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Re: Clarification on blank node idioms

From: Gregg Kellogg <gregg@kellogg-assoc.com>
Date: Mon, 2 Apr 2012 12:46:47 -0400
To: Danny Ayers <danny.ayers@gmail.com>
CC: Markus Lanthaler <markus.lanthaler@gmx.net>, "public-linked-json@w3.org" <public-linked-json@w3.org>
Message-ID: <35C4D1E8-F0A6-4963-9FAF-FCBFF5FAE5A6@greggkellogg.net>
On Apr 2, 2012, at 9:24 AM, Danny Ayers wrote:

> 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.

Danny, do you think that [1] isn't clear? Or that it is referenced too far down in the document?

[[[
At times, it becomes necessary to be able to express information without being able to specify the subject. Typically, this type of node is called an unlabeled node or a blank node. In JSON-LD, unlabeled node identifiers are automatically created if a subject is not specified using the @idkeyword. However, authors may provide identifiers for unlabeled nodes by using the special _ (underscore) prefix. This allows to reference the node locally within the document but not in an external document.
]]]

> 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.

Typically, you wouldn't use both the expanded form and a compact form if you specify the datatype in the context, but it's certainly allowed.

Gregg

[1] http://json-ld.org/spec/latest/json-ld-syntax/#identifying-unlabeled-nodes

> 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
>> 
>> 
>> 
> 
> 
> 
> -- 
> http://dannyayers.com
> 
> http://webbeep.it  - text to tones and back again
> 
Received on Monday, 2 April 2012 16:54:47 GMT

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