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Re: [Core] new safeness condition

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Mon, 26 Jan 2009 18:25:30 +0100
Message-ID: <497DF20A.4030405@inf.unibz.it>
To: Axel Polleres <axel.polleres@deri.org>
CC: Sandro Hawke <sandro@w3.org>, RIF WG <public-rif-wg@w3.org>

Axel Polleres wrote:
> 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. 

Not true. The naive fixpoint computation will simply not terminate

> I know at least 2 implementations, probably more
> that would choke on unbound usage of built-ins a la
>   a(X).

I assume you mean a(1)?

>   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.

Could you explain why?

Your ruleset is the same as:

a(Y) :- a(X), add(Y,X,1).

And (u,b,b) is a completely reasonable binding pattern for an add/3
built-in predicate. Given two integers, you can compute their addition.

Best, Jos

> 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.
> Axel


Jos de Bruijn,        http://www.debruijn.net/
Many would be cowards if they had courage
  - Thomas Fuller
Received on Monday, 26 January 2009 17:26:19 UTC

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