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RE: [xmlChunk-44] Chunk of XML - Canonicalization and equality

From: Elliotte Rusty Harold <elharo@metalab.unc.edu>
Date: Tue, 3 Feb 2004 12:22:20 -0500
Message-Id: <p06010205bc458788f1af@[]>
To: "Williams, Stuart" <skw@hp.com>
Cc: "'www-tag@w3.org'" <www-tag@w3.org>

At 3:21 PM +0000 2/3/04, Williams, Stuart wrote:
>There seems to be some question as to whether xml:lang (and maybe xml:base)
>survive the canonicalisation process. See [1] and thread.

I don't think there are any questions about this. If I recall 
correctly, they don't unless they're a part of the canonicalized 
chunk in which case they do.

Perhaps what's being asked is really whether canonicalization 
invented the right semantics for equality. Personally, I suspect no 
one notion of deep equality will satisfy everyone. For instance, in 
my XOM work I do test base URIs for equality (which XML 
canonicalization does not) but consider two bases to be equal if one 
might be a relative form of the other. That's necessary for my unit 
tests to work, but it may well be not what everyone needs all the 
time. Similarly I compare document type declarations when comparing 
documents, which canonicalization doesn't do.

I don't think the xml:lang and xml:base cases are particularly 
special. As long as you have less than a complete document being 
singed, there could always be ancestor attributes that have meaning 
in a particular local context and which are not signed. For instance, 
imagine a process which uses verified, approved, or confirmed 
attributes to describe the content of an element. If the elements 
descendants are signed without their ancestor it would be easy to 
change any of these from false to true or vice versa. I'm sure you 
can conceive of many similar cases.

I don't think the predefined semantics of xml:base and xml:lang are 
so special that they are justified in being treated in a different 
way than any other attribute.

   Elliotte Rusty Harold
   Effective XML (Addison-Wesley, 2003)
Received on Tuesday, 3 February 2004 12:29:16 UTC

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