W3C home > Mailing lists > Public > public-linked-json@w3.org > February 2013

Re: The default graph, @id, and @graph

From: Erik Isaksson <erikis@kth.se>
Date: Tue, 5 Feb 2013 14:17:50 +0100
Message-ID: <CAK9bEGxDO3pFdO_c1ExG-TU8-494c3yAdL8+CoDzRxQoeWPhkg@mail.gmail.com>
To: Manu Sporny <msporny@digitalbazaar.com>
Cc: JSON-LD CG <public-linked-json@w3.org>
On Sat, Feb 2, 2013 at 8:14 PM, Manu Sporny <msporny@digitalbazaar.com> wrote:
>> Niklas Lindström: .. "When @graph is used in a document's top-level
>> object which has no other properties that are mapped to an IRI or a
>> keyword it is considered to express the otherwise implicit default
>> graph."
>>
>> Niklas Lindström: my friend had a problem with this wording. … If I
>> understand him, he wasn't sure it was enough to add @id; he wanted to
>> make sure he had a named graph.
>>
>> Manu Sporny: The text is wrong, but let's try to fix it now. I'll
>> make a change right after the call.
>
> Actually, the text is correct Niklas. The sentence is talking about how
> to express the 'default graph'. If he added @id, it would no longer
> express the default graph.
>
> Is there some other wording we could introduce to make this more clear
> to your friend?

"When @graph is used in a document's top-level object which has no
other properties that are mapped to an IRI or a keyword it is
considered to express the otherwise implicit default graph. This
mechanism can be useful when a number of nodes do not directly relate
to one another through a property or where embedding is not desirable
to the application."

My thoughts here were: Wouldn't it be better (mainly as in easier to
understand intuitively) to use a separate keyword (e.g., @default)
when the intention is to provide the default graph?

Also, isn't it possible (although an unusual case) that one would
actually like to have a top-level object that has a @graph, but no
other properties (not even @id, i.e., it is a blank node)? This would
currently result in the contained graph being interpreted as the
default graph, which isn't the intention here. Even without this
(perhaps strange) case, it may be confusing that (seemingly) unrelated
properties affect the semantics of @graph (i.e., whether it's the
default graph or a named graph).

In my own use case, I'm fine with inserting a blank node (or perhaps
urn:uuid:) identifier into the top-level object to make sure that any
contained @graph is never interpreted as the default graph. However,
I'd like to avoid the semantic confusion described above (it did
confuse a colleague of mine when he was looking at the spec recently).
Whether to allow @graph for blank nodes is probably a separate
discussion; I'm using blank nodes as graph names when describing
resources as part of creation requests (i.e., the resource hasn't been
assigned an identifier yet, but it will have an IRI after creation).

The part of the spec quoted above could be rewritten as "...or a
keyword (e.g., @id)" and "...or where embedding is not desirable to
the application, but a shared @context is still to be used for all of
the nodes". I know that shared context is mentioned in the next
paragraph, but I think the sentence makes more sense if shared context
is mentioned there already. (Otherwise, an array can equivalently, and
I think preferably, be used at the top-level.) By the way, if an array
is at the top-level, how should "a document's top-level object" be
interpreted?

I hope this is useful to you. Thank you for all of your great work on
JSON-LD, and I hope I can be of a little help as well with these and
perhaps other, future comments.

Best regards,
Erik
(aka Niklas' friend)

>
> -- manu
>
> --
> Manu Sporny (skype: msporny, twitter: manusporny, G+: +Manu Sporny)
> Founder/CEO - Digital Bazaar, Inc.
> blog: Aaron Swartz, PaySwarm, and Academic Journals
> http://manu.sporny.org/2013/payswarm-journals/
>
Received on Tuesday, 5 February 2013 13:18:23 GMT

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