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Re: Clarifications needed for the Collection construct (with CR)

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 26 Feb 2003 11:59:25 -0600
Message-Id: <p05111b00ba82af5f5122@[]>
To: "Karsten Tolle" <tolle@dbis.informatik.uni-frankfurt.de>, Graham Klyne <Graham.Klyne@MIMEsweeper.com>
Cc: www-rdf-comments@w3.org

>----- Original Message -----
>From: "Graham Klyne" <GK@ninebynine.org>
>To: "pat hayes" <phayes@ai.uwf.edu>
>Cc: "Karsten Tolle" <tolle@dbis.informatik.uni-frankfurt.de>;
>Sent: Tuesday, February 25, 2003 1:53 PM
>Subject: Re: Clarifications needed for the Collection construct (with CR)
>>  At 05:43 PM 2/21/03 -0600, pat hayes wrote:
>>  >>Would it not make more
>>  >>sense to enter a rdf:Bag instead? But there is also another question: Do
>>  >>need the collection construct at all?
>>  >
>>  >It was specifically requested by the Webont working group, as a necessary
>>  >requirement for OWL. So the answer is yes.
>>  I have also found in my own use of RDF that it is sometimes important to
>>  have a collection construct that is "closed";  i.e. to which no new
>>  elements can be added through RDF graph merging.  Not having this would
>>  make it impossible (I think) to express certain things without violating
>>  the basic RDF semantics.
>>  #g
>As you can read in my notes (and also in the comments of Pat), that
>collections per se
>are not "closed".
>>  At 05:43 PM 2/21/03 -0600, pat hayes wrote:
>>  >>The effect is that by entering a non-blank node someone could enter also
>>  >>to the collection construct elements from outside. This means without
>>  >>any restrictions this construct is not fixed!
>>  >Right, it is not. Nothing is 'fixed' in this sense in RDF.
>The constructs collection or container are only "closed" or "fixed" when
>blank nodes are used.

I think there is a misunderstanding here. One can use this word 
'closed' in two senses.

In the sense that Graham is using it, and the sense that was used in 
the request to the RDF WG from the Webont WG, it means that it is 
*possible* to write an RDF graph which fully determines the members 
of a collection. Note that this is not possible for an RDF container, 
hence the need for the collection syntax.

In the sense that Karsten is using it, 'closed' means (I think) that 
*any* syntactically correct use of the collection syntax will define 
a closed collection. Indeed, RDF collections are not 'closed' in this 
sense, since RDF imposes almost no syntactic restrictions on RDF 
graphs. That is why we always use scare quotes when referring to 
"well-formed" lists, since even "non-well-formed" lists are perfectly 
correct, ie well-formed, RDF syntax. RDF is very forgiving in its 
syntax: it allows you to assert nonsensical things.


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Received on Wednesday, 26 February 2003 12:59:31 UTC

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