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RE: [XPath Filter 2.0] Problem with Transform model

From: John Boyer <JBoyer@PureEdge.com>
Date: Wed, 29 May 2002 10:55:23 -0700
Message-ID: <7874BFCCD289A645B5CE3935769F0B52328785@tigger.PureEdge.com>
To: "merlin" <merlin@baltimore.ie>, <reagle@w3.org>
Cc: <w3c-ietf-xmldsig@w3.org>



Hi all, 

An empty node-set as input to an Xpath filter 1.0 yields no output.  For
Xpath filter 2.0 intersection and subtraction, this result continues to
make sense.  Only union can cause a problem if the input node-set is
empty since the node-set must have an implicit knowledge of the document
from which the node-set selects nothing.

As for union, perhaps it would be best for the filter to act exactly
like XPath would under the same circumstances.  The union filter is
supposed to perform a union of an empty set S with a node-set obtained
by evaluating a given Xpath expression E with an initial context node of
the root node R relative to the input node-set S.

A pure XPath expression that performs the same sequence of operations
is:

(S | (S)/ancestor::node()[last()]/E)

The ancestor::node()[last()] gets the root node as the starting context
for E, but the subexpression only works if set S is non-empty.  The way
XPath works is that once a location step results in an empty node-set,
there are no context nodes on which to perform the next location step so
the result of the subexpression S/ancestor::node()[last()]/E is an empty
node-set.  Since S is an empty node-set, the result of the final union
is an empty node-set, regardless of what E would have selected had we
been able to obtain the document root.

In conclusion, this means that if an empty node-set is given as input to
an Xpath 2.0 filter, returning an empty node-set would be behavior
consistent with the XPath 1.0 recommendation and with the XPath filter
1.0 in the XML DSig recommendation.  To throw an error would be
inconsistent.

Thanks,
John Boyer
Received on Wednesday, 29 May 2002 13:58:18 GMT

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