Re: Building limited types through nested creator functions

[Simon Belmont <sbelmont700@gmail.com>]

On Feb 6, 2:11 pm, Shark8 <onewingedsh...@gmail.com> wrote:
> On Feb 5, 4:03 pm, Simon Belmont <sbelmont...@gmail.com> wrote:
>
>
>
>
>
> > Hi,
>
> > Consider two limited record types, inner and outer, with the former
> > nested inside the latter.  If the records are public, then code can
> > initialize the outer by specifying an aggregate for the inner, as in:
>
> > type Inner is limited
> >   record
> >     e : Integer;
> >   end record;
>
> > type Outer is limited
> >   record
> >     i : Inner;
> >   end record;
>
> > o : Outer := Outer'(i => Inner'(e => 42));
>
> > However, if types are made private, suitable functions must be
> > provided to make the appropriate objects.  If just Inner is private,
> > then this can be done (assuming simple creator functions that just
> > create the objects with the given values):
>
> > o : Outer := Outer'(i => Make_Inner(arg => 42));
>
> > but if both are private, then the following:
>
> > o : Outer := Make_Outer (arg => Make_Inner(arg => 42));
>
> > ends up being illegal, because in the code:
>
> > function Make_Outer (arg : Inner) return Outer is
> > begin
> >   return Outer'(i => arg);
> > end Make_Outer
>
> > would end up trying to copy a limited type.  For the cases in which an
> > existing object is passed in, this would be the appropriate action,
> > but for cases where the object is built-in-place into the argument,
> > clearly the intended behavior is to build it in place to the resultant
> > object.  It's easy enough to use an access value, but as unnecessary
> > use of access values is generally discouraged, I was just curious if
> > there was an alternative mechanism to achieve the desired effect.
>
> > -sb
>
> Try this:
>
> Function Make_Inner( I: Integer ) Return Inner is
> begin
>     Return Result : Inner:= Inner'( E => I );
> end Make_Inner;
>
> Function Make_Outer( I: Integer ) Return Outer is
> begin
>     Return Result : Outer:= Outer'( I => Make_Inner(I) );
> end Make_Outer;
>
> The 'trick' is to use all the parameters used in make_inner plus
> whatever is needed in the Outer record for the parameter-list of
> Make_Outer; thus you avoid all copying restrictions as you are really
> building-in-place.- Hide quoted text -
>
> - Show quoted text -

This was my original plan, but the worry is that this doesn't reflect
the appropriate responsibilities; If A creates B and gives it to C,
then C should have no say in how (or if) it's created.  For instance,
the Make_Outer code could easily alter the supplied arguments and
Make_Inner in any way it chooses.  I suppose using an access value has
essentially the same problem, as it could simply eschew the supplied
value and allocate whatever it wants, but such is life I guess

Thank you

-sb