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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada vs Fortran for scientific applications
Newsgroups: comp.lang.ada,comp.lang.fortran,comp.lang.pl1
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Date: Mon, 10 Jul 2006 09:38:47 +0200
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Xref: g2news2.google.com comp.lang.ada:5590 comp.lang.fortran:11872
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Date: 2006-07-10T09:38:51+02:00
List-Id: <comp.lang.ada>

On Sun, 09 Jul 2006 21:36:07 GMT, James Giles wrote:

> All the stuff I elided is interesting.  Many of the features are even
> good things for languages to have.  None of them were checks for
> undefined variables.
> 
> Given the Ada program fragment:
> 
>    COUNT, SUM   :   INTEGER;
> 
>    [... lots of code ...]
>    [... some paths through which assign to SUM ...]
>    [... and some don't ...]
> 
>    COUNT := SUM+1;  -- is SUM defined here or not?
> 
> In most Ada implementations, as for most other languages, all
> the bit patterns in the representation of an INTEGER data type
> are valid integer values.  There is no bit pattern representing
> NOI (Not An Integer) corresponding to the IEEE float idea of
> a NAN.  Determining whether a variable is defined or not is a
> complex problem.  It's made worse by the fact that the user can
> make the error message go away (though not usually the problem)
> by initializing the variable in the declaration.

Yes, unfortunately this is one case, where Ada's default is not safe. A
better design would be to require explicit initialization for all variables
of types with assignment. If the programmer wanted to leave something
uninitialized, he should do it explicitly:

Sum : Integer := 0;
Count  : Integer := <>;
Bar : Integer; -- Error: no public default constructor visible

But not-a-value is IMO not a good idea. Firstly, it is run-time, i.e. too
late. Secondly, it does not work for all types. What is not-a-bit,
not-an-array, not-a-user-defined-type?

If types like Integer had user-definable constructors, one could easily
achieve not-a-value functionality using subtypes.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de