[Versa] Question about query

[Chimezie Ogbuji]

> The following is a legal expression in Versa:
> 
>    <http://foo.com>
> 

This doesn't look like current Versa syntax for identifying resources
(unless you had the previously suggested <..uri ..> form in mind -
from N3), so I'll assume you meant (@'http://foo.com').

> It can be used as a filter expression in a filter or traversal and it
> returns a boolean value if the resource matches, etc.
> 
> My question, is what if this is the top level expression and executed
> on an empty model.  What are the results of the expression?
> Is it a boolean?

I would think not, since < .. uri .. > (or @' ..uri..') explicitely
identifies resources.

> Is it null because the resource does not exist?

It *should* be, since Versa is *bound* to an RDF graph and so should
only be able to resolve resources within that graph.  So, either empty
set or owl:Nothing (see below).

> 
> Is it just the resource's URI regardless of whether or not it exists?

I wouldn't think you would want to return a resource if it didn't
exist in the underlying RDF graph.  In which case the < .. uri .. > /
@'.. uri ..' expression would be more of a filter/pattern (in the same
way it is used in SPARQL) than actually expressing the existence of a
resource with that uri.  It should return null, or nothing.

> 
> I think it should be the empty set.

Same, (since we have no concept of 'null'/'None').  Perhaps it should
return owl:Nothing - and do this anytime an expression evaluates to
'nothing'? But then you might have to account for owl:Nothing in the
datatype comparison matrix? i.e. empty Versa set/list are equivalent
to owl:Nothing since list(owl:Nothing) should be equivalent to list()?

> 
> Mike
> 
> 
> 
> 
> 
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> Mike Olson                                                Principal
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