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Re: On production rules and phase I&II

From: Michael Kifer <kifer@cs.sunysb.edu>
Date: Tue, 07 Mar 2006 18:56:04 -0500
To: Frank McCabe <frank.mccabe@us.fujitsu.com>
Cc: public-rif-wg@w3.org
Message-ID: <7957.1141775764@kiferdesk>


I am with Peter P-S on this one.

Why did you say that the PR subset won't include recursion?

The mapping to Horn that we had in mind in Harold's and my presentations was

C <- A,B

as Peter said. Your explanations purporting to show that this semantics
needs to be retracted in Phase 2 (for PRs that use no negation in the body
and actions other than assert in the head) do not look convincing to me at all.


	--michael  


> Part of the proposed plan is to partition the handling of production  
> rules across the phases, with phase I being the 'pure' subset of  
> production rules.
> 
> The pure subset corresponding to an extremely small subset of horn  
> clause logic (no recursion!) and an extremely small subset of PR (no  
> modify or remove in the action part of the rule)
> 
> Quite apart from the fact that that to isolate this small subset of  
> PRs is to completely miss the point & approach of PRs, there is a  
> further technical issue.
> 
> It is possible to map a rule of the form
> 
> when A & B then assert C
> 
> into a 'horn clause' of the form
> 
> isTrue(C) <- A ^ B
> 
> A point to bear in mind: to support chaining, it will be nec. to  
> conclude from
> 
> isTrue(C)
> 
> to
> 
> C
> 
> This appears to imply a Kripke-style possible world semantics.
> 
> A much more serious point is this: in order to support modify and  
> remove (phase II aspects) it will be necessary to undo this reading  
> of PRs. I.e., to model PRs in terms of predicate logic, it will be  
> necessary to get into modeling causal relationships/actions etc. A  
> simple straw man approach being to use the situation calculus:
> 
> isTrue([C|W]) :- show(W,A), show(W,B)
> 
> (using Prolog here...:-)
> 
> where show is analogous to
> 
> show(W,C) :- C in W.
> show(W,C) :- originallyTrue(C).
> 
> Actually it would have to be considerably more complex than this and  
> one would not use situation calculus anyway because it cannot model  
> serendipitous reasoning.
> 
> The point I am trying to make is that the characterization of PRs  
> including modify and remove has to undo the prior characterization  
> *even of the 'pure subset' of PRs*.
> 
> In effect, the bottom line is:
> 
> if there is a Phase I spec published as planned, any Phase II spec  
> will not be monotonic wrt Phase I. In addition to the usual health  
> warning attached to preliminary specs being incomplete we would have  
> to declare that we already know that its wrong.
> 
> In any case, I think that the modeling of PRs (if we bother to do it)  
> will not be based on a simplistic mapping of PR rules onto a horn  
> rule: we must take the state into account. And, of course, that  
> raises the nasty frame axiom.
> 
> Frank
> 
> 
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> 
Received on Tuesday, 7 March 2006 23:56:17 GMT

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