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Newsgroups: comp.lang.ada
Subject: Re: Very confused by Ada tasking,
 can not explain the execution  outcome.
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From: Stephen Leake <stephen_leake@stephe-leake.org>
Date: Sun, 31 Aug 2008 09:37:20 -0400
Message-ID: <uljyde25r.fsf@stephe-leake.org>
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Date: 2008-08-31T09:37:20-04:00
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Damien Carbonne <damien.carbonne@free.fr> writes:

> climber.cui@gmail.com a �crit :
>> Thank you Jim for modifying the program and making it work. I saw how
>> you modify the guarding conditions for the procedures. In fact, the
>> conditions are fine by themselves, because i wrote the same program in
>> Java. However, the modified program is working now. It indeed reminds
>> me that Ada probably follows the 'strict evaluation' rule when it
>> comes to the boolean expressions, so in
>>   if Next=1 and State=0 and D<N and Needs(D)  then ...
>>   Ada probably would evaluate all the predicates connect by logical
>> AND, but as soon as D=N is reached, array index for Needs(_) would go
>> out of bound (because Needs is defined to be  array(0..N-1) of
>> boolean). I am just guessing here.
>
> Use "and then" instead of "and" to obtain what Java does with "and" :
> --> if Next=1 and then State=0 and then D<N and then Needs(D)  then ...

A better style is to use 'and then' only when it is actually necessary
to protect against an exception. In this case, that would be:

    if Next=1 and State=0 and (D<N and then Needs(D))  then ...

since Needs(D) will raise an exception if D<N is false.

That lets the compiler more completely optimize the full expression;
'Next=1' and 'State=0' can be done in parallel register operations. 

It also tells the reader that they should consider exceptions for the
D part of the expression, but they _don't_ have to consider them for
the other part.

Ada is a very expressive language; it helps both the reader and the
compiler if the code is precise.

-- 
-- Stephe