Re: Making sense of predicates

[Jacob Sparre Andersen <jacob@jacob-sparre.dk>]

Dmitry A. Kazakov wrote:

> Considering this:
>
>    type T is tagged null record;
>    procedure P (X : T);
>
>    type S is new T with null record;
>    procedure Q (X : T);
>
>    subtype Subclass is T'Class with
>       Dynamic_Predicate => Subclass in S'Class;
>       -- Should have been Static_Predicate, but illegal
>
> Now:
>
>    X : Subclass := S'(null record);
> begin
>    X.P;
>    X.Q; -- Illegal
>
> Well, predicates were not supposed to work in an intelligent way, so
> why not to add a kind that will?

But they do.

* Predicates limit the set of possible values of a type.
* Extensions expand the set of possible values of a type.

You just have to use the right tool for the job.

> There are number of cases where this would be very useful. E.g. smart
> pointers:
>
>    type T1 is tagged ...;
>    type T1_Ptr is access all T1'Class;
>    type H1 is new Ada.Finalization.Controlled with record -- Handle type
>        Ptr : T1_Ptr;
>    end record;
>
>    type T2 is new T1 with ...;
>
> Presently it is impossible to derive H2 from H1 constrained to point
> to T2'Class. (Neither it were possible with implicit dereferencing)
>
> With the above, one would create a subtype of H1 constrained to
> T2'Class and then derive from it (better in one step, of course).

I think I understand what you want.  I suspect that it would be rather
hard to implement in a compiler.  Do you have an idea for how it could
be done?  (Just asking for fancy stuff without thinking if it is
possible is something we should leave to architects.)

Greetings,

Jacob
-- 
"War does not determine who is right - only who is left."
                                         -- Bertrand Russell