08:58:51 UTC

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