W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > October to December 2004

Re: UNSAID - two test cases (dawg:unbound, issues#useMentionOp)

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 22 Dec 2004 14:53:47 -0800
Message-Id: <p06001f11bdefaa185fa8@[]>
To: Steve Harris <S.W.Harris@ecs.soton.ac.uk>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>

>On Wed, Dec 22, 2004 at 05:26:01PM +0000, Andy Seaborne wrote:
>>  >Yes, but SQL for eg. has tri-value logic (true, false and NULL), so you
>>  >can meaningfully apply operators and functions to unbound values (NULL).
>>  It doesn't quite work out that simply.  It's fine for operators and
>>  functions but pattern matching isn't so straight forward.
>>  OPTIONAL (<x> ?p ?o)
>>   (?o ?q <y>)
>>  so ?o may be NULL then we have the (?o ?q <y>) and it needs to handle
>>  ?o = NULL differently.  NULL is different.
>Yes, bun in RDF you cant have a triple like (NULL ?q <y>), so that match
>will always fail. Unless I'm missing something.

Er...seems to me that you are missing a use/mention confusion. NULL 
here is supposed to be a value (right?), not a piece of syntax. So 
what does it even mean to put NULL into a triple? Or are you 
suggesting that 'NULL' - that is an actual piece of syntax, a 
four-letter character string - is used as a kind of dummy binding to 
stand for the binding in cases where a variable doesn't have a 
binding? If so, I think this really is a very bad idea. All sorts of 
problems will arise right away: eg two different unbound variables 
will be the same. It would be much better to simply leave the 
variable in the pattern in cases like this.

>  > Talking about NULLs, with all it special cases for matching and function
>>  handling, like NULL != NULL, is no different to talking about unbound
>>  variables.  Both need special handling.
>There are no special cases. Any arithemtic operation involving NULL is
>NULL, so NULL == NULL is NULL, NULL > 3 is NULL, ...

Does NULL match NULL? If ?x and ?y are both bound to NULL, are the 
bindings the same? And so on.


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Received on Wednesday, 22 December 2004 22:55:58 UTC

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