Re: [Core] new safeness condition

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).
   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.

Axel

-- 
Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
Galway
email: axel.polleres@deri.org  url: http://www.polleres.net/

Received on Monday, 26 January 2009 17:16:52 UTC