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Date: Mon, 07 Jun 2010 13:13:18 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Ada noob here! Is Ada widely used?
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Date: 2010-06-07T13:13:19+02:00
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On 07.06.10 09:56, Martin Krischik wrote:
> Am 06.06.2010, 23:22 Uhr, schrieb Georg Bauhaus
> <rm-host.bauhaus@maps.futureapps.de>:
> 
>> On 6/6/10 3:10 PM, Dmitry A. Kazakov wrote:
> 
>> It seems worthwhile mentioning that DbC's primary purpose
>> is not to augment programs with run-time checks; rather, DbC asks
>> for programmers who write as if there was no assertion monitoring
>> but who explain their code with pre/post/inv.  The operator may
>> turn on run-time monitoring so that he/she is notified if something goes
>> wrong (disproving the programmers' assumptions; the monitor stops
>> the program or runs the remains in a debugger).
> 
> Sounds a lot like Java assertions. We makes me think:
> 
> If the captain thinks that there might be icebergs on route he orders to
> take on live boats. If not he leaves them at shore to save petrol and
> gain extra speed. The later is default if the captain forgets to make a
> decision.

A captain requesting lifeboats does not prepare for exception
handling. You use lifeboats when the ship sinks, the ship's
components have failed, possibly after an impact of unexpected
I/O (an iceberg popping up, say).
By analogy, sinking is when the program stops.

DbC, however, has to do with designing components of the ship.
You can have rescue clauses (exception handlers)
in DbC.  But the idea is that a correct program is better.
Yet, praising one-self to be preparing for exceptional
situations is only as safe normal flow of control within
and from exception handlers---if they are at all prepared
to handle a situation.

Exceptional situations is "unprogrammed" exceptional state,
since this is what "exceptional" is about, if anything.
And if a hammer  turns out to be made of wood,
some contract is broken. If no one thought of such a preposterous
choice of material for a hammer, it will be reasonable
to expect there is no handler for this outcome.
Hence no rescue clause.  Next ship will have a type, though,
that allows distinguishing different kinds of hammer.