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Re: Converting RDF to JSON-LD : shared lists between graphs

From: Peter Ansell <ansell.peter@gmail.com>
Date: Wed, 23 Jul 2014 09:28:03 +1000
Message-ID: <CAGYFOCRvR-9n+yf+ZU6KkULYGAM42AOBL8j7zPNYXi0+qmX+Qw@mail.gmail.com>
To: Andy Seaborne <andy@apache.org>
Cc: Linked JSON <public-linked-json@w3.org>, "public-rdf-comments@w3.org" <public-rdf-comments@w3.org>
On 22 July 2014 19:21, Andy Seaborne <andy@apache.org> wrote:
> We are encountering an issue when converting RDF Datasets to JSON-LD.
>
> The problem is with blank nodes that are shared between graphs and lists.
>
> In TriG (yes, this is a synthetic reduced test case that captures a smaller
> example that might appear for real):
>
> ---------------------
> # Bnode references across graph and lists
> PREFIX :        <http://www.example.com/>
> PREFIX rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
>
> :G {
>   # Written in short form it would be:
>   # :z :q ("cell-A" "cell-B")
>   # but we want to share the tail ("cell-B")
>
>   :z :q _:z0 .
>
>   _:z0   rdf:first "cell-A" .
>   _:z0   rdf:rest  _:z1 .
>
>   _:z1   rdf:first "cell-B" .
>   _:z1   rdf:rest rdf:nil .
> }
>
> :G1 {
>    # This references the tail  ("cell-B")
>    :x :p _:z1 .
> }
> ---------------------
>
> The triple in :G1 references into the list in :G.
>
> But as we understand the conversion algorithm, section 4 only considers each
> graph in turn and so does not see the cross graph sharing.
>
> Is this a correct reading of the spec text?
>
> Part 4 of the conversion algorithm has
> "For each name and graph object in graph map: "
>
> so 4.3.3.* walks back up the list in one graph only.
>
> (Conversion generated by jsonld-java : it does not matter if compaction is
> applied or not):
> ---------------
> {
>   "@graph" : [ {
>     "@graph" : [ {
>       "@id" : ":z",
>       ":q" : {
>         "@list" : [ "cell-A", "cell-B" ]
>       }
>     } ],
>     "@id" : ":G"
>   }, {
>     "@graph" : [ {
>       "@id" : ":x",
>       ":p" : {
>         "@id" : "_:b1"
>       }
>     } ],
>     "@id" : ":G1"
>   } ],
>   "@context" : {
>     "@base" : "http://www.example.com/",
>     "" : "http://www.example.com/",
>     "rdf" : "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>   }
> }
> ---------------
>
> There is no _:b1 in :G to refer to because the algorith generated @list and
> its implicit bNodes don't have labels.
> This is a different dataset with no shared bNode.
>
> If it is all the same graph (s/:G1/:G/), the RDF dataset structure is
> correctly serialized.
>
>         Andy
>

Just to add a reference from RDF-1.1 that may be relevant:

"Blank nodes can be shared between graphs in an RDF dataset."

http://www.w3.org/TR/rdf11-concepts/#section-dataset

The RDF dataset to JSON-LD algorithm doesn't seem to allow for that right now.

Cheers,

Peter
Received on Tuesday, 22 July 2014 23:28:30 UTC

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