W3C home > Mailing lists > Public > semantic-web@w3.org > December 2012

Re: Well Behaved RDF - Taming Blank Nodes, etc.

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 12 Dec 2012 23:00:18 -0800
Cc: David Booth <david@dbooth.org>, semantic-web <semantic-web@w3.org>
Message-Id: <D02C0057-1D90-4719-B693-F3C58CD77FA9@ihmc.us>
To: Hugh Glaser <hg@ecs.soton.ac.uk>

On Dec 12, 2012, at 10:23 AM, Hugh Glaser wrote:

> OK, blank nodes :-)
> Hi Pat,
> On 12 Dec 2012, at 17:43, Pat Hayes <phayes@ihmc.us>
> wrote:
> 
>> 
>> On Dec 12, 2012, at 9:01 AM, David Booth wrote:
>> 
> <snip>
>>> A Well-Behaved RDF graph is an RDF graph that can be serialized 
>>> as Turtle without the use of explicit blank node identifiers. 
>>> I.e., only blank nodes that are implicitly created by the 
>>> bracket "[ ... ]" or list "( ... )" notations are permitted. 
>> 
>> That is too restrictive. There is a real need to be able to describe things such as "Joe's father" or "a woman in a red dress" which are naturally phrased as bnodes with identifying descriptors attached to them. 
>> 
> I don't think I understand, or I may disagree :-)
> Why is the node for "Joe's father" any different in character from the node for "Joe"?
> (Assuming this is my data I'm publishing, and I'm not reusing external URIs.)

Ah, but I *am* re-using external URIs. Isn't that one of the points of linked data, to re-use pre-existing URIs where possible? Take a real example which came up in our work. I have a photograph of a statue taken inside Manchester Town Hall, and I want to say this. There is a URI in DBpedia for Manchester Town Hall, and I plan to use it. But I don't want to say that my picture is of MTH, becuase its actually of a room inside MTH. I don't have a URI for this room and I don't want to coin one, because that URI would never get re-used by anyone and won't serve any useful purpose, and in order to make it useful I would have to invent some kind of global naming discipline for URIs denoting rooms, and I don't know how to do that sensibly. I just want to say the RDF equivalent of "**a** room in Manchester Town Hall". And that is exactly what blank nodes are for, so I would like to use them to do that. 

Another example: a picture of some celebrity standing next to a horse. I have a URI for the celebrity, but I don't have and don't need one for the horse: and if I were to invent one for each horse, then I could no longer query for retrieval of a picture of that person with "a horse", but would have to remember the URi for each of the bloody horses. But nobody gives a damn about the particular horse. 

LIfe is full of things which are important only by virtue of their existence and the relationships in which they stand, not by virtue of their particular identity. My car needs **a** battery to work, It does not need a particular battery identified by a URI. The conditions for my car to start involve a blank node, not a URI. Putting a URI to denote the battery would actually be misleading and inaccurate. Similarly my swiffer needs **a** refill, and the patient needs **a** course of antibiotics, and I need **a** drink when I am thirsty. 

> I had to make up a URI for Joe; it isn't a great hardship to ask me to make one up for his father as well.
> The only difference between the two resources seems to be that you have a property for one that you don't have for the other (perhaps "name" in this case).
> That doesn't seem a great way to choose what URIs you decide to mint and publish.

Well, frankly, my rationale for creating and publishing URIs is my business, not yours. I want to create them only when they are likely to be useful for re-use. In descriptions like the ones I am mentioning, they will never serve any useful purpose. 

> And the downside (apart from any processing problems that David might have), is that no-one can make statements about the resource I am referring to as Joe's father.

Sure they can. They can use the very same RDF construction that you use to refer to it. It that involves blank nodes, they can use blank nodes in the same way. This argument you are using (that a URI is necessary in order to refer to something) is complete baloney.

Pat


> The only reason (apart from laziness in thinking up a URI) I can see for a bnode here is if I want to prevent people making statements about the resource, which is surely not something we want to encourage in a simple RDF profile description.
> 
> Cheers

------------------------------------------------------------
IHMC                                     (850)434 8903 or (650)494 3973   
40 South Alcaniz St.           (850)202 4416   office
Pensacola                            (850)202 4440   fax
FL 32502                              (850)291 0667   mobile
phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
Received on Thursday, 13 December 2012 07:03:18 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 1 March 2016 07:42:38 UTC