Re: Full view of a private partial view cannot be a subtype

[Jere <jhb.chat@gmail.com>]

On Monday, December 18, 2017 at 5:47:51 PM UTC-5, Randy Brukardt wrote:
> "Dmitry A. Kazakov" wrote in message 
> > On 2017-12-17 16:26, Jere wrote:
> >
> >> my assertion is that:
> >>
> >>     subtype Thing is Core_Pkg.Thing;
> >>
> >>     procedure Do_Something(The_Thing : in out Thing)
> >>        renames Core_Pkg.Do_Something;
> >>         is easier to both maintain and read than:
> >>
> >>     type Thing is new Core_Pkg.Thing with null record;
> >>     procedure Do_Something(The_Thing : in out Thing);
> >
> > But these are two semantically different concepts. Ada's subtype declares 
> > an equivalent type [it inherits everything from and exports everything to 
> > the base]. Ada's new tagged type declares a new instance of a class. It 
> > only inherits.
> >
> > I don't understand how can you exchange one for another.
> 
> You can't really; they're very different concepts. The OP is showing a 
> confusion, because the renames he wants is perfectly legal, assuming proper 
> visibility. And if the items in question are private, then he is trying to 
> make a privacy leak -- which the language will not make easy for obvious 
> reasons.
> 
>                             Randy/

I'm not really trying to make a privacy leak on purpose.  I'm just trying to
provide a simpler interface to a much more complex and dangerous generic
so that someone doesn't accidentally use something they shouldn't.  Any 
privacy leaks are not intentional.  I was just asking a question.  Using
either extension, derivations, or composition all seemed very heavy handed
for just wanting to mimic the same specification as another generic.  Subtypes
and renames expressed exactly the relationship I wanted, but the only thing
I couldn't do was hide a deallocation operation that should never be called
directly (it was an input to the base generic).  That one items is what 
got me to this question.  I wanted to see if there was a way to keep the
subtype/rename method while hiding the operation and the answer looks to be
no.  But I wasn't sitting around twiddling my fingers looking for ways to
break Ada privacy in the process.  I just wanted to understand if there was
a way and if not, why.  That's all, nothing nefarious on my part, at least
not intentionally.