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Re: [Core] new safeness condition

From: Axel Polleres <axel.polleres@deri.org>
Date: Mon, 26 Jan 2009 17:15:45 +0000
Message-ID: <497DEFC1.9040304@deri.org>
To: Sandro Hawke <sandro@w3.org>
CC: Jos de Bruijn <debruijn@inf.unibz.it>, RIF WG <public-rif-wg@w3.org>

Sandro Hawke wrote:
>> Looks sound, but implies that finiteness is no longer is requested.
>> Was that what was agreed? (Am just catching up with reading minutes)
> My sense of the group was that everyone present prefered to give up on
> finiteness, but we told that you had an argument for it, so we wanted to
> wait until we'd at least heard that argument.  (Something about datalog
> implementations, I think.)
>      -- Sandro

In principle, bottom-up evaluation is only possible if a finite Herbrand 
universe is guaranteed. I know at least 2 implementations, probably more 
that would choke on unbound usage of built-ins a la

   a(X+1) :-  a(X).

I mean, these engines would just not be able to handle those kind of 
rules. That's the simple argument. Obviously, these engines could handle

   a(Y) :-  a(X), Y = X+1, HU(Y).

but I see that this is restrictive. It is just question of what then 
core compliance means. I thought that there was some agreement that Core 
should be something like safe datalog, i.e. something that could be 
handled both with bottom-up and top-down engines.


Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
email: axel.polleres@deri.org  url: http://www.polleres.net/
Received on Monday, 26 January 2009 17:16:52 UTC

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