Re: Why no Propagates?

[gauthier@unilim.fr (Michel Gauthier)]

In article <mheaney-ya023180001710961746550001@news.ni.net>,
mheaney@ni.net (Matthew Heaney) wrote:

>>  In article <326391B6.4B23@gsfc.nasa.gov>, Mike Stark
>>  <michael.e.stark@gsfc.nasa.gov> wrote:
>>  
>> [...]
>>  
>>  The designers of Ada 83 considered including explicitly naming the
>>  exceptions propagated by a subprogram:
>>  
>>     procedure P (X : Integer) propagates A, B, C;
>>  
>>  This feature, and the reasons it was rejected, is discussed in section
>>  14.5.2  of the Ada 83 Rationale.
>>   [...]
>>  Personally, I think it would be a cool feature to have.  Let me the
>>  programmer decide whether I want to incur the code-expansion penalty.  (And
>>  if the procedure tries to propagate something other than what's stated,
>>  then raise Program_Error.)
>>  
>>  To the designers of Ada 95: Was this feature considered for inclusion?  Is
>>  it a candidate for inclusion in Ada 0X?

I was not in the Ada9X team, hence I can't give any information about 
the question itself, but I guess you could reflect about some cases :

 - Consider a generic power function. It has a "*" parameter which can
"propagate X" for an overflow. Consequently, the power function can 
propagate the same exception, which must be a parameter of the construct.
Ada95 can do this, of course, but it is rather complex.

 - Consider a generic iterator. You reach the same problem, but complicated
by the fact that the iterator can raise exceptions by itself (for example
Device_Error if you scan a file).

 - Consider some use of "exception identities" in order to transmit an
exception through rendezvous, and more generally any use of an
exception identity variable. Then you need to make"propagate the
exception identified by V at some moment of the execution" explicit .
How will you write this ?

I fully agree that " it would be a cool feature to have", but I concluded
long ago that it would be very difficult to make it compatible with
all the other features of the language. Possibly an additional effort
on ANNotated Ada in this direction would have been useful, but
we cannot hope a full solution.

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