Re: NULLs: theoretical problems?

Jon Heggland wrote:
> Keith Duggar wrote:
>
> > I'm not trying to have anything. I'm trying to understand what
> > the "write DEF" prescription buys us over say the "Interactive
> > Mathematical Proof System" of Farmer, Guttman, and Thayer that
> > has exactly the property I described that any formula is false
> > if any variable is NULL. Again, I am failing to grasp what the
> > "write DEF" prescription buys us. I would like to understand.
>
> In this system, how can you check whether a given variable actually is
> NULL or not?

I'm not so sure what you mean by "check". Any proposition in IMPS is FALSE if any of its arguments is undefined. So as an example (x OR TRUE) is false if x is NULL. Does this suffice for what you mean by "check"?

KHD Received on Sat Sep 01 2007 - 17:43:29 CEST