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   Re: [xml-dev] The Myth of Explicit Relationships [Was: 3 XML Design Prin

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On Sat, Feb 05, 2005 at 03:11:27PM -0500, Roger L. Costello wrote:
> In this message I would like to reverse my stand and argue that
> it is not possible for XML markup to state explicit relationships.

>    Picker is nested within Lot
> No other statements can be made regarding the relationship
> between the Lot and the Picker.


> Thus, it is erroneous to state this relationship: 
>    The Picker is located on the Lot

It may or may not be erroneous, but the markup iteslf does not
license that inference.

> I make these two assumptions for this discussion:
> 1. The processing application is an XML-aware application.
> 2. The processing application is completely ignorant of our vocabulary.

> 1. No matter how you design your XML it won't make any difference in
>    semantics.
>    Specifically, nested elements will always yield the "nested within"
>    relationship and nothing else.
There may also be other derivable relationships such as "is followed
by" or "has the same content as" of course.

> 2. There is no such thing as markup that enables you to specify an
>   "explicit relationship" between components.  Any relationship
>   semantics beyond "nested within" is entirely a product of the
>   application processing the markup.  
Let's take away the word "application" -- a human can also process
XML by (for example) printing it out and reading it, but we don't
usually think of humans as applications.

I think a clearer formulation might be this:
    Inferences beyond the lexical level can only be made using
    external information.

> 3. XML places the whole burden of semantics squarely upon the shoulders of 
>    processing applications. Consequently, the sender and receiver must
>    necessarily be tightly coupled (i.e., have shared semantics).

Yes and no, respectively.  Shared semantics are needed only if you want
to achieve communication -- which is, I admit, a common desideratum.

> 4. All message exchanges are nothing more than a series of encodings and
>    decodings.
Shannon would probably agree with you; Marshall Mcluhan might not have.

> 5. Is it conceivable that a code could be processed by many different
>    decoders?
Yes.  By asking the question you have conceived of such a thing, had it
not already been commonplace :-)

> 6. Are there such things are "self-decoding codes"?  (I guess that a virus
>    is an example.)
perhaps a meaningless code is an example?

> 7. Is it reasonable that a generic message format (i.e., XML) should
>    carry at least some of the burden of semantics?  Should XML 2.0
>    possess more semantics than is currently found in XML 1.0?

XML is about sharing labels for things.  It has not in the past been
about constraining the inferences one can make based on the labels
themselves.  Other specifications (e.g. RDF, OWL) work in that area.

I do think that more interaction between Semantic Web research and
XML Markup research is very much in order.  Each group can learn
things from the other.


Liam Quin, W3C XML Activity Lead, http://www.w3.org/People/Quin/


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