Re: A function that cannot be called?

[Shark8 <onewingedshark@gmail.com>]

On Friday, October 19, 2018 at 12:19:07 PM UTC-6, marc...@earthlink.net wrote:
> On Thursday, October 18, 2018 at 8:14:32 AM UTC-4, G.B. wrote:
> > There is a type declaration, I rememberd, in Ada, that
> > does not allow any object of the type to be declared.
> > I think the type should be along the lines below. Is
> > this correct?
> > 
> > Then, a function that nominally returns objects of
> > this type should be impossible to call. Is this right,
> > too?
> > 
> > generic
> >      type T(<>) is limited private;  -- gen. arg. type
> > 
> > package What is
> > 
> >      type Void (<>) is private;
> > 
> >      function Impossible (X : T) return Void;
> > 
> > private
> > 
> >      package Inner is
> >          type Void (<>) is private;
> >      private
> >          type Void is  record
> >              null;
> >          end record;
> >      end Inner;
> > 
> >      type Void is new Inner.Void;
> > 
> > end What;
> > 
> > (If current Ada could still pass by reference in both call
> > direction and return direction, then I'd have added *limited*
> > to the Voids' declarations. Without it, a body of Impossible
> > is closer to becoming possible by using Convention => Ada
> > together with an 'Address clause on a Result object. I think.)
> 
> Before Ada 95 or Ada 2005 it would have been a package
> spec that had the following (with P1,P2,etc. being valid types): 
> 
> type Not_Objects_Just_for_Parameters is limited private;
> function Return_A_Corresponding_Value(p1:t1;p2:t2;etc...)
>   return Not_Objects_Just_for_Parameters;
> procedrure Force_Call_Of_Function_1
>           (Function_Call:Not_Objects_Just_for_Parameters;
>            Other_Param3:p3;etc...);
> procedrure Force_Call_Of_Function_2
>           (Function_Call:Not_Objects_Just_for_Parameters;
>            Other_Param4:p4;etc...);
> 
> I miss that way of forcing/ensuring new execution of some
> code as a prerequisite of use of a subprogram. ;-(
> Being able to hang on to the result of such a function
> by using renames wrecks this usage.

Isn't there a similar effect that can be had via

  Type Just_For_Parameters(<>) is limited private;
  Function Get_Param return Just_For_Parameters;
  --...
PRIVATE
  Type Stub_Record is null record;
  Type Just_For_Parameters is not null access Stub_Record
    with Size => 0;
  --...

Or am I misunderstanding what you're getting at? (Quite possible as I've never had occasion to use Size 0 "access"-types.)