Visibility of variables in nested Group(OrUnion)GraphPatterns

Dear community,

I’ve repeatedly observed inconsistent behaviour across SPARQL engines when it comes to the visibility of variables in nested groups (or in UNIONs). Let me illustrate the issues through a couple of queries, to be executed over a simple dataset consisting of the two triples:

<http://example.com/Alice <http://example.com/Alice>> a <http://example.com/ <http://example.com/>Person> .
<http://example.com/Flipper <http://example.com/Flipper>> a <http://example.com/Animal <http://example.com/Animal>> .


1a.) Visibility of FILTERs

SELECT ?s ?type WHERE { 
  BIND(<http://example.com/Person <http://example.com/Person>> AS ?type)
  { ?s ?p ?o FILTER(?o=?type) }
}

As far as I understand the spec, the variable ?type is not visible in the inner subgroup, so it is unbound when evaluating the FILTER. As a consequence, this query should return the empty result. Is this correct?


1b.) A slight variant of this example is where we do not consider a GroupGraphPattern but a GroupOrUnionGraphPattern:

SELECT ?s ?type WHERE { 
  BIND(<http://example.com/Person <http://example.com/Person>> AS ?type)
  { ?s ?p ?o FILTER(?o=?type) }
 UNION
 { ?s ?p ?o FILTER(?o!=?type) }
}

Again, variable ?type should not be visible in any part of the UNION, so both filters evaluate to error and the result is empty again. Is this correct?


2a.) Let’s now consider examples where we reuse variables from outside in BIND expressions rather than FILTERs:

SELECT ?s ?type WHERE { 
  BIND("http://example.com/ <http://example.com/>" AS ?typeBase)
  {
    BIND(URI(CONCAT(?typeBase,"Person")) AS ?type)
   ?s ?p ?o
   FILTER(?o=?type)
  }
}

The query tries to bind variable ?type in the nested group based on variable ?typeBase, which is defined outside. In my understanding, this outer variable (?typeBase) should not be visible when executing the inner BIND, so this query would return the empty result?


2b.) A slight variant of the previous one with a GroupOrUnionGraphPattern:


SELECT ?s ?type WHERE { 
  BIND("http://example.com/ <http://example.com/>" AS ?typeBase)
  {
    BIND(URI(CONCAT(?typeBase,"Person")) AS ?type)
    ?s ?p ?o
    FILTER(?o=?type)
  }
  UNION
  {
    BIND(URI(CONCAT(?typeBase,"Animal")) AS ?type)
    ?s ?p ?o
    FILTER(?o=?type)
  }
}

Also here I’d expect the empty result.

Any thoughts, comments/clarifications would be highly appreciated.

Best,
Michael


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