[Versa] More on literal queries

[Uche Ogbuji]

On Tue, 2005-09-06 at 09:19 -0400, Chimezie Ogbuji wrote:
> Okay, I have a suggestion regarding the need to query for a resource
> by name and a literal by value.  This came from an exchange Uche and I
> had on IRC.  If we leave the current Versa data model as is, we have
> URI references (not 'resources' as we currently call them, which is
> misleading)  -

I don't think it's misleading at all.  Do we really want to replay years
of discussion about what an RDF (or more precisely URI) resource
"means"?  I certainly don't.  Surely you're not telling me that if I do
a Versa query for "http://4suite.org" that I should get back a magical
abstraction that can dispense representations, such as the Web page
itself or whatever else I may send in an HTTP Accept header.

No, what we get is the URI reference, which for the purpose of RDF *is*
the resource.

The closest I could come to agreeing on this is "resource name".  Not
"URI reference".


> which may or may not refer to rdf resources  in the
> underlying model - and literal values which only exist in the query
> space and may not have any corresponding literal value in the
> underlying model.
> 
> There is a need to query or match resources by URI and match literals
> by value w/out having to resort to convoluted queries such as:
> 
> filter(all(),type(rdfs:Literal),eq("3.0"))
> filter(all(),eq('<mailto:foo at yahoo.com>'))

Yes, and we all agree with that.  My point has always been that there
are better ways of doing so.


> We could define a function that takes a set of Versa data types
> (resource or literal) and 'grounds' them to the underlying model.
> 
> For each data type leave it in the set if there is a corresponding RDF
> resource in the graph (by identity operation).  Otherwise, remove it
> 
> So:
> 
> filter(all(),type(rdfs:Literal),eq("3.0"))
> 
> becomes
> 
> ground("3.0")    => empty set or set("3.0")  - depending on if there
> is a "3.0" in the graph
> 
> filter(all(),eq('<mailto:foo at yahoo.com>'))
> 
> becomes
> 
> ground(<mailto:foo at yahoo.com>) => empty set or set(<mailto:foo at yahoo.com>)
> 
> I'm suggesting the term 'ground' because it is used to describe the
> process of assigning unused URIs to blank nodes in a graph, which is
> (somewhat) similar in some ways to what is being done here, resolving
> a URI reference or literal in the query space to actual
> resources/literals in the underlying graph.
> 
> We probably would also need to make sure our definition of eq is clear
> about the semantics of comparing a resource in the underlying model
> with a URI ref.  Do we mention that the model has the right to
> determine how it calculates the identity operation, since this
> apparently may involve more than just a string comparison of the
> URI's?

I'm fine with this.


-- 
Uche Ogbuji                               Fourthought, Inc.
http://uche.ogbuji.net                    http://fourthought.com
http://copia.ogbuji.net                   http://4Suite.org
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