tagged types

[Sami Evangelista <sami.evangelista@wanadoo.fr>]

i have declared an abstract type with two derived types like this

type father is null record;
type son1 is new father with record
	son1_field : integer;
end record;
type son2 is new father with record
	son2_field : boolean;
end record;
type access_son1 is access son1;
type access_son2 is access son2;

then i introduced a list of these elements and a procedure to add an 
element to the list like this:

type access_father_class is access all father'class;
type list;
type access_list is access list;
type list is record
	element : access_father_class;
	next : access_list;
end record;

procedure add(The_list : in out access_list;
               The_Element : access_father_class) is
begin
       if The_list = null then
          The_list := new list;
          The_list.element := The_Element;
          The_list.next := null;
       else
          add(The_list.next, The_element);
       end if;
end

until there no problem. but how is it possible to add a son1 object to a 
list. I have tried this:

my_object : access_son1 := new son1;
my_object.son1.field := 0;
my_list : access_list;
...
add(my_list, my_object);

and it gives me the following error message:

expected type "access_father"
found type "access_son"

Thanks for any help

Sami Evangelista