Update dataset examples

[gkellogg]

As discussed for #182, we split out the dataset examples, based on the decision to have the default graph be used for consumers expressing a graph. This implies that uses of datasets for describing provenance information would better store such information in a named graph, and assert information against the default graph, rather than the other way around. For example, Example 50 in the syntax spec includes the following:

{
  "@context": {
    "generatedAt": "http://www.w3.org/ns/prov#generatedAtTime",
    "Person": "http://xmlns.com/foaf/0.1/Person",
    "name": "http://xmlns.com/foaf/0.1/name",
    "knows": "http://xmlns.com/foaf/0.1/knows",
    "xsd": "http://www.w3.org/2001/XMLSchema#"
  },
  "@id": "http://example.org/graphs/73",
  "generatedAt": { "@value": "2012-04-09", "@type": "xsd:date" },
  "@graph":
  [
    {
      "@id": "http://manu.sporny.org/i/public",
      "@type": "Person",
      "name": "Manu Sporny",
      "knows": "http://greggkellogg.net/foaf#me"
    },
    {
      "@id": "http://greggkellogg.net/foaf#me",
      "@type": "Person",
      "name": "Gregg Kellogg",
      "knows": "http://manu.sporny.org/i/public"
    }
  ]
}

As this asserts information in a named graph, and the describes how metadata about that (generatedAt) in the default graph, this would not be appropriate to use when expecting a graph rather than a dataset, as only the <http://example.org/graphs/73> generatedAt "2012-04-09"^^xsd:date would result, when that's clearly not the intention.

It's not up to this group to describe how to use datasets to describe document provenance, but it makes sense that our examples are in line with the expected behavior. As an alternative, consider the following statement of this example:

{
  "@context": {
    "generatedAt": "http://www.w3.org/ns/prov#generatedAtTime",
    "Person": "http://xmlns.com/foaf/0.1/Person",
    "name": "http://xmlns.com/foaf/0.1/name",
    "knows": "http://xmlns.com/foaf/0.1/knows",
    "xsd": "http://www.w3.org/2001/XMLSchema#"
  },
  "@graph": [
    {
      "@id": "http://manu.sporny.org/i/public",
      "@type": "Person",
      "name": "Manu Sporny",
      "knows": "http://greggkellogg.net/foaf#me"
    },
    {
      "@id": "http://greggkellogg.net/foaf#me",
      "@type": "Person",
      "name": "Gregg Kellogg",
      "knows": "http://manu.sporny.org/i/public"
    },
    {
      "@id": "",
      "@graph": {
        "@id": "",
        "generatedAt": { "@value": "2012-04-09", "@type": "xsd:date" }
      }
    }
  ]
}

This states the same information, but does so using the default graph for the data. The provenance information is in a graph named the same as the document location. Basically, in TriG, this would like like the following:

@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix prov: <http://www.w3.org/ns/prov#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

{
  <http://greggkellogg.net/foaf#me> a foaf:Person:;
     foaf:knows: "http://manu.sporny.org/i/public";
     foaf:name: "Gregg Kellogg" .

  <http://manu.sporny.org/i/public> a foaf:Person:;
     foaf:knows: "http://greggkellogg.net/foaf#me";
     foaf:name: "Manu Sporny" .
}

<> {
  <> prov:generatedAt "2012-04-09"^^xsd:date .
}

It's likely that more triples would be needed to locate the provenance information, and type it as such, but that's consistent with the existing example.

[lanthaler]

Maybe it would be simpler to change the content of the examples altogether!? For example state something in the default graph and say it was derived from some named graph.. just an idea.

[lanthaler]

RESOLVED: Add a note to the "Relationship to RDF" section to specify that if clients that do not understand Datasets are to be supported using JSON-LD, that the primary information should go in the default graph.

[lanthaler]

According the resolution I've added a note to the spec making it clear that the primary data has to be put in the default graph if publishers want to support both dataset and graph syntaxes.

Unless I hear objections I will therefore close this issue in 24 hours.