Questions about Blank Nodes about rdf4h HOT 4 OPEN

Hi @tmciver and @koslambrou (from #106),

We have two options:

1. Validate blank node labels in the bnode function e.g.

bnode "_:foo" => Just (BNode "_:foo")
bnode "foo"   => Nothing
bnode "_foo"  => Nothing
bnode ":foo"  => Nothing

2. Generate a valid blank from an arbitrary label e.g.

bnode "foo" => BNode "_:foo"

The issue with the 2nd option is that the blank node label should be unique in a graph, but the bnode function is not associated with an RDF graph:

bnode :: Text -> Node

There could be a method added to the Rdf type class to associate bnode with graphs to allow option 2:

class (Generic rdfImpl, NFData rdfImpl) => Rdf rdfImpl where
   ...
   bnodeGen :: RDF rdfImpl -> Text -> (Node, RDF rdfImpl)

e.g. something like

let g1 = empty
let (n1, g2) = bnodeGen g1 "foo"
let (n2, g3) = bnodeGen g2 "foo"

where n1 == "_:foo1" and n2 == "_:foo2".

but this burdens all instances of Rdf so my instinct is to go for option 1 instead. The main limitation of option 1 is that the burden is on the programmer to ensure that blank node labels are unique, when used in triples added to an RDF graph.

Thoughts?

from rdf4h.

@robstewart57 I don't mind option 1 with bnode. However, what about BNodeGen ? I doesn't seem to autogenerate a an identifier. Should it be removed ?

from rdf4h.

@tmciver and @koslambrou

This is addressed in the bnodeGen branch. See this commit message: 3da3a9f

Thoughts?

from rdf4h.

I'd like to see a solution that guarantees blank node uniqueness for a given graph and so I'm in favor of the solution that adds a blank node creation function to the type class. But the sample code provided by @robstewart57:

let g1 = empty
let (n1, g2) = bnodeGen g1 "foo"
let (n2, g3) = bnodeGen g2 "foo"

suggests use of State or StateT which was also suggested by @koslambrou here. I like this solution as it solves the unique blank node issue as well as giving a nice interface for graph manipulation.

Thoughts on such a solution?

from rdf4h.