call a Sparql command in a rule, is it possible?

[Miguel Bento Alves]

Is it possible call a Sparql command in a rule? Can we do that with a custom
built-in function?

An example:

let's consider that I have the classes Student, Course and Lesson. Lesson is
related with Course. I have an ObjectProperty wasFailedBy that defines that
a given student fails to a given lesson (I list my schema and my data
example below). I want to define an ObjectProperty isDiligent that defines
that a given student is diligent to a given course. I consider that a given
student is diligent to a given course if he didn't fails more than a given
number of lessons (let's consider 1). I can't get this information with OWL
or OWL2 inference, I only can get with a Sparql command.

I'm thinking to define my rule in this way:

(?Student ex:isDiligent ?Course) <-
(?Student rdf:type ex:Student),
(?Course rdf:type ex:Course),
execSparql('
prefix exa: <http://www.example.org/example#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT (COUNT(1) AS ?nCount)
WHERE {   
?V1_Student ex:failsTo ?Lesson .
?Lesson exa:isLessonOf ?V2_Course .
}', ?Student, ?Course, ?NCount),
le(?NCount, 1). 

In my idea, the custom built-in function execSparql receives as parameters a
string with a Sparql command, a list of instanced variables to be replaced
inside the function and will return the result in the last variable passed
as parameter. I didn't implemented yet because I have doubts if this is the
better approach. My main doubts is:
Can I execute a Sparql command inside a custom built-in function using de
RuleContext?
Can I use a custom built-in function only to be an external access from the
rule and call the Sparql command, for instance via Web Service, to my
inference engine? Any better ideas?


<?xml version="1.0"?>


<!DOCTYPE rdf:RDF [
    <!ENTITY owl "http://www.w3.org/2002/07/owl#"; >
    <!ENTITY exa "http://www.example.org/example#"; >
    <!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"; >
    <!ENTITY rdfs "http://www.w3.org/2000/01/rdf-schema#"; >
    <!ENTITY rdf "http://www.w3.org/1999/02/22-rdf-syntax-ns#"; >
]>


<rdf:RDF xmlns="http://www.example.org/example#";
     xml:base="http://www.example.org/example#";
     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#";
     xmlns:owl="http://www.w3.org/2002/07/owl#";
     xmlns:xsd="http://www.w3.org/2001/XMLSchema#";
     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
     xmlns:exa="http://www.example.org/example#";>
    


    <!-- 
    
////////////////////////////////////////////////////////////////////////////
///////////
    //
    // Object Properties
    //
    
////////////////////////////////////////////////////////////////////////////
///////////
     -->

<owl:ObjectProperty rdf:about="&exa;isLessonOf">
        <rdfs:label>wasFailedBy</rdfs:label>
        <rdfs:domain rdf:resource="&exa;Lesson"/>
        <rdfs:range rdf:resource="&exa;Course"/>
    </owl:ObjectProperty>


<owl:ObjectProperty rdf:about="&exa;wasFailedBy">
        <rdfs:label>wasFailedBy</rdfs:label>
        <rdfs:domain rdf:resource="&exa;Lesson"/>
        <rdfs:range rdf:resource="&exa;Student"/>
    </owl:ObjectProperty>
    
    <owl:ObjectProperty rdf:about="&exa;failsTo">
        <rdfs:label>failsTo</rdfs:label>
        <owl:inverseOf rdf:resource="&exa;wasFailedBy"/>
    </owl:ObjectProperty>
    
    
    <!-- 
    
////////////////////////////////////////////////////////////////////////////
///////////
    //
    // Classes
    //
    
////////////////////////////////////////////////////////////////////////////
///////////
     -->

   <owl:Class rdf:about="&exa;Student">
        <rdfs:label>Student</rdfs:label>
    </owl:Class>
   
   <owl:Class rdf:about="&exa;Course">
        <rdfs:label>Course</rdfs:label>
    </owl:Class>
     
   <owl:Class rdf:about="&exa;Lesson">
        <rdfs:label>Lesson</rdfs:label>
    </owl:Class>
 


    <!-- 
    
////////////////////////////////////////////////////////////////////////////
///////////
    //
    // Individuals
    //
    
////////////////////////////////////////////////////////////////////////////
///////////
     -->

    
    <owl:Thing rdf:about="&exa;Databases">
        <rdfs:label>Databases</rdfs:label>
        <rdf:type rdf:resource="&exa;Course"/>
    </owl:Thing>



    <owl:Thing rdf:about="&exa;ComputerProgramming">
        <rdfs:label>Computer Programming</rdfs:label>
        <rdf:type rdf:resource="&exa;Course"/>
    </owl:Thing>
    

<owl:Thing rdf:about="&exa;Michael">
        <rdfs:label>Michael</rdfs:label>
        <rdf:type rdf:resource="&exa;Student"/>
    </owl:Thing>

<owl:Thing rdf:about="&exa;Richard">
        <rdfs:label>Richard</rdfs:label>
        <rdf:type rdf:resource="&exa;Student"/>
    </owl:Thing>

<owl:Thing rdf:about="&exa;LessonDB1">
        <rdf:type rdf:resource="&exa;Lesson"/>
        <isLessonOf rdf:resource="&exa;Databases"/>
    <wasFailedBy rdf:resource="&exa;Michael"/>
    </owl:Thing>

<owl:Thing rdf:about="&exa;LessonDB2">
        <rdf:type rdf:resource="&exa;Lesson"/>
        <isLessonOf rdf:resource="&exa;Databases"/>
    <wasFailedBy rdf:resource="&exa;Michael"/>
    <wasFailedBy rdf:resource="&exa;Richard"/>
    </owl:Thing>
    
    <owl:Thing rdf:about="&exa;LessonCP1">
        <rdf:type rdf:resource="&exa;Lesson"/>
        <isLessonOf rdf:resource="&exa;ComputerProgramming"/>
    <wasFailedBy rdf:resource="&exa;Richard"/>
    </owl:Thing>
    
    <owl:Thing rdf:about="&exa;LessonCP2">
        <rdf:type rdf:resource="&exa;Lesson"/>
        <isLessonOf rdf:resource="&exa;ComputerProgramming"/>
        <wasFailedBy rdf:resource="&exa;Michael"/>
    <wasFailedBy rdf:resource="&exa;Richard"/>
    </owl:Thing>
    
</rdf:RDF>