Re: User-defined type attributes

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:pqup5kf1u34t$.19kwms4bw12hs.dlg@40tude.net...
> On Thu, 13 Mar 2008 20:46:41 -0500, Randy Brukardt wrote:
>
> > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> > news:965suhjl0bxt$.74se2ylyksin.dlg@40tude.net...
> > ...
> >> Why? The only thing you need is formal generic ADTs. Ada's generics
lack
> >> ADTs, which is the reason why you need to pass operations together with
the
> >> formal types. There exist formal built-in types like "private", "<>",
> >> "digits <>" etc, but there is no user-defined ones. Add these and all
> >> primitive operations will go with the actual type just per magic. It
would
> >> be IMO a natural step for anybody who wanted generics to evolve in an
> >> Ada-way. (Not for me, as you know, I prefer generics removed from the
> >> language.)
> >
> > Generic formal derived types are the way to bring an ADT into a generic.
> > After all, they do bring in all of the primitive operations of the ADT.
> >
> > The only problem is that they can't be used on unrelated types, but
that's
> > not very relevant if you are using tagged types (and you might as well
being
> > doing so if you're defining any sort of composite type).
>
> Right, but this also implies that there is no need to have generics at
all.
> A class-wide object does all what needed:
>
>    generic
>       type S is new T with private;
>    procedure Foo (X : in out S);
>
> can be replaced by:
>
>    procedure Foo (X : in out T'Class);

Not if you need other generic parameters along with the type. And this is a
way to do mixins, which otherwise can't be done in Ada 95 because type can't
derive from a class-wide type:

   generic
      type S is new T with private;
   package Foo is
      type New_S is new S with private;
      procedure New_Operation (Op : in out New_S);
   end Foo;

It's a nice way to add persistence, for instance. (Interfaces only allow you
to add, umm, interfaces, not code.)

                     Randy.