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From: "David Cressey" <dcressey@verizon.net>
Newsgroups: comp.databases.theory
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Subject: Re: 3vl 2vl and NULL
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Date: Mon, 12 Dec 2005 15:42:41 GMT
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Xref: dp-news.maxwell.syr.edu comp.databases.theory:35078


"JOG" <jog@cs.nott.ac.uk> wrote in message
news:1134395485.579347.115980@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execution time,  the variables have been replaced by "values".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.