- From: Chris Welty <cawelty@gmail.com>
- Date: Sun, 30 Nov 2008 22:23:57 -0500
- To: kifer@cs.sunysb.edu
- CC: Jos de Bruijn <debruijn@inf.unibz.it>, RIF WG <public-rif-wg@w3.org>
Michael Kifer wrote: > > On Sat, 29 Nov 2008 22:39:24 -0500 > Chris Welty <cawelty@gmail.com> wrote: > >> >> Thanks, Jos. >> >> This gets me wondering. Clearly the converse of Jos' test case also holds, >> e.g. that: >> >> Document( >> Prefix(ex http://example.com/example#) >> Prefix(pred http://www.w3.org/2007/rif-builtin-predicate#) >> Group( >> ex:p(ex:a) >> Forall ?x (q(?x) :- And (ex:p(?x) External(pred:isInteger(ex:a)))))) >> >> |= (or q(ex:a) External(pred:isNotInteger(ex:a))) > > Why is this outside the language? By outside the language I mean that you cannot say (or q(ex:a) External(pred:isNotInteger(ex:a))) in BLD, it is syntactically invalid. The whole point of Jos' test cases was being able to entail something that can't be expressed in the language (in this case disjunction in the conclusion). > The problem here is that without > any kind of closed-world assumption or a unique name assumption the above leads > to a disjunction, which cannot be taken apart, unlike in logic programming. The problem I'm talking about is that the language has entailments that fall outside its expressivity. > So, with negative guards we are introducing negation through the back door, but > do not inject the standard antidotes to keep disjunction away. I'm suggesting negative guards don't create a new problem here. We already know implication is negation + disjunction, so even without negative guards you can write rules whose entailments are outside the language, including disjunction in the conclusion. So, why not just take the usual route in such cases and say that anything outside the expressiveness of the language is not entailed (or not necessarily entailed, I guess). -Chris -- Dr. Christopher A. Welty IBM Watson Research Center +1.914.784.7055 19 Skyline Dr. cawelty@gmail.com Hawthorne, NY 10532 http://www.research.ibm.com/people/w/welty
Received on Monday, 1 December 2008 03:24:39 UTC