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Re: [DTB/Abridged syntax] ISSUE on -equals and -not-equals predicates?

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Thu, 04 Dec 2008 17:29:46 +0100
Message-ID: <4938057A.2050702@inf.unibz.it>
To: Axel Polleres <axel.polleres@deri.org>
CC: "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>


Axel Polleres wrote:
> 
> Hi,
> 
> I introduced, as for the other datatypes (along with an editors note),
> -equals and -not-equals predicates for the rdf:XMLLiteral datatype.
> 
> There is an obvious overlap between the -equals predicates and '=', i.e.
> the '=' built-ins are superfluous, it seems, for dialects that support
> equality.
> 
> That is not the case for the not-equals predicates, though:
> 
> All the -not-equals predicates, i.e.  pred:numeric-not-equal,
> pred:string-not-equal, pred:dateTime-not-equal, pred:date-not-equal,
> pred:time-not-equal, pred:duration-not-equal, pred:XMLLiteral-not-equal,
> pred:text-not-equal
> 
> are defined on the same domains as their positive counterpart with
> reversed truth-values.
> 
> That has the consequence though, that they are not possibly emulated by
> negation as failure, i.e. it is in general not true to say that the
> 
> -not-equals version is true, whenever the -equals version is NOT true.
> 
> 
> An example:
> 
>  _a:-  External (pred:numberic-equals("blabla", 1) )
>  _b:-  External (pred:numberic-not-equals("blabla", 1) )
> 
> entails neither _a nor _b, however

Both rules contain errors, and since errors mean arbitrary truth values,
you don't know whether _a or _b are entailed, and in practice I suspect
applications will terminate with some error message.

> 
>  _a:-  "blabla" = 1
>  _b:-  naf ( "blabla" = 1 )
> 
> would entail _b.
> 
> So, 2 questions:
> 
> 1) Do we want this behavior or do we want the negation as failure
> behavior of inequality?

We want this behavior, since this is the only reasonable way
to define these built-in predicates.

> 2) Do we want the redundancy of equals predicates?

Yes, for dialects that do not supported equality.


Best, Jos

> 
> I think we need these things clarified, to be able to determine how
> '=' and "!=" shortcuts in an abridged syntax should be defined.
> 
> Axel
> 
> 
> 

-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
----------------------------------------------
No one who cannot rejoice in the discovery of
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scholar.
  - Donald Foster


Received on Thursday, 4 December 2008 16:29:50 GMT

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