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From: stefan-lucks@see-the.signature
Newsgroups: comp.lang.ada
Subject: Re: and then... (a curiosity)
Date: Thu, 4 Sep 2008 08:45:03 +0200
Organization: Bauhaus-Universitaet Weimar
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Date: 2008-09-04T08:45:03+02:00
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> > 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.

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. 

Perhaps that is the reason why people at AdaCore always write "and then" 
instead of "and". 


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