Re: 3vl 2vl and NULL

"JOG" <jog_at_cs.nott.ac.uk> wrote in message news:1134395485.579347.115980_at_g43g2000cwa.googlegroups.com...
> David Cressey wrote:
> > There is a 3VL whose values are [TRUE,FALSE,MEANINGLESS] when a formula
> > comes up MEANINGLESS, it means that it is not a "well formed formula"
or
> > WFF. The definition of a WFF is outside this discussion. But an
assertion
> > of MEANINGLESS is a denial of TRUE and FALSE.
>
> Hi David, am I correct in saying that in a domain like {1,2,3,null}
> something like 1<null is an example of one of these non-WFF?

My answer is, I'm really not sure.

Let's say the programmer coded:

IF (X < Y) THEN EXIT

When reading the source, X and Y stand for "variables". However, at execution time, the variables have been replaced by "values". The "arithmetic element" of the CPU is capable of doing both integer arthmetic and "boolean arithmetic", but it doesn't do "algebra". It works only with "known values" for the inputs, and uses the hardware equivalent of "truth tables" or "addition tables" or what have you to generate outputs.

If Y is NULL, then I expect the arithmetic element to generate an exception, instead of generating either TRUE or FALSE. From there, I don't know what happens.

In reality, I expect the operation to be intercepted long before it gets down to the bare metal. But that's another discussion.

A non well formed formula might look more like this:

IF (X<) THEN EXIT

But that one can be flagged at parse time, instead of waiting till execution time. I *think* but I'm not sure that languages like Lisp can generate non well formed formulas that are indetectable as such until evaluation is attempted. I *know* that a Lisp programmer can generate and execute Lisp code dynamically.

Sorry I can't be more helpful. Received on Mon Dec 12 2005 - 16:42:41 CET