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Need Union and RE: Should I organize a call on the XPath Filter?

From: John Boyer <JBoyer@PureEdge.com>
Date: Wed, 20 Mar 2002 10:35:20 -0800
Message-ID: <7874BFCCD289A645B5CE3935769F0B52328496@tigger.PureEdge.com>
To: "merlin" <merlin@baltimore.ie>, <reagle@w3.org>
Cc: "Christian Geuer-Pollmann" <geuer-pollmann@nue.et-inf.uni-siegen.de>, "TAMURA Kent" <kent@trl.ibm.co.jp>, <w3c-ietf-xmldsig@w3.org>

Hi Joseph and Merlin,

though there is still some confusion: Christian asking about what
exactly Merlin is proposing [1], and my confusion about multiple
transforms operating on the *original* document, not their preceding
transforms output [2]. So before we have a call or resort to a poll I'd
recommend we gather our thoughts and first reflect these changed in the
document. Then we can further discuss as needed.

To answer 2, in case it's not clear from my last message (which
addresses 1), the XPath expression is evaluated in the context of the
input document and then the result is applied to the input node set.

Yes, and I would further assert that the set operation transforms *are*
operating over the output result of the preceding transform in a natural
way, esp. given that we want to avoid running lots of XPath expressions.
An Xpath evaluation context takes one node.  If we want to run the Xpath
over each node of the input, then use Xpath filter 1.0.  Otherwise, we
run one Xpath over the document from which the input node-set is drawn,
then we perform a set operation between the Xpath result and the input

Also, I think the fact that Xpath has a union operator has tricked us
into believing that we don't need union as a third set operation.  I
think it's easy to implement and useful, e.g. Christian's examples can
be solved if we add union as a third operation.  I also think we should
do it for the sake of completeness.

I think, as an aside, that a sequence of XPath, XPath filter and
signature transforms can thus be reordered without altering the
node set.

This is a neat property, but I don't think it a necessity, esp. not
compared to having a union operation, which would eliminate the

John Boyer
Received on Wednesday, 20 March 2002 13:35:53 UTC

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