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Robert Koberg wrote:
> Thomas B. Passin wrote:
>> In RDF, a Property is a kind of Resource, and you can in fact make
>> statements about it. However, apparently, this kind of practice
>> prevents many kinds of logical reasoners (or maybe all, I am not
>> expert enough to know) from being "sound" and /or "complete", so most
>> of the restricted logics (like various forms of Description Logics)
>> restrict the kinds of constraints you can place on properties (which
>> is in effect restricting what you can say about them). The goal of
>> such restrictions is to make sure that a reasoner can complete any
>> assignment in a reasonable time (such a polynomial time as opposed to,
>> say, exponential) and be sure that any answer is correct.
>
>
> Doesn't this represent a flaw somewhere in RDF?
>
No, RDF itself does not retrict you from making statements about any
"resource", including a property. Restrictions on various subsets of
full first order logic (like OWL, which can be considered to be a
subset) are usually put in place to make reasoning tasks more tractable.
> Why are the examples in RDF of things that do not correspond to
> hierarchies (unless striping (or perhaps more semantically, 'grouping')
> is used)? Simplistic examples of things that will never have a hierarchy
> do not help.
Because the RDF model is not inherently hierarchical, although you can
represent hierarchies. Subgraphs are more natural ways to look at RDF
groupings, and a hierarchy is one kind of subgraph. Here is one example
of a hierarchy
root
child1
subchild1a
subchild1b
child2
In rdf, how you do this may depend on whether the order within any level
is important. If it is not, then in a striped format -
root
hasChild
child1
has Child
subchild1a
subchild1b
child2
If order is important, using rdf:parseType='Collection' causes the
processor to produce something eqivalent to this -
root
rdf:first
child1
rdf:first
subchild1a
rdf:rest
(something untyped)
rdf:first
subchild1b
rdf:rest
rdf:nil
rdf:rest
(something untyped)
rdf:first
child2
rdf:rest
rdf:nil
The addition of the "nil" term makes this kind of list non-extendable
(there are other list idioms that can be extended), which can be good if
you don't want someone else claiming that there are more list items than
you put in.
Cheers,
Tom P
--
Thomas B. Passin
Explorer's Guide the the Semantic Web (Manning Books)
http://www.manning.com/catalog/view.php?book=passin
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