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Issue with top-down and bottom-up semantics

From: Francis McCabe <frankmccabe@mac.com>
Date: Thu, 25 Oct 2007 21:33:12 -0700
Message-Id: <3D2982EE-7896-4B84-B6C6-A93B53B64482@mac.com>
To: public-rdf-dawg-comments@w3.org

I believe that is important that SPARQL have a declarative semantics.  
This both reflects the fundamental purpose of a query language -- it  
is not a programming language -- and will make it easier to  
communicate to non-professionals the merits and benefits of using it.

In the case of a query language for RDF, this is doubly the case as  
the base language is inherently declarative. (It even has a model  
theory!)

It is therefore something of a disappointment to discover that SPARQL  
does not have a truly declarative semantics. It is not possibly to  
firmly state that the results of satisfying a SPARQL query are based  
on some sound inference process backed up by a model theoretic  
interpretation.

I believe that the OPTIONAL feature may be one of the causes of this.  
Following a recent email conversation, I became aware that its  
semantics do not fit well with the current model for the  
quantification of variables. Certainly, the idea that a top-down  
evaluation (or a left-to-right versus left-to-right) would give  
different answers than a bottom-up evaluation is strong evidence of  
the weakness of the semantic framework.

The specification hints at this, the query:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX dc:   <http://purl.org/dc/elements/1.1/>
SELECT ?name
  WHERE { ?x foaf:givenName  ?name .
          OPTIONAL { ?x dc:date ?date } .
          FILTER (!bound(?date)) }

is described as being equivalent to negation-as-failure. Giving NAF a  
declarative semantics is a non-trivial task (first done by Keith  
Clark). It involves assuming a 'completion semantics' for the  
predicates: the definitions must be interpreted as if-and-only-if;  
and furthermore, inequality of symbol must become inequality of  
denoted individuals.

Both of these assumptions are antithetical to the nature of the  
semantic web which depends on the so-called Open World assumption --  
primarily because information on the SW can never be assumed to be  
complete.

Although there may appear to be compelling pragmatic reasons for  
retaining the OPTIONAL feature; I believe that they are outweighed by  
the conflicts that they raise with the fundamental nature of the  
Semantic Web.

Thank you for your attention

Frank McCabe
Received on Friday, 26 October 2007 04:33:51 GMT

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