Re: Issue with top-down and bottom-up semantics

Frank,

Thank you for your comment. This is an issue that has been discussed 
many times by the working group, and I do not see any new information in 
your message that should cause the Working Group to reconsider the 
design at this point in time. OPTIONAL, in particular, has been in the 
language since July of 2004[1], and is in extremely wide use by the 
SPARQL user community and is widely implemented. [2] Note that the 
Working Group has on record a long-standing objection [3] about the 
adoption of OPTIONAL [4], and also an objection to the use of an 
algebraic rather than declarative semantics [5].

Please let us know if you are satisfied with this response. If not, I 
will be glad to add your comment in support of either or both of the 
above objections.

Lee

[1] http://www.w3.org/2001/sw/DataAccess/ftf2#initdn3
[2] http://www.w3.org/2001/sw/DataAccess/tests/implementations
[3] http://www.w3.org/2005/10/Process-20051014/policies.html#FormalObjection
[4] http://www.w3.org/2001/sw/DataAccess/crq350#optional
[5] http://www.w3.org/2001/sw/DataAccess/crq350#algabraicsemantics

Francis McCabe wrote:
> 
> 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 06:02:15 UTC