[dotNetRDF-bugs] SERVICE calls from BIND-bounded variables [SEC=UNCLASSIFIED]

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UNCLASSIFIED
Hi all,

I'm working off the following example, but using BIND instead of a triple pattern to give a value to ?service (actually BIND + CONCAT, but the problem can be shown with just BIND).
http://www.w3.org/TR/sparql11-federated-query/#variableService

The BIND call works, as illustrated by this query:

SELECT ?service ?project ?projectName
WHERE
{
                                BIND ("http://projects1.example.org/sparql" as ?service) .
}

This results in one value for ?service - http://projects1.example.org/sparql

However, if I then add a SERVICE statement the query returns zero results:

PREFIX  doap: <http://usefulinc.com/ns/doap#<http://usefulinc.com/ns/doap>>
SELECT ?service ?project ?projectName
WHERE
{
                                BIND ("http://projects1.example.org/sparql" as ?service) .
                                SERVICE ?service
                                {
                                                ?project  doap:name ?projectName .
                                }
}

I am confident that the SERVICE call is never made as:
Options.HttpDebugging = true; + no output to console.
No traffic is seen in Wireshark.

So then I spent some time debugging. Line 113 of Service.cs checks the type of node generated by BIND. This is always a NodeType.Literal, and NodeType.Uri is required. I believe this is because nodes generated by BIND are always given NodeType.Literal?

I fixed this (for my purposes) by modifying SparqlExpressionParser.cs line 468, in the case Token.LITERAL I added a check to see if the string was actually a valid URI, which for me was just a string starting with "http://", then invoked the Token.URI code:

if (next.Value.StartsWith("http://"))
{
return this.TryParseIriRefOrFunction(tokens);
}

This caused the expected service call.

I don't know what best fix would be, but it would be nice to pass the results of BIND to a SERVICE statement.

I had a second issue when putting SERVICE in an OPTIONAL block like this:

PREFIX  doap: <http://usefulinc.com/ns/doap#<http://usefulinc.com/ns/doap>>
SELECT ?service ?project ?projectName
WHERE
{
                                BIND ("http://projects1.example.org/sparql" as ?service) .
                                OPTIONAL
                                {
                                                SERVICE ?service
                                                {
                                                                ?project  doap:name ?projectName .
                                                }
                                }
}

The following exception is thrown:

VDS.RDF.Query.RdfQueryException was unhandled by user code
  HResult=-2146233088
  Message=Query execution failed because evaluating a SERVICE clause failed - this may be due to an error with the remote service
  Source=dotNetRDF
  StackTrace:
       at VDS.RDF.Query.Algebra.Service.Evaluate(SparqlEvaluationContext context) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\Algebra\Service.cs:line 235
       at VDS.RDF.Query.SparqlEvaluationContext.Evaluate(ISparqlAlgebra algebra) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\SparqlEvaluationContext.cs:line 399
       at VDS.RDF.Query.Algebra.LeftJoin.Evaluate(SparqlEvaluationContext context) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\Algebra\AlgebraJoinClasses.cs:line 286
       at VDS.RDF.Query.SparqlEvaluationContext.Evaluate(ISparqlAlgebra algebra) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\SparqlEvaluationContext.cs:line 399
       at VDS.RDF.Query.Algebra.Select.Evaluate(SparqlEvaluationContext context) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\Algebra\Project.cs:line 85
       at VDS.RDF.Query.SparqlEvaluationContext.Evaluate(ISparqlAlgebra algebra) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\SparqlEvaluationContext.cs:line 399
       at VDS.RDF.Query.LeviathanQueryProcessor.ProcessQuery(IRdfHandler rdfHandler, ISparqlResultsHandler resultsHandler, SparqlQuery query) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\LeviathanQueryProcessor.cs:line 280
       at VDS.RDF.Query.LeviathanQueryProcessor.ProcessQuery(SparqlQuery query) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\LeviathanQueryProcessor.cs:line 102
       at VDS.RDF.Storage.InMemoryManager.Query(String sparqlQuery) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Storage\InMemoryManager.cs:line 325
       at ABC.RDF.RdfAdaptor.executeQuery(String query) in c:\projects\vsprojects\RDF_Tester\RdfHandler.cs:line 90
       at ABC.RDF. RDFTesterForm.<executeQueryButton_Click>b__1(Object o, DoWorkEventArgs args) in c:\projects\vsprojects\RDF_Tester\RdfManagerForm.cs:line 279
       at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
       at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)
  InnerException: VDS.RDF.Query.RdfQueryException
       HResult=-2146233088
       Message=Cannot evaluate a SERVICE clause which uses a Variable as the Service specifier when the Variable is unbound
       Source=dotNetRDF
       StackTrace:
            at VDS.RDF.Query.Algebra.Service.Evaluate(SparqlEvaluationContext context) in c:\projects\vsprojects\lib\dotnetrdf_library_source_107_stable\Libraries\core\net40\Query\Algebra\Service.cs:line 108
       InnerException:

After reading a little on how dotNetRdf does SPARQL Optimisation (https://bitbucket.org/dotnetrdf/dotnetrdf/wiki/DeveloperGuide/SPARQL/SPARQL%20Optimization) I suspect that the SERVICE block is being executed before the BIND statement. This is supported by the fact a query containing only the BIND statement returns the URI as expected. However, I have not spent enough time with the debugger to confirm this.

I am using version 1.0.7.

Thanks in advance.

Regards, Chris.

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