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

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Christopher

Firstly you can create a URI node from a BIND simply by using the built-in
URI() function - http://www.w3.org/TR/sparql11-query/#func-iri

BIND(URI("http://example.org") AS ?service)

If you place the SERVICE inside an OPTIONAL block then indeed the ?service
variable will not be considered bound at that point in the query.  The
SERVICE block is being executed after the BIND but in the case of the
OPTIONAL we can't stream the existing bindings for ?service into the
OPTIONAL block because that breaks the semantics of the query.

Note that if an error occurs executing a SERVICE block then a SPARQL engine
is required to produce an error unless the SILENT keyword was specified on
the SERVICE block.

Regards,

Rob Vesse

From:  "Penny, Christopher" <Chr...@ds...>
Reply-To:  dotNetRDF Bug Report tracking and resolution
<dot...@li...>
Date:  Thursday, 19 February 2015 23:18
To:  "dot...@li..."
<dot...@li...>
Subject:  [dotNetRDF-bugs] SERVICE calls from BIND-bounded variables
[SEC=UNCLASSIFIED]

> 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://"))
> {
> returnthis.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%2
> 0Optimization) 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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