Re: and then... (a curiosity)

[stefan-lucks@see-the.signature]

On Thu, 4 Sep 2008,  wrote:

> > On Thu, 4 Sep 2008, stefan-lucks@see-the.signature wrote:
> >> No, you misread me again. If you had read some earlier posts from me in
> >> this thread, what I am trying to express is that "and" should better do
> >> what "and then" actually does. That is it, so simple!
[...]
> I personally could life with the idea that in Ada, 'and' would have had
> conditional formantics (now  and-then)and some other operator would have
> unconditional formantics (now and). That is just swapping tokens at the
> lexical level. Fine. What is your point?

That was my point. I don't know why this raised such a lengthy thread. 

> > To be precise, here is a cut-and-paste from what I wrote in another
> > posting:
> >
> > 1. If the A- or the B-part in "if A and B" raises an exception, but the
> >   other part is false, the "right thing" (TM) to do would be to transfer
> >   control to the else clause (or below "end if", when there is no else).
> >   The intermediate result can be viewed as a three-valued logic
> >   expression, but the final outcome of the Boolean expression must be
> >   Boolean, of course.
> 
> Thank you, there it is.

Please don't take me out of context! As I pointed out, there are reasons 
not to do exactly the "right thing" in a pragmatic language. I had been 
very explicit, that if I where proposing a language change (I am not, as 
this would break compatibilty), I would propose the following:

> > 3. I would be willing to pragmatically sacrifice mathematical purity for a
> >   shortcut rule: If A is false, the expression is false, whatever B does.
> >   If A raises an exception, of if A is true and B raises an exception,
> >   the exception is propagated.
> 
> Congratulations, you just reinvented the and-then in Ada.

I didn't invent or re-invent anything. I just pointed out that the fact 
that "and then" does, what "and" should do. This is un-intiutive and 
error-prone. Exceptions may be raised which the programmer would not 
really expect to be raised. 

I described an "ideal" interpretation. You would not need "mind reading" 
capabilities to impleent it, but it would still be too heavy-weight and 
complex. And I pointed out an easy and pragmatic approach which could 
handle the issue, and which is used in quite a few other programming 
languages. 

> 1+2 may appear ideal for a mathematician, because of the symmetry it
> provides. [...]
> 
> Ideal behavior is a fata morgana from lala-land. We have to deal with
> fallible developers and machinery.

Thank you for making my point! 


-- 
------ Stefan Lucks   --  Bauhaus-University Weimar  --   Germany  ------
               Stefan dot Lucks at uni minus weimar dot de
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------