"The function returns no binding and therefore ?user is unbound and will
be
iterating over all possible subjects of the firstName triples."
Hmmm, not what I was expecting. I guess I would have expected it to act
more like normal SPARQL variable chaining occurrs, e.g. in the following
query:
SELECT ?user ?value
WHERE {?user a :NonExistantClass .
?user :firstName ?value}
?user is "unbound" after the first line (NonExistantClass doesn't really
exist), and this query returns no results instead of everything in the
model with a firstName property. Even if you reverse the order:
SELECT ?user ?value
WHERE {?user :firstName ?value .
?user a :NonExistantClass .
}
You still get no results. But change it to something like:
SELECT ?user ?value
WHERE {(LET ?user := :filterOnClass(:NonExistantClass)
?user :firstName ?value}
And you do get everything in the model with a firstName property.
Be that as it may, using subqueries I think I'm able to get the behavior
I'm looking for. A little verbose, but if I understand things, I think
you'd have to do this anytime you wanted to use a re-usable SPIN
filtering function:
SELECT ?user ?value
{
{SELECT ?user
WHERE {
LET(?user := ModelManagement:getProjectUser("DEFAULT_PROJECT",
"xxx"))
}
}
{SELECT ?user ?value
WHERE {
?user ModelManagement:firstName ?value
}
}
}
In the above query, if the user specified in the LET (xxx) doesn't
exist, nothing is returned.
> -----Original Message-----
> From: Holger Knublauch [mailto:[email protected]]
> Sent: Wednesday, May 27, 2009 9:38 PM
> To: [email protected]
> Subject: [tbc-users] Re: [SPIN] Using FILTER with function arguments
>
>
>
> On May 27, 2009, at 12:21 PM, Schmitz, Jeffrey A wrote:
>
> >
> > It's getting curiousor and curiousor...
> >
> > Using the TBC SPARQL tab, when I call a function that filters
> > according to an argument, e.g.:
> >
> > SELECT ?user
> > WHERE {
> > ?project ModelManagement:hasUser ?user .
> > ?user Common:name ?username .
> > FILTER (?username = ?arg1) .
> > }
>
> If the function returns no binding then it will be comparable
> to a wildcard in the query below.
>
> >
> >
> > As long as I pass in an argument that has a match:
> >
> > SELECT ?value
> > WHERE {LET(?user := ModelManagement:getProjectUser("john"))
> > ?user ModelManagement:firstName ?value}
> >
> > the function returns just one result as expected (in this case for
> > some
> > reason the blank result rows aren't returned). However, if I pass
> > in a
> > value that has NO matches:
> >
> > SELECT ?value
> > WHERE {LET(?user := ModelManagement:getProjectUser("xxx"))
> > ?user ModelManagement:firstName ?value}
> >
> >
> > It acts like the FILTER statement isn't there, and all
> User's names in
> > the model are returned. I would have expected no results to be
> > returned. Btw, I'm seeing this same behavior in my code too. Am I
> > thinking about this wrong, or is there perhaps a problem with how
> > FILTER
> > works within a Function?
>
> No, the behavior is correct (as far as I understand your
> example). The
> function returns no binding and therefore ?user is unbound
> and will be
> iterating over all possible subjects of the firstName triples.
>
> In your first email, please keep in mind that the execution order of
> ARQ might be different from what you have typed it. In particular, I
> find often that LET statements are executed at the end, and
> therefore
> lead to more iterations than expected.
>
> Holger
>
>
> >
>
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