Re: tagged types extensions - language design question

[Charles Hixson <charleshixsn@earthlink.net>]

That should work, but the question was why couldn't it be done in one
step.  (I don't know.)

Matthew Heaney wrote:

> In article <s90mu0hg7q538@corp.supernews.com> , "Vladimir Olensky"
> <vladimir_olensky@yahoo.com> wrote:
>
> Why don't you just declare these types in separate packages?
>
> package P is
>
>   type T is tagged private;
>
> private
>   type T is tagged record ...;
> end P;
>
> package P.C is
>
>   type T1_Public is
>     abstract new T with record
>       Something : Some_Other_Type;
>     end record;
>
>   type T1 is new T1_Public with private;
>
> private
>   type T1 is new T1_Public with record ... ;
> end P.C;
>
> > package P is
> >
> >    type T is tagged private;
> >
> >    type T1 is new T with
> >       record with private
> >                  something: some_type;
> >       and record;
> >
> > private
> >
> >    type T is tagged with
> >       record
> >            ID : Integer;
> >       end record;
> >
> >    type T1 is new T with
> >       record
> >           thing : some_other_type;
> >       end record;
> >
> > end P;
> >
> > Instead I should do this in two steps.
> >   1. to extend  private part   and then
> >   2. to extend  public part
> > or vice versa.
> >
> > Due to that type T1 has inheritance_depth = 3
> > instead of inheritance_depth =  2.
> >
> > LRM (3.9.1) says:
> >   Every type extension is a tagged type,
> > and is EITHER a record extension OR a private
> > extension of some other tagged type.
> >
> >     It would be interesting to know what were
> > the reasons not to allow to have BOTH a record
> > extension AND a private extension at the same time.
> >
> > Regards,
> > Vladimir Olensky
> >
> >