W3C home > Mailing lists > Public > public-linked-json@w3.org > April 2012

Re: Clarification on blank node idioms

From: Danny Ayers <danny.ayers@gmail.com>
Date: Mon, 2 Apr 2012 18:24:55 +0200
Message-ID: <CAM=Pv=REGGp=q_y++NGPUZndQ+uAxF8G=fSgcTUL3n1vZ8Yjiw@mail.gmail.com>
To: Markus Lanthaler <markus.lanthaler@gmx.net>
Cc: public-linked-json@w3.org
Hi Markus,

Thanks. The use of "_:blank" is clear now I've seen an example, but
really wasn't obvious from the spec, I'd still suggest a doc tweak.

Re. typing of objects, right, it makes sense now you mention
literals/scalars. In this case when the context says the object of a
statement is an @id as well as the actual statement it's redundant,
but I guess that still might be useful as a sanity-checking
annotation.

Cheers,
Danny.

On 2 April 2012 12:28, Markus Lanthaler <markus.lanthaler@gmx.net> wrote:
> Hi Danny,
>
>> After not having looked at the spec for months, I'm in the process of
>> manually converting a bit of Turtle into JSON-LD. I stumbled at this
>> point:
>>
>> <http://hyperdata.org/Hello>
>>       foaf:maker [ foaf:nick "danja" ] .
>>
>> Text-searching the spec ("blank nodes") I found:
>>
>> http://json-ld.org/spec/latest/json-ld-syntax/#identifying-unlabeled-
>> nodes
>>
>> ...which didn't seem to help at all. How would the above be expressed
>> using the "_:blank" style?
>
> Generally blank nodes are generated automatically when you convert to RDF. If you want to label an object with a specific blank node you use the idiom described in the spec.
>
> So the above example (which also just has implicit blank nodes) would be expressed as follows:
>
> {
>  "@context": {
>    "maker": "http://xmlns.com/foaf/0.1/maker",
>    "nick": "http://xmlns.com/foaf/0.1/nick"
>  },
>  "@id": "http://hyperdata.org/Hello",
>  "maker": {
>    "@id": "_:blank",
>    "nick": "danja"
>  }
> }
>
>
>> One thing I'm still uncertain about is that although the JSON-LD above
>> produced the triples I was after, the context part doesn't distinguish
>> what kind of a node the object of maker/subject of nick was. This
>> appears to be possible using the context:
>>
>>       "@context": {
>>               "maker": {
>>                       "@id": "http://xmlns.com/foaf/0.1/maker",
>>                       "@type": "@id"
>>       },
>>
>> - but doesn't make any difference to the resulting triples.
>
> The reason for this is that you set the value to an object and not a literal (string, number, boolean in JSON). If you would have set it to "http://example.org/maker" you would have to add that @type declaration.
>
>
>> Leading to the question: why would/should anyone bother putting node
>> type info into the context?
>
> It's used for type coercing with scalars/literals. As explained above. Since you use an object as value, JSON-LD automatically generates an identifier for that object and "knows" that it is an IRI (when converted to Turtle for example).
>
>
> Hope this helps,
> Markus
>
>
> --
> Markus Lanthaler
> @markuslanthaler
>
>
>



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Received on Monday, 2 April 2012 16:25:28 GMT

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