User defined operations and PREMISES keyword

Hi,

 

I am currently trying to create user-defined spatial operators for SPARQL. 

For example, I would like to define the following operator:

 

withinDistance(G1,G2,d,uom)

 

G1 and G2 are instance of rdf:type geo:Geometry 

d is a distance

uom is a Unit of Measure (but optional argument).

 

I have found a number of issues by trying to use the current specification.
I will try to explain through a use-case that does not seem to be captured
in RDF Data Access Use <http://www.w3.org/TR/rdf-dawg-uc/>  Cases and
Requirements ).

 

Let's suppose the user wants to find all the instances of Geometry within a
user defined Polygon p1 (subclass of Geometry) and returns the complete
description of Geometry)

The query will look like more or less like this:

 

PREFIX user: <http://www.foo.org/userdefined#>
PREFIX geo: <http://www.opengis.org/geometry#>
PREFIX uom: <http://www.foo.org/uom#>
PREFIX fn: <http://my.example.org/geo/functor#>

 

DESCRIBE ?g
WHERE { ?g1  rdf:type <http://www.foo.org/geometry#Geometry> .   
        FILTER fn:withinDistance(?g1,
<http://www.foo.org/userdefined#p1>,10, <http://www.foo.org/uom#meter>) 
      }

 

 

 

 

ISSUE1: PREMISES TRIPLES

 

 

It seems to me that the  current specification does not provide a way to
pass the description of p1 in the query:

 

Using the SPARQL triple syntax, p1 could be defined by the user like the
following:

 

I assume geo:SimplePolygon to be a subclass of geo:Geometry and is already
known by the server.

I also assume that geo:coordinates is a datatype defined by the ontology and
understood by the server.

 

 

{

     <http://www.foo.org/userdefined#p1>   rdf:type
<http://www.opengis.org/geometry#SimplePolygon>   ,

     <http://www.foo.org/userdefined#p1>   geo:crs
<http://www.epsg.org#_4326>   , 

      <http://www.foo.org/userdefined#p1>   geo.exterior
<http://www.foo.org/userdefined#lr1>   , 

      <http://www.foo.org/userdefined#lr1>  geo.coordinates "0,0 0,10 10,10
0,10"^^geo:coordinates

}

 

To support this query, SPARQL needs a mechanism to pass the user defined
triples, so they could be added as premises in the query engine (DQL is
using such concept) prior to the evaluation on any graph. The handling of
the premises would be pretty straightforward to implement by any SPARQL
query engine. 

 

I propose to add a new keyword in the language called PREMISES to support my
use case.

 

This would look like this:

 

PREFIX user: <http://www.foo.org/userdefined#>
PREFIX geo: <http://www.opengis.org/geometry#>
PREFIX uom: <http://www.foo.org/uom#>
PREFIX fn: <http://my.example.org/geo/functor#>

 

PREMISES: {

      <http://www.foo.org/userdefined#p1>   rdf:type
<http://www.opengis.org/geometry#SimplePolygon>   ,

      <http://www.foo.org/userdefined#p1>   geo:crs
<http://www.epsg.org#_4326>   , 

      <http://www.foo.org/userdefined#p1>   geo.exterior
<http://www.foo.org/userdefined#lr1>   , 

      <http://www.foo.org/userdefined#lr1>  rdf:type
<http://www.opengis.org/geometry#LinearRing>  

      <http://www.foo.org/userdefined#lr1>  geo.coordinates "0,0 0,10 10,10
0,10"^^geo:coordinates.

}

DESCRIBE ?g
WHERE { ?g1  rdf:type <http://www.foo.org/geometry#Geometry> .   
        FILTER fn:withinDistance(?g1,
<http://www.foo.org/userdefined#p1>,10, <http://www.foo.org/uom#meter>) 
      }

 

ISSUE 2: OPTIONAL OR NULL PARAMETERS.

 

The other issue is how optional or null  parameter are set in the functions.
I can see two alternatives: using null or using blank parameter

 

Using null,

 

        FILTER fn:withinDistance(?g1,
<http://www.foo.org/userdefined#p1>,10, null) 
 
Using blank:
 
        FILTER fn:withinDistance(?g1,
<http://www.foo.org/userdefined#p1>,10, )   (note the comma after 10).
 

 

 

ISSUE 3: DESCRIPTION OF SUPPORTED FUNCTIONS IN SPARQL PROTOCOL.

 

The SPARQL <http://www.w3.org/TR/rdf-sparql-protocol/>  Protocol for RDF
document is not clear how the ServiceDescription describes the supported
functions and operations by the SPARQL Service. 

For client operations, it is important not only to know the URI of the
functions and operations but also to get a description of their arguments
types, cardinality  and order.

An additional optional query type could be added to describe Functions and
Operands such GetOperationDescription and takes an optional parameter name
which can take a list of operand or function uri,

 

http://www.foo.com/sqs?query=GetOperationDescription&name=http://my.example.
org/geo/functor#WithinDistance, http://my.example.org/geo/functor#Intersects

 

The response should be in RDF format. This assumes an ontology for functors
and operands needs to be elaborated.

 

 

In conclusion, I would like the DAWG elaborates more the section 11.3 of the
current SPARQL Query Language for RDF document, because I anticipate many
user-defined functions would be defined in the future. 

 

 

I am looking forward to get feedback on this topic.

 

Best regards

 

Stephane Fellah

Image Matters LLC