Re: Bus error

[Georg Bauhaus <bauhaus.rm.tsoh@maps.futureapps.de>]

Robert A Duff wrote:
> Georg Bauhaus <bauhaus.rm.tsoh@maps.futureapps.de> writes:
> 
>> Dmitry A. Kazakov wrote:
>>> On Fri, 29 Jun 2007 17:44:22 +0200, Georg Bauhaus wrote:
>>>
>>>> Dmitry A. Kazakov wrote:
>>>>>    function Very_Positive return Integer is
>>>>>    begin
>>>>>        return -1;
>>>>>    end Very_Positive;
>>>>>
>>>>>    Oops : Positive renames Very_Positive; -- This is OK!
> 
>> Oops will at some point raise Constraint error;...
> 
> Dmitry's complaint is the above will NOT raise C_E.

I see, yes, it won't raise at that point. I didn't jump to conclusions about
faulty language design because (1) Ada has new <type> when needed and
(b) using subtypes will, I guess, require some compromises, i.e. range
checks at run time.
So I will get a (possibly surprising) Constraint_Error when I write

  Some_String(Oops).

Positive'image is just as "tolerant". 


>> ...how can a compiler
>> be supposed to known that some Integer function will *always*
>> return non-Positives?
> 
> Indeed!  Likewise, how is the _programmer_ supposed to know?

A programmer may be using Positive when it is clear from the logic
of a program that in a region of the program, function Very_Positive
will indeed compute positive return values from its inputs (and thus
be aptly named). Otherwise, yes, mentioning subtype Positive in the
renaming declaration would not be the best choice.  When in other
circumstances, the function returns values not necessarily positive,
I expect the renaming to mention a different subtype. But OK, 
this subtype name is just a hint of varying value for the reader.


>> ...What makes you think that the programmer
>> who has written the Oops doesn't know what he is
>> doing using a subtype, not a new type?
>>
>> I think it is a program design fault, if a fault at all (or a flaw as
>> Bub Duff notes).
> 
> Yes, I agree with you and with my evil twin Bub ;-)

I'm sorry!  <:-|


-- Georg
Too many scripting language programs, log files, and no sunshine for
too many days here
http://www.boersenspiel.de