I find it helpful and inspiring to remember a milestone of intellectual history, which is the expression, in the sense of "a syntactical shape which has a value". Here is the shape, there is the value, one may change while the other remains.
From our elemantary school days, we worked with expressions, like "1 + 1" which can be resolved to the value "2".
Studying the XDM, we learn that 1 + 1 is an expression resolving to a value consisting of a single atomic item of type "xs:integer".
Both perspectives make sense. The example suggests a basic rule stating that a data model is not owned by the syntax, but applied to a syntax. Given an instance of syntax (data), dependent on context the application of different models is appropriate.
So it makes much sense to apply the XDM to a record of CSV data, parsing them into a tree representation (with a root element representing the data set, child elements representing rows, and grandchild elements representing cells). Why should one do that? Because it enables us to apply powerful operations to the information contained by the data. For example, the XPath expression
distinct-values(/*/*/count(*))
gives us the distinct values of how many items the csv rows contain. It is very important to see things in context: to see the relationship between the application of a data model and the operations (if any) we want to apply to the information contained in the data.
I prefer the view that the data which we can see on paper or screen are an *expression*, the value of which is given by the data model which we choose to apply. The CSV notation, an XML notation, a JSON notation can be regarded as different expressions with a common value.
If somebody does not see a practical use of such a view, he need not bother with it. It all depends on the operations one is up to apply and the available tools one is aware of.
Hans-Jürgen
"Costello, Roger L." <costello@mitre.org> schrieb am 18:12 Samstag, 16.April 2016:
Michael Kay wrote:
> The Namespaces in XML spec came from the XML Core working group.
> As with the XML spec itself, the group chose (in my view unwisely) to
> define a syntax with no underlying data model.
I read Michael's words to say:
XML has no underlying data model.
Isn't the XML Infoset considered to be XML's underlying data model?
I am looking at the specification for the XQuery and XPath Data Model (XDM) and it says:
The [XDM] data model is based on the [XML] infoset.
That seems to say that the XQuery and XPath Data Model (XDM) builds on top of the XML data model (XML infoset).
Am I reading Michael's words incorrectly and the XDM specification's words incorrectly?
/Roger
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