Re: and then... (a curiosity)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 4 Sep 2008 08:45:03 +0200, stefan-lucks@see-the.signature wrote:

>>> True. But, at the logic level, the Ada-statement
>> 
>> What is the "logic level"?
>> 
>> I you mean "abstract Boolean logic, like in college", then there is no
>> such thing as Constraint_Error, nor side effects. So of course it
>> gives you a bad understanding of programming languages.
> 
> Actually, there is Constraint_Erorr, though under a different name. ;-)
> 
> If your Boolean function is allowed to raise an exception, this is the 
> same as returning "undefined" in three-valued logic, as I pointed out in 
> another posting in this mail.

Why "undefined", it is rather "contradictory" (in four-state Belnap logic)

> This is the point I had been trying to make in all my postings to this 
> thread: "False and Undefined" is False, not Undefined! Thus, "if X/=0 and 
> Y/X<100" should go into the else-branch, instead of raising an exception.
> The shortcut version "if X/=0 and then Y/X<100" does exactly the right 
> thing. 

Well, indeed

    0 /\ _|_ = 0
    0 /\ T = 0

Here 0 = False, 1 = True, _|_ = Uncertain, T = Contradiction. False under
/\ always renders False.

Nevertheless Boolean logic /= Belnap logic, though the former is contained
in the latter. 

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de