Re: Different encoding for RDF-star queries as S-Expressions with Jena

Jena parses and evaluates Daniel's original example - it is using a 
(triple) as an expression. SPARQL-star TRIPLE.

The two expression are not identical - one has quoted triple term <<>> 
pattern, the other has an expression that evaluates to a quoted triple 
-the TRIPLE(expr, expr, expr) function in SPARQL-star.

So there are three cases:

1 : triple pattern - 3-tuple of RDF-terms/variables.
2 : quoted triple pattern, an RDF term + may have variables
3 : TRIPLE/3 function - arguments are expressions.

triple != quoted triple.  strict typing :-)

On 20/01/2022 22:10, Gregg Kellogg wrote:
 > While, Ruby RDF outputs:
 >
 > (prefix ((:<http://bigdata.com>)
 >           (foaf:<http://xmlns.com/foaf/0.1/>)
 >           (ex:<http://example.org/>))
 >   (project (?age ?c)
 >    (bgp
 >     (triple ?bob foaf:name "Bob")
 >     (triple (triple ?bob foaf:age ?age) ex:certainty ?c))))
 >
 > I’m not sure if this is intentional on Andy’s part, as  `triple << 
?bob foaf:age ?age >> ex:certainty ?c)` doesn’t really look like an 
S-Expression to my eyes,

"intention" is too strong :-)

<<>> is another kind of RDF term, like <uri> or "string". I'd happily 
swap to a ()-form. It's messy and special case to parse.

There again, parsing <<>> is a bit fiddly. Nested <<>> is allowed.

Without a separate word, those three uses of (triple) have context 
sensitive meaning and have to be coded around at each usage site which 
is not nice.

A word for quoted triple? (qtriple)?

On 21/01/2022 11:13, Daniel Hernandez wrote:
> 
> Hi all,
> 
> Gregg Kellogg writes:
> 
>> Of course, there is no real standard for expressing queries as
>> s-expressions, but Ruby RDF generally follows Jena. (Creating a
>> normative S-Expression representation would be interesting, though).
> 
> I agree on that a normative for S-Expressions would help for
> interoperability.
> 
>> I’m not sure if this is intentional on Andy’s part, as `triple << ?bob
>> foaf:age ?age >> ex:certainty ?c)` doesn’t really look like an
>> S-Expression to my eyes, and the “triple” expression has already been
>> established, but using that embedded triple expression made the most
>> sense to me. It does lead to some complications in re-serializing, as
>> SPARQL-star does have a TRIPLE function, which gets the same
>> representation in Ruby RDF, and distinguishing between their use took
>> some doing, but internally, it’s never been an issue.
> 
> Gregg, I agree with you on that the notation <<...>> doesn't look as
> look as an S-Expression.  Hence, this notation requires implementing an
> additional type of parenthesis.  Indeed, if S-Expressions allow for the
> notation <<...>>, then it is also needed to support nested triple
> expressions, for example << << ?s ?p ?o >> ?q ?r >> ?s ?v.

Yes. It does.

>> I wasn’t able to get qparse to parse either form of S-Expression, but
>> that’s likely an operator error.

qparse does not parse SSE. It is only an output format.

> 
> I have tested how Jena parse both forms, and seems that Jena support
> both forms. It translates << ?s ?p ?o >> as an embedded triple
> expression << ?s ?p ?o >>, whereas (triple ?s ?p ?o) is translated as
> the triple function triple(?s, ?p, ?o).
> 
> Daniel
>

Received on Friday, 21 January 2022 17:39:14 UTC