Re: [dotNetRDF-Support] Leviathan - SPARQL grammar extensions

[Rob V. <rv...@do...>]

Hi Dennis

No there is currently no support or APIs for extending the grammar

Having had a look at that paper it would seem like most of what they wanted
to do can be done purely with ORDER BY especially now that there is a much
broader and more powerful set of functions available in SPARQL 1.1 (IF,
COALESCE and IN are three very useful functions that spring to mind) so this
may be worth exploring as a viable alternative.  Keeping within the
specification is always preferable IMO.

Just as an example their PREFERING clause in section 4 could be rewritten as
follows:

ORDER BY 
DESC( IF( ?rating = pt:excellent, 1, 0) )
DESC( IF( ?end <= '16:00' || start >= '18:00', ?start, '00:00') )

For the ?rating type conditions where you may have more complex mappings of
values to rankings you could either define an extension function (dotNetRDF
and other APIs have support for this) or you could use the VALUES clause in
your WHERE clause to assign rankings to constants e.g.

WHERE
{
  ?t pt:has-rating ?rating
  VALUES (?rating ?ratingRank)
  {
    ( pt:excellent 5 )
    ( pt:good 3 )
    ( pt:poor 1 )
  }
}

Then you can just do a ORDER BY DESC(?ratingRank), again this latter
approach has the advantage of being within the specification (and was
actually the motivation for this feature)

Hope this helps,

Rob Vesse

From:  Dennis Ludl <Den...@St...>
Reply-To:  dotNetRDF User Help and Support
<dot...@li...>
Date:  Thursday, November 15, 2012 5:49 AM
To:  <dot...@li...>
Subject:  [dotNetRDF-Support] Leviathan - SPARQL grammar extensions

> Hi,
> I try to find out, if it's possible to extend SPARQL grammar used in your
> Leviathan engine. I want to add an preference modifier to the SPARQL grammar,
> like proposed by Siberski et al. (W. Siberski, J. Z. Pan, U. Thaden: Querying
> the Semantic Web with Preferences. In Proc. of the 5th Int. Semantic Web
> Conf., 2006).
> 
> As far as I know there are no API's for such extensions in dotNetRDF, am I
> right with this?
> 
> Thank you,
> Dennis
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