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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Gnat 2013 is out!
Date: Fri, 31 May 2013 14:03:08 +0200
Organization: Tidorum Ltd
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Date: 2013-05-31T14:03:08+02:00
List-Id: <comp.lang.ada>

On 13-05-31 13:30 , Stefan.Lucks@uni-weimar.de wrote:
> On Thu, 30 May 2013, Randy Brukardt wrote:
> 
>> "Adam Beneschan" <adam@irvine.com> wrote in message
>>
>>> I've always wanted some kind of feature in Ada that would allow a
>>> caller to provide a "dummy" for OUT parameters, without having to
>>> declare a new variable.  The compiler would allocate a temporary
>>> object (and a separate one for each use of a "dummy") and then
>>> discard it after the call.  It wouldn't work well when parameter
>>> types are unconstrained array or discriminant records, though.
>>
>> Hmm, that seems like a good idea to me.

To me too, but only of the "nice to have" priority.

>> But what would the syntax be? <>
>> maybe?
>>
>>         My_Proc (Obj1, Obj2, Result => <>);
>>
> 
> This has two issues:
> 
>   1. This won't work if the type of Result is unconstrained, as, e.g., in
> 
>      procedure My_Proc(Obj1, Obj2: String; Result : String);

This is probably a very rare case.

> 
>   2. This would sometimes break overload resolution -- which procedure
>      should be called by "My_Proc (Obj1, Obj2, Result => <>);" if there
>      happens to be another procedure in scope, such as
> 
>      procedure My_Proc(Obj1, Obj2: String; Result : Unbounded_String);
> 
> I'd suggest
>  
>         My_Proc (Obj1, Obj2, Result => String(1 .. 80));
> 
> or in general
> 
>         My_Proc (Obj1, Obj2, Result => type Name_of_Type);

It seems more consistent with the present language to allow type
qualification of the box, as in

   My_Proc (Obj1, Obj2, Result => Name_of_Type'(<>));

> as syntactic sugar for
> 
>    declare
>      Dummy : Name_Of_Type;
>    begin
>      My_Proc (Obj1, Obj2, Result => type Name_of_Type);

You probable meant this to be:

      My_Proc (Obj1, Obj2, Result => Dummy);

>    end;

If this proposal is taken up, I would like it to be extended with some
mechanism by which the called subprogram can query if a given out
parameter is actually a dummy <>, so that the subprogram can avoid
computing a useless value into that out parameter. However, that could
easily lead to distributed overhead.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .