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Re: car color defaults: a story about Scoped Negation As Failure/log:notIncludes

From: Michael Kifer <kifer@cs.sunysb.edu>
Date: Thu, 07 Jul 2005 14:58:48 -0400
To: Dan Connolly <connolly@w3.org>
Cc: "public-rule-workshop-discuss@w3.org" <public-rule-workshop-discuss@w3.org>
Message-Id: <20050707185848.77C2CCB5D3@kiferserv.kiferhome.com>



Dan,

You are talking about it as if SNAF is a new idea or that anybody disagrees
that default negation needs to have scope.

In fact, every use of default negation *always* had a clear scope and I am
not aware of anyone who (thoughtfully) advocated default negation and
imagined otherwise.

Because the KB that is the Web doesn't have defined boundaries, no known
formalization of default negation (that I know of, anyway) can deal with
scope-less default negation.

Now, regarding N3, the use of SNAF in N3 is very limiting and ad hoc. There
are systems that supported SNAF before N3 (without calling it SNAF) and
where this idea is much more developed. (E.g., FLORA-2.)

It is good, however, that people are now beginning to see the light (i.e.,
the need for default negation :-).

Finally, I would like to say once again that the term "negation as failure"
is bad and misleading. Traditionally, negation as failure meant the
particular strategy used in Prolog. NAF is just one of the known forms of
default negation.


	--michael  

> 
> Here's a sort of story we've written to explain and motivate
> log:notIncludes, aka Scoped Negation As Failure[1].
> 
> [[
> Because a formula is a finite size, you can test for what it does not
> say, with log:notIncludes. Here, we have a rule that is the
> specification for a car doesn't say what color it is then it is black.
> 
> this log:forAll :car.
> { :car.auto:specification log:notIncludes {:car auto:color []}}
>     => {:car auto:color auto:black}.
> 
> Note the use of [] here in the nested formula as a blank node. If the
> spec said that a car had color green, then that would mean that the car
> had color something, so we would say that the formula included :car
> auto:color [] . A statement with a [] in it you can think of as weaker
> version of one with a value for the color.
> 
> This is a way to do defaults. Notation3 as it is doesn't have defaults,
> because on the web, you can't say "if nothing says it is another color".
> You can never know in the whole web whether anyone has given a color.
> Also, if we start to just loosely talk about defaults in the sense of if
> you don't already know a color, then different agents will end up
> drawing different conclusions from the same data, which is not a good
> foundation for a scalable web. So, you handle defaults by first running
> rules to work out everything which is specified, and then on the result
> of that do a notIncludes rule like that above to implement the default
> values.
> 
> ]]
>  -- section "Implementing defaults and log:notIncludes" 
>   of part "Reaching out onto the Web"
>   of the Semantic Web Tutorial Using N3
>   http://www.w3.org/2000/10/swap/doc/Reach#Implementi
> 
> 
> 
> [1] "The term Scoped Negation As Failure (SNAF) was proposed to indicate
> NAF where the scope of the search failure is well defined."
> http://www.w3.org/2004/12/rules-ws/report/#negation-as-failure
> 
> -- 
> Dan Connolly, W3C http://www.w3.org/People/Connolly/
> D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E
Received on Thursday, 7 July 2005 18:58:55 GMT

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