Re: Modular types inside records

[niewiap@widzew.net (Pawe� Niewiadomski)]

>
>   X := X + 1;
>   if X = max then X := 0; end if;
>
>will work just fine
>
>And no, there is clearly no way to use a modular type in
>this case. To understand why, consider that the model of
>generics is always organized to allow shared generics, and
>we can't have base types whose range is dynamic at runtime!
>
How about this one?

generic
    type item_type is private;
    type mod_n_max is mod <>;
package queues is
    type queue is tagged private;
    function empty (q: queue) return boolean;
    function full (q: queue) return boolean;
    procedure add (q: queue; item: in item_type);
    procedure remove (q: queue; item: out item_type);
private
    type arr is array (natural range <>) of item_type;
    procedure move_to_0 (var: out natural);
    procedure move_to_last (var: out natural; values: in arr); 
    
    type queue is tagged record
        values: arr (0..mod_n_max'modulus-1);
        tail: mod_n_max:=0;
        head: mod_n_max:=1;
    end record;
end queues;

Pretty neat, huh?  Not as flexible as my previous (erroneous) idea (you 
need to instantiate the generic every time you want to change the size of 
your queue), but still gets rid of constraint verification problem.  It is 
just a rough, untested idea, but I think it will work.
Thanks for your answer,

Pawel