sparql construct: unbound variables in blank node notation

[justin2004]

Version

6.0.0

Question

If any such instantiation produces a triple containing an unbound variable or an illegal RDF construct, such as a literal in subject or predicate position, then that triple is not included in the output RDF graph.

https://www.w3.org/TR/sparql11-query/#construct

Executing this:

prefix u: <uri:>
construct {
  u:thingA u:a [ u:time ?then ] .
  u:thingB u:time ?then .
  [ u:timeB ?then ]
}
where {
  bind(now() as ?now)
}

With an empty a.ttl...

$ /path/apache-jena-6.0.0/bin/sparql --data=a.ttl --query=./a.rq
PREFIX u: <uri:>

u:thingA  u:a   []  .

Based on reading the SPARQL 1.1 doc I don't expect any triples to be produced.
Applying the rule from the inside out...

• ?then is not bound
  • therefore don't produce [ u:time ?then ]
    • if [ u:time ?then ] isn't produced
      • then u:thingA u:a [ u:time ?then ] shouldn't be produced

[namedgraph]

I think it would be clearer if you rewrote the template as separate triples with explicit bnode labels.

[justin2004]

@namedgraph

$ cat a.rq
prefix u: <uri:>
construct {
  u:thingA u:a _:b0 .
  _:b0 u:time ?then .
}
where {
  bind(now() as ?now)
}
$ /path/apache-jena-6.0.0/bin/sparql --data=a.ttl --query=./a.rq
PREFIX u: <uri:>

u:thingA  u:a   []  .

Maybe you're right.

Though it would be really nice to have a convenient way to build structure around a variable but don't output any of it if the variable is unbound.

[afs]

It is what the spec says - for better or worse.

In SHACL Rules, there are wellformed-ness conditions, one of which is a variable in the head must be defined in the body, and one which requires every variable in the body to be bound. The body pattern is not full SPARQL.

[namedgraph]

Though it would be really nice to have a convenient way to build structure around a variable but don't output any of it if the variable is unbound.

What if you make the bnodes variables as well?

[justin2004]

What if you make the bnodes variables as well?

@namedgraph

prefix u: <uri:>
construct {
u:a u:b ?b .
?b u:time ?then .
}
where {
  bind(now() as ?now)
  bind(if(bound(?then),bnode(), 1/0 )   as ?b)
}

it works but it is more verbose and i have to specify lots of intermediate structure.

the [ ] notation in the construct head is so close to doing the ideal thing for terse conditional structure.