Re: Problem trying to implement generics.

[Tucker Taft <stt@averstar.com>]

Robert A Duff wrote:
> 
> "Ayende Rahien" <Dont@spam.me> writes:
> 
> > BTW, is there any reason why I can't use out parameter with a function?
> 
> Because it's not allowed.  ;-)
> 
> Hmm, are we going have this argument again?  Oh well, it's kind of fun,
> in a time-wasting sort of way.  Here goes:
> 
> If I ran the circus, functions would be allowed to have 'out' and 'in
> out' parameters.

It would be interesting to see where such functions would
typically be used.  My experience is that the most common
uses for such functions are:
   1) Returning a status value (probably better handled with
        exceptions in Ada)
   2) Using in a "while" loop, such as:
         while Get_Next(Iterator, Val) loop
             ...
         end loop;

My own objection to having functions with OUT parameters is
that it increases the possibility for confusion for the
reader.  When you see a procedure call, there is little doubt
that it is being called for its side-effects.  When you see
a function call (except in certain stylized locations such
as the "while" condition above), you presume it is being
called for its value, or at least it won't directly change
the value of some elementary parameter passed to it.

I wonder if we might create a new loop syntax to accommodate
the "while" case above, without opening up the function-with-
out-parameters box.

Alternatively, just get used to one of the idioms:

    loop
        Get_Next(Iterator, Val, Done);
      exit when Done;
        ...
    end loop;

or
    while More(Iterator) loop
        Get_Next(Iterator, Val);
        ...
    end loop;

Interestingly enough, the standard Iterator in Java
uses:
     while (Iter.hasNext()) {
         X = Iter.next();
         ...
     }

even though Java has functions with side effects (although
in fact, only with side-effects on by-reference operands,
since it has no parameter modes at all!).

-Tuck

> 
> - Bob
> 
> P.S. One workaround is to use access parameters.

Note, that "C" uses essentially the same "workaround" since
C has no "out" parameters.

-- 
-Tucker Taft   stt@avercom.net   http://www.averstar.com/~stt/
Chief Technology Officer, AverCom Corporation (A Titan Company) 
Burlington, MA  USA (AverCom was formerly the Commercial Division of AverStar:
http://www.averstar.com/services/ebusiness_applications.html)