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SD2 - Structured Attributes

From: Jean Paoli <jeanpa@microsoft.com>
Date: Thu, 15 May 1997 21:14:16 -0700
Message-ID: <78DFE33066ABD0118B9200805FD431BA5EC14A@RED-16-MSG.dns.microsoft.com>
To: "'w3c-sgml-wg@w3.org'" <w3c-sgml-wg@w3.org>
SD2 - Structured Attributes:
----------------------------------------

XML uses containment to hold data between tags. This data can be
internally 
subdivided, for example the author can be marked to indicate internal
parts, as 
in <AUTHOR><LAST>Dantzig</LAST>, <FIRST>Tobias</FIRST></AUTHOR>. These
divisions 
don't change the contents of AUTHOR, them merely identify constituent
parts.

Contrast this with adding a digital signature, as in the following:
<AUTHOR 
XML-DSIG="ABCDE">Dantzig, Tobias</AUTHOR>. Here, the signature is an
attribute 
of the author element, that is, supplementary information, but not
actual 
contents: Adding a signature does not change who the author is. The
distinction 
between contents and attributes is important when tools (particularly
down-level 
versions of products) need to process data without fully understanding
its 
meaning: unknown attributes are completely ignored, while the contents
of 
unknown elements are retained. In both cases above, an uninformed tool
could 
retrieve the value of AUTHOR as "Dantzig, Tobias".

However, XML attributes inside the tag cannot have structure, nor can
they 
themselves have attributes, nor can they be easily extended.
Consequently, for 
example, a digital signature cannot have any structure (except by
private 
conventions and private syntax) nor can attributes be digitally signed.
The 
current attribute syntax is insufficient.

Proposal: I'm going to say something radical here: We should invent a
way to add 
structure and attributes to attributes. A special character sequence
"<*" in an 
opening tag (the exact characters used are to-be-determined) signals
that the 
contents are an attribute, not a subdivision.

<ORDERS>
	<LINEITEM XML-ID="L1" >
		<NAME>Number, the Language of Science</NAME>
		<AUTHOR><First>Dantzig</>, <Last>Tobias</>

<*XML-DSIG><DIGEST>5432109876</><SIGNER>AdamB@microsoft.com</></XML-DSIG
>
			</AUTHOR>
		<PRICE>5.95</PRICE>
		<AVAILABILITY>Out of Stock</AVAILABILITY>

<*XML-DSIG><DIGEST>1234567890</DIGEST><SIGNER>AndrewL@microsoft.com</></
XML-DSIG>
		</LINEITEM ></ORDERS>

In this example, the AUTHOR element has a digital signature attribute,
as well 
as subelements for First and Last parts of the name. Similarly, LINEITEM
has a 
digital signature. Finally, the digital signatures themselves have
subelements, 
one of which has a attribute, XML-TYPE.

An attribute that identifies an element should be expressed within the
element's 
tag. (That is, the XML-ID attribute.) All other attributes can be
expressed in 
either form, so there is a smooth upgrade path as structural needs show
up. The 
grove will reflect both syntactical representations the same way.


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Received on Friday, 16 May 1997 00:14:19 EDT

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