Re: Ada2012 : Expression functions and extended return statements

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 6 Jan 2012 03:37:27 -0800 (PST), Martin wrote:

> On Jan 6, 11:23�am, "Peter C. Chapin" <PCha...@vtc.vsc.edu> wrote:
>> On 2012-01-06 05:02, Martin wrote:
>>
>>> I still think it would be nice to have a short-hand e.g. 'return
>>> Result :<> �do'. If anything it would then draw the eye to the more
>>> interesting case when the 2 are different.
>>
>> You need to repeat the type for ordinary returns as well.
>>
>> function F return Integer is
>> � �Result : Integer; �-- Duplication of return type here.
>> begin
>> � �Result := 1;
>> � �return Result;
>> end F;
>>
>> To be consistent it seems like you'd need to be able to use '<>'
>> throughout the body of the function.
>>
>> function F return Integer is
>> � �Result : <>; �-- Shorthand for function's return type.
>> begin
>> � �Result := 1;
>> � �return Result;
>> end F;
>>
>> Considering Ada's tendency to require one to repeat information in
>> earlier declarations, this seems a bit unAda-like to me. Also this would
>> allow '<>' to show up in a lot of strange looking places such as type
>> conversions and type qualifications.
>>
> The difference is that the declaration of 'Return' in the extended
> return is that the compiler 'knows' that this is the declaration of
> something that's going to be returned - it can't know that in the
> ordinary return case.

This is not a declaration, because the result is not an object.

Note that your argument applies to other parameters of a subprogram:

   procedure Foo (X : Integer);  -- Declares X

   procedure Foo (X : Integer) is -- Reiterates what is already known
      ...
   end Foo;

> It would certainly be a departure from current Ada - adding a little
> bit of "Type Inferencing" to the language. But I'm not adverse to
> that...so long as the inferencing is safe. I like SML/NJ :-)

Maybe. There are lots of cases where repetition is boring and error prone.
E.g.:

   if X in T'Class then
      declare
         XX : T'Class renames T'Class (X);

or

   if X /= null then
      declare
         XX : T renames X.all;

or

   X : Subarray renames Array_Thing;  -- Legal, but broken

As for return, the second best (the first one is to ditch return statement
altogether) would be:

   X : return -- Loop label syntax
      ...
   end X;

BTW, Ada could have the result explicitly named like other parameters are.
E.g.

   function F return X : Integer := 1 is
   begin
      if Something then
        X := 2;
      end if;
   end F; -- No explicit return needed, obviously

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de