Re: Design By Contract

[nospam@thanks.com.au (Don Harrison)]

Jon S Anthony wrote:

:In article <EFx966.8LB@syd.csa.com.au> nospam@thanks.com.au (Don Harrison) writes:
:
:> System validity, in its broadest sense, means that all usage of entities 
:
:Well, you can define it anyway you wish, I suppose, but I was simply
:refering to its "traditional" usage vis-a-vis Eiffel.

Sure.

:> While Eiffel currently fails WRT polymorphic usage, Ada fails WRT other 
:> usage. See Deja News for details.
:> 
:> So, your claim that Ada doesn't have system validity problems is false.
:
:As I recall, that scenario required the _concious_ and _explicit_
:choice to violate certain aspects via Unchecked_Conversion.  

No, I'm not referring to Unchecked_Conversion. As we both know, all bets are 
off when you choose to do naughty things.

I recall we agreed that neither Ada nor Eiffel stopped you statically from 
trying to attach an out-of-range value at runtime - for Ada, to a subtype;
for Eiffel to a precondition-constrained type. 

However, I recall Norman Cohen (I think) volunteering a hole in Ada's type
system - don't remember the details, though. 

BTW, where is Norm these days? Haven't seen him in c.l.a. lately.

:Not at all the same as the concealed land mines of Eiffel.  But, I'm not
:going to say these are somehow ultra-bad or something.  In practice
:they are probably not that big of a deal if you are aware and careful
:to avoid the situations.  

Quite true. Essentialy, being able to break polymorphism isn't any worse
than the uncertainty of not knowing whether a precondition will always be 
satisfied. It can justifiably be regarded as a variant of just that. There's
no question, of course, that it would be preferable to exclude such 
errors statically, rather than having to deal with them dynamically.

:Besides, you snipped the one of two smileys
:I had in there...

True.


Don.                     (Reverse to reply)
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Don Harrison             au.com.csa.syd@donh