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Re: [XQuery] 3.2 Path expressions returning non-nodes

From: Don Chamberlin <chamberl@almaden.ibm.com>
Date: Tue, 14 Sep 2004 11:19:56 -0700
To: swilkin@apple.com
Cc: public-qt-comments@w3.org
Message-ID: <OF3C4298A9.C739AF92-ON88256F0F.0063C8BB-88256F0F.0064A6B0@us.ibm.com>
Dear Sarah,
Thanks for your comment of Feb. 3, 2004 on this subject (attached below). 
After considerable discussion, the XML Query and XSLT working groups have 
decided to adopt your proposal. In the next edition of the XQuery and 
XPath 2.0 specifications, the definition of a path expression will be 
edited as follows:

"Each operation E1/E2 is evaluated as follows: Expression E1 is evaluated, 
and if the result is not a (possibly empty) sequence of nodes, a type 
error is raised. [err:XP0019] Each node resulting from the evaluation of 
E1 then serves in turn as an inner focus for an evaluation of E2, as 
described in [Dynamic Context]. The sequences resulting from all the 
evaluations of E2 are combined as follows:
(a) If every evaluation of E2 returns a (possibly empty) sequence of 
nodes, these sequences are combined, and duplicate nodes are eliminated 
based on node identity. <xpath-only>The resulting node sequence is 
returned in document order.</xpath-only><xquery-only>If ordering mode is 
"ordered", the resulting node sequence is returned in document order; 
otherwise it is returned in implementation-dependent order.</xquery-only>
(b) If every evaluation of E2 returns a (possibly empty) sequence of 
atomic values, these sequences are concatenated, in order, and returned.
(c) Otherwise (that is, if the multiple evaluations of E2 return at least 
one node and at least one atomic value), an error is raised.
NOTE: Since each step in a path provides context nodes for the following 
step, in effect, only the last step in a path is allowed to return a 
sequence of atomic values."

Please let us know whether you consider this to be an acceptable response 
to your comment.
Thanks and best regards,
--Don Chamberlin

----------------(referenced note)------------------

3.2 Path Expressions
"Each evaluation of E2 must result in a (possibly empty) sequences of 
nodes; otherwise a type error is raised. [err:XP0019]"

We feel XQuery is limited by its focus on nodes. The evaluation of E2 
should be able to contain nodes or atomic values.

The main purpose of this is to allow for a function at the end of a 
path. Generally this saves writing a loop. For example:

let $root := <b><a>   foo  bar</a><a>baz     faz</a></b>
return $root/a/normalize-space(.)

instead of
let $root := <b><a>   foo  bar</a><a>baz     faz</a></b>
let $seq := $root/a
let $result := for $item in $seq
                 return normalize-space($item)
return $result

In addition, without this functionality ugly workarounds are required 
to obtain the value of context functions. For example:
("a", "b", "c" )/text{ position() }

instead of the straightforward:
("a", "b", "c" )/position()

--Sarah
Received on Tuesday, 14 September 2004 18:20:33 UTC

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