Triples containing Variable result in invalid turtle

[mrkvon]

When creating a triple with Variables in it, and then writing it as text/turtle, the outcome is invalid turtle.

const writer = new Writer({ format: 'text/turtle' })
writer.addQuad(new Quad(new Variable('a'), new NamedNode(foaf.knows), new Variable('c')))
writer.end((err, result) => {
  console.log(result)
})

This yields

?a <http://xmlns.com/foaf/0.1/knows> ?c.

which is invalid turtle.

Along the argumentation in #165, this could rather throw error or gracefully drop the triples containing Variable.

[RubenVerborgh]

Working as intended (writer always expects valid input), but we can indeed accept a PR along the lines of #165 to perform optional input sanitization.

[mrkvon]

Indeed relevant only if a feature like #339 (comment) is included.

[bergos]

May I throw in one of my recent blog posts RDF/JS for Data Processing. Summary of the blog post: Validation is a spectrum, and what is valid RDF depends on the context. That's why I like a modular approach, and rdf-validation could be one building block. Nevertheless, if a media type is given, it's legit that a serializer validates that the output is according to the spec. But I would recommend using flags, as mentioned in #339, to control the behavior, and ideally there is no performance impact.

[nichtich]

With N3 it's easy to accidently transform valid RDF into invalid RDF if the output format supports a smaller part of RDF. Examples include variables (this issue), graphs (#339) and relative IRIs (not mentioned yet). I'd welcome a general flag to make sure a writer only emits valid data.

[nichtich]

A filter function for each output format to maps a Quad to a (possibly reduced) Quad or null would also help. For instance TurtleFilter would remove the named graph of a Quad and return null if any of subject, object, predicate is a variable.

In any case the documentation should mention current behaviour here similar to the statement on parsing:

By default, N3.Parser parses a permissive superset of Turtle, TriG, N-Triples, and N-Quads.
For strict compatibility with any of those languages, pass a format argument upon creation:

The current documentation is misleading:

By default, N3.Writer writes Turtle (or TriG if some quads are in a named graph).

yes.

To write N-Triples (or N-Quads) instead, pass a format argument upon creation:

This does not ensure strict compatibility like the Parser does. It's a design choice but reading the documentation one might assume strict writing as well.