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From: stefan-lucks@see-the.signature
Newsgroups: comp.lang.ada
Subject: Re: and then... (a curiosity)
Date: Tue, 2 Sep 2008 09:48:57 +0200
Organization: Bauhaus-Universitaet Weimar
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Date: 2008-09-02T09:48:57+02:00
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On Mon, 1 Sep 2008, Steve wrote:
> If short circuit evalution is permitted then more complex logic is required 
> to assure that both arguments are evaluated:
> 
>   resultA := A(x);
>   resultB := B(z);
>   if resultA and resultB then
>     ... do something
>   end if;

If I actually require to run both functions A and B, I always write 
something like the above (typically inside a declare ... begin ... end), 
because I think that makes my intention clearer than by just writing 
"and" (or "or"). 

> With Ada you can choose whether or not you want short circuit evaluation. 
> If you require short circuit evaluation you must convey that information to 
> the compiler.  If you do not require short circuit evaluation you can convey 
> that information as well.  The more the compiler knows about your 
> requirements, the greater the potential for optimization.

True. But the case that you want both expressions to be evaluated is a 
rare exception. On a *logic* level, "A and B" implies "if A is false, the 
result is false, regardless of B". So short-circuit evaluation is a 
natural and good thing for a programming language, and it is regrettable 
that Ada needs a more complex syntax for this. (Well, "A and B" also 
implies "if B is false, don't care about A", but in a programming language 
you can't have it both ways.)

But, as others pointed it: IF the default "and" supported 
short-circuit-evaluation, THEN this "and" could not be an ordinary Ada 
operator. At the language semantic level, it is much more reasonable to 
treat "and" like any other operator or function, such as "+" and "-". So 
the inventors of Ada decided on a special syntax for short-circuit 
evaluation. But I wish Ada would support some sort of lazy evaluation for 
subprogram arguments. 

So long

Stefan

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