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Re: Draft response to: Re: major technical: blank nodes

From: Enrico Franconi <franconi@inf.unibz.it>
Date: Fri, 27 Jan 2006 09:52:56 +0100
Message-Id: <FA510D91-3240-46D9-856F-07102CE20A6A@inf.unibz.it>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
To: Pat Hayes <phayes@ihmc.us>

It seems a perfect answer to us.
--enrico & sergio

On 26 Jan 2006, at 23:50, Pat Hayes wrote:

>
> <<After volunteering for this I noticed that Dan had already  
> responded to this message with an [OK?], so this might now be  
> redundant. But here goes anyway.>>
>
> Fred, greetings.
>
> You make several points about blank nodes in SPARQL queries, and we  
> will respond to them in sequence. Your first point:
>
>> Blank nodes of the form _:a and [ ] do not add anything to the  
>> language.
>> Everything that can be expressed with such blank nodes can be  
>> expressed
>> with variables.
>
> is correct. The language has a syntactic redundancy. Some members  
> of the working group agree with your conclusion. We considered  
> prohibiting blank nodes in queries, but this would impose an extra  
> syntactic burden on someone wishing to form query patterns by  
> editing query variables into RDF. We also considered not having  
> unselected variables and requiring what are now unselected  
> variables to be replaced by blank nodes, but again this imposes a  
> burden on users while providing no extra utility. In neither case  
> did the conceptual simplification seem worth the operational burden  
> on users.
>
> There is however a deeper reason for distinguishing query blank  
> nodes from query variables, which addresses your next point:
>
>> What is the difference semantically between
>> _:a and ?a ?
>
> Extending SPARQL to richer entailment modes can make them  
> semantically different. When simple entailment is replaced by OWL  
> entailment in the SPARQL basic definitions, it is possible for an  
> existential to be OWL-entailed by a graph which contains no token  
> which would be a binder for a query variable: OWL supports  
> 'genuinely existential' entailments. For one of many possible  
> examples, if the OWL asserts that :a is in a restriction class  
> of :p to :c with cardinality one, this entails the assertion
>
> :a :p _:x .
> _:x rdf:type :c
>
> but provides no term to bind the query variable ?x to in the query  
> pattern
>
> :a :p ?x .
> ?x rdf:type :c
>
> so the query
>
> SELECT ?y WHERE { ?y :p _:u , _:u rdf:type :c }
>
> would succeed with x bound to :a, but the corresponding query
>
> SELECT ?y WHERE { ?y :p ?u , ?u rdf:type :c }
>
> might rationally be said to fail; all when using OWL entailment.   
> Admittedly, this case is controversial. One could argue that even  
> in the second case, it would be sensible to require that the query  
> engine provide a blank node identifier as an answer binding. But  
> the working group felt that it would be prudent to leave the option  
> open for future designers of OWL versions of SPARQL, which  
> motivates keeping the blank-node/variable distinction in the syntax.
>
> Your next point is best addressed by discussing blank node scopes.
>
>>  The only difference I can see is that _:a can not be
>> placed in the SELECT list (and there does not appear to be any
>> motivation for this).  Thus if the user, in the course of writing a
>> query, later decided he wants to receive the value of the blank node,
>> he must rewrite the query with a variable in place of the blank node.
>> The user might as well just write the query without blank nodes from
>> the beginning.
>
> There really is no such thing in SPARQL as the 'value' of a query  
> blank node. Blank node identifiers in queries are scoped to the  
> query, and indicate an existential assertion.
>
> In the course of checking the simple entailment relationship  
> between the target graph and the pattern instance such a blank node  
> must be 'mapped' to some term in the target graph, to be sure, but  
> this mapping is distinct from the variable-to-binding instance  
> mapping: it does not identify that term in any sense; rather, the  
> presence of the mapped term simply confirms the truth of the  
> existential claim made by the presence of the blank node. This also  
> gets to your next point:
>
>> In addition, the term "blank node" creates a false analogy with RDF.
>> An RDF blank node is a node in a graph with no IRI.  A SPARQL  
>> blank node
>> is not a node at all, it is actually a variable that cannot be  
>> named in
>> the SELECT list.
>
> We disagree. It is exactly an RDF blank node, and the analogy is  
> not false. Do not think of a query bnode as a 'blank variable':  
> think instead of the entire query basic graph pattern as an RDF  
> graph with some 'named holes' in it, the query variables. The query  
> answer is a vector of pieces of RDF syntax which, when  
> syntactically substituted for the variables, produces (an  
> appropriate lexicalization of ) an RDF graph which is simply  
> entailed by the target graph[*]. All of this is purely syntactic,  
> but the entailment relationship between this instance and the  
> target graph, that makes the answer a genuine answer, is semantic.  
> Blank nodes in the query pattern are genuine RDF blank nodes in the  
> entailed instance, and the entailment relationship holds between  
> two RDF graphs.
>
> Simple entailment is indeed so simple that it can be defined in  
> terms of a mapping from blank nodes to RDF terms: A simply entails  
> B just when B has an RDF instance (gotten by mapping from blank  
> nodes to terms) which is a subgraph of A. So, to check the required  
> relationship between a target graph A and a basic graph pattern C,  
> we need an instance mapping M on the variables in C and then  
> another N on the blank nodes in M(C) such that N(M(C)) is a  
> subgraph of A. In this simple case, then, this is equivalent to  
> asking for a single mapping on variables and blank nodes which  
> produces an instance [N+M](C) which is a subgraph of A, then  
> ignoring part of it.  But there is a real conceptual distinction,  
> which is reflected in the definitions, between the two parts of  
> this composite mapping; and when simple entailment is replaced by  
> more advanced forms of entailment, the distinction can become  
> operationally important.
>
> Pat
>
> [*] (In fact, it is simply entailed by a 'scoping graph' which is  
> graph-equivalent to the target graph under a blank node  
> substitution, but this complication is just to allow blank nodes to  
> be scoped separately in the answer document.)
>
> Pat
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>
Received on Friday, 27 January 2006 08:53:05 GMT

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