Re: and then... (a curiosity)

["Karel Th�nissen" <thoenissen@garabit.nl>]

<stefan-lucks@see-the.signature> schreef in bericht 
news:Pine.LNX.4.64.0809040835270.21815@medsec1.medien.uni-weimar.de...

>> 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.

Exactly. Programming is not mathematics and the resemblance is so close that 
it is dangerous.

> 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.


> This is the point I had been trying to make in all my postings to this
> thread: "False and Undefined" is False, not Undefined!

That depends on the type of ternary logic that you consider. If the third 
value stands for 'unknown' then:
   false and unknown => false
However, if the third value is used for contradiction, then:
   false and contradiction => contradiction
That is what Ada does with exceptions.

> 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.

That would be a very bad idea. With that in place, an exception raised by a 
simple programming error, /any/ programming error, would give control to the 
else-branch. Nice if the exception is the one that you know that could 
happen, otherwise disasterous. It has become a exception handler in 
disguise, but one that confuses falsity with exceptions. If this is what you 
want you should invent a 3-branched selection statement.

I agree with other posters that the choice between conditional and 
unconditional AND should be explicit and documented. One may not like the 
syntax chosen in Ada, but it is the only sensible design.

Groeten, Karel