Attr+: Integrating attributes and elements

Sat Aug 21 00:47:26 BST 1999

I would say the key advantage of attributes over elements is that attributes
can have defaults. I can define a set of elements to be optional and allowed
in any order by using (Attr1 | Attr2 ...)*. With schemas, I can type the
content of an element. Default values seem the only missing capability.

Marc B. McDonald
Principal Software Scientist
 <<...>> 
Design Intelligence, Inc.
1111 Third Avenue, Suite 1500
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marc.mcdonald at design-intelligence.com
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> ----------
> From: 	Paul Prescod[SMTP:paul at prescod.net]
> Sent: 	Friday, August 20, 1999 8:56 AM
> To: 	w3c-xml-schema-ig at w3.org; www-xml-schema-comments at w3.org; xml-dev
> Subject: 	Attr+: Integrating attributes and elements
> 
> This proposal goes one step beyond the one in "&-compromise". It goes a
> step farther towards making XML behave as a property/value language like
> most "OO" languages.
> 
> One of the hardest questions to answer about XML DTD design is:
> "element  or attribute." Attributes have certain virtues relating to
> lexical typing and convenience and elements have the primary virtue of
> being able to contain sub-structure.
> 
> XML schemas are set to reduce some of the benefits of attributes but not
> all. Attributes will continue to be more convenient because they can
> come in any order and because they are easier to type.
> 
> The problem is: if you choose to represent something as an attribute,
> for user convenience and "intuitiveness," you can never change your mind
> and allow it to have substructure later.
> 
> I propose a new concept called a "structured attribute" (attr+). An
> attr+ is a property of an element. In most respects it is just like an
> attribute. In fact, not much in the schema spec would change at all.
> 
> Am attr+ is different in that it can be syntactically fulfilled in a
> document by EITHER an XML 1.0 attribute OR a sub-element element of the
> same name. All such elements must precede the "real content" of an
> element. All characteristics must have names that are different from any
> element type name allowable in the element. A processor knows it has
> shifted from processing characteristics to processing "content elements"
> merely by looking at the generic identifiers ("tag names", for
> DOM-heads).
> 
> Let me be clear that there is no new syntax in the document instance. A
> characteristic "foo" of type "IDREF" on element "bar" can be expressed
> as:
> 
> <bar foo="abc">...</bar>
> 
> or
> 
> <bar><foo>abc</foo>...</bar>
> 
> Attr+es can have full content models. If a particular instance of a
> characteristic on a particular element happens to be all text (no
> sub-elements) then it can be expressed as an attribute instead of as an
> element. Over time, the word "attribute" would come to be synonymous
> with the term "characteristic in the minimized syntax."
> 
> The working group can decide whether to keep the concept of "classic
> attribute" alive in the schema -- perhaps for backwards compatibility. A
> classic attribute would be an Attr+ that is constrained to only the
> minimized syntax. You could have an attribute called "StringOnly" or
> something. This would be useful for backwards compatibility. Attr+es
> would give these attributes "a future" in that they could become
> structured later.
> 
> The information set contribution of an attr+ would be an information set
> item just like an Attribute information item except that its value would
> be an arbitrarily sub-strucured node list instead of just references and
> characters as it is now. An extra property would specify whether the
> attribute+ was expressed syntactially as an element or as "classic
> attribute." 
> 
> Practically speaking, this means that the XPath @foo could return a node
> with elements as its children. Thanks to the XPath concept of "value",
> this is 100% backwards compatible. Given
> <foo>abc<emph>def</emph>ghi</foo>, @foo would return "abcdefghi" in any
> context expecting a string and the structured content anywhere else.
> 
> It is debatable whether attr+es would show up in the information set as
> element content (or in the child:: axis of XPath). Having them NOT show
> up is probably more convenient because why would they be in two axes? On
> the other hand, this means that schema-using processors would see a very
> different information set than XML 1.0 processors. That may or may not
> be a "big deal." It depends on how much the information set is otherwise
> modified by XMLSchema (archetypes, datatypes etc.).
> 
> There are various ways of making the information set backwards
> compatible and yet making it easy to filter "true content" from
> "attribute content."
> 
>  Paul Prescod
> 
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