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Re: Error definition

From: <Faisal.Alkhateeb@inrialpes.fr>
Date: Thu, 15 Jun 2006 09:59:20 +0200
Message-ID: <1150358360.449113580c74f@listes-serv.inrialpes.fr>
To: Pat Hayes <phayes@ihmc.us>
Cc: public-rdf-dawg-comments@w3.org

Quoting Pat Hayes <phayes@ihmc.us>:

> >Hi,
> >
> >While reading the last version of SPARQL, I am surprised with the
> following
> >definition. I don't know if the semantics of RDF (and consequently SPARQL)
> is
> >changed.
> >
> >
> >Definition: Triple Pattern
> >
> >A triple pattern is member of the set:
> >     (RDF-T union V) x (I union V) x (RDF-T union V)
> >
> >>From the above definition, the literals are allowed in the subject
> position.
> >However, they are not allowed in RDF. So, I want to know if it is a mistake
> or
> >not?
> It is not a mistake. The text mentions the 
> difference and explains the reasons for it.
> It is now widely believed that forbidding 
> literals from subject position was a strategic 
> error in the RDF design, and many RDF 
> 'extensions' allow it. It simplifies the syntax 
> and makes reasoning easier. This change to RDF 
> would not change the RDF semantics at all, only 
> the syntax. The RDF semantics would apply without 
> alteration to a fully 'free' RDF syntax in which 
> any kind of expression could occur in any 
> position. The syntactic constraints in RDF all 
> arise from difficulties with encoding the 
> resulting triples in the XML striped syntax, not 
> from any semantic requirements.

SPARQL goes before RDF! but ok.

> >- For blanks in SPARQL, I want to know if they act as variables or not,
> that
> >is, if a blank can be mapped to any RDF term (or just to a resource)? I
> need
> >some explannation on that, because blanks = anonymous variables. What is a
> >resource? Is it a literal or an IRI or just an IRI?
> >
> You ask many questions :-) I will try to answer them all, briefly.
> The word 'resource' in the RDF spec just means 
> 'the thing that an identifier identifies'. So you 
> can replace "resource" by "thing" and the spec 
> would mean exactly the same. The use of this 
> rather odd word has become traditional in W3C 
> documents, but it has no special meaning. So, to 
> answer your question directly, a resource is 
> anything at all: it is whatever the RDF is 
> supposed to be describing. So, yes, an IRI could 
> be a resource, and so could a literal (because 
> the name of a thing is itself a thing); but it is 
> more usual to think of the resource as the thing 
> referred to by the IRI or literal. So for example 
> this is a literal:
> xsd:number^^"34"
> which refers to the number thirty four, which 
> would normally be called the resource when 
> discussing RDF-style languages.
> During query answering, variables are bound to 
> *names* of resources, not to the resources 
> themselves. So a query answer mapping is a 
> binding of IRIs or literals (or blank nodes) - 
> all RDF atomic expressions which *refer to* 
> resources - to variables.
> In SPARQL, blank nodes do indeed act very 
> similarly to variables, and for simple examples 
> you can think of them as being 'blank variables', 
> but strictly speaking one has to distinguish 
> them, since they behave differently in 
> complicated cases. Their identifiers are scoped 
> differently. In a group query pattern for 
> example, if triples in two different simple 
> patterns have the same variable then it is really 
> the same variable; but the bnodeIDs are scoped to 
> the local simple pattern. Also, in future 
> extensions to SPARQL where it would apply to more 
> powerful inference regimes such as OWL (and which 
> are intended to be covered by the definitions in 
> section 2.5), there is a real difference in 
> meaning between a variable (which has to be bound 
> to a value) and a blank node (which only 
> indicates that something can be inferred to 
> exist). However, all that being said, if you are 
> reading the spec for the first time, I suggest 
> that you first try reading it with the 
> simplifying idea that blank nodes are like 
> anonymous variables, bearing in mind that you 
> will have to give up this temporary mental crutch 
> when you get more skilled.

I know all this, but in fact  I was confused with the following message:

In which we can find "Where a variable
	matches any resource or literal, blank nodes only match resources.

> >- Is the case that blanks are not allowed in the predicate position?
> Right, they are not. This is another possible 
> extension to the RDF model which DAWG thought 
> about, but decided NOT to include in SPARQL.


> I hope this helps. Feel free to get back with more questions.
> Pat Hayes
> >Best regards,
> >Faisal Alkhateeb
> >Ph.d. student, INRIA Rhône-Alpes
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Received on Thursday, 15 June 2006 08:16:36 UTC

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