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Newsgroups: comp.lang.ada
Subject: Re: contracted exceptions
From: Georg Bauhaus <bauhaus+rm.tsoh@maps.futureapps.de>
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Date: Sun, 10 Jun 2007 20:14:16 +0200
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Date: 2007-06-10T20:15:52+02:00
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On Sun, 2007-06-10 at 12:49 +0100, Simon Wright wrote: 
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
> > On Sat, 09 Jun 2007 22:04:24 +0100, Simon Wright wrote:
> 
> >> Also, this check is about a precondition, if the caller violates
> >> the precondition she has no right to expect me not to raise
> >> unheralded exceptions!
> >
> > Oh my. "If you don't do that, then I ..." (:-))
> >
> > No, if the precondition was violated by the caller, then how this
> > caller is supposed to handle the exception it does not know?

If the precondition was violated by the caller then, yes, an assertion
error will ensue; obviously, the caller is not prepared to handle the
error because that's the point of contracts: that caller fulfills the
precondition of callee and this will get it the postcondition, not the
precondition or its violation. If not, there is a bug that the party
that *states* the contract cannot take care of, being a callee whose
contract was not fulfilled.

Obviously, the caller has a bug. I have not yet heard of a way
to document unknown caller bugs whatever contracts.