Re: Extending discriminant types

[anon@anon.org (anon)]

--  Your talking about "Variant Parts and Discrete Choices" (RM 3.8.1) 
--  which is validated during runtine, if the variable is reference.
--
--  Now with a smart compiler it could detect the error, but the 
--  compiler in GNAT is not that smart.

procedure t is

   --  Change "Base" to "KBase" to prevent confusion with predefine 
   --  Attribute 'Base, which als works.

   type Kind is (A, B);

   type KBase (K : Kind) is abstract tagged record   
      case K is
         when A => null;
         when B => Dummy : Natural;
      end case;
   end record;

   type Base_Access is access all KBase'Class;

   --  type "Child" is bound to KBase ( K => B ) so any reference 
   --  of "Child" to ( K => A ) should cause an error condition 
   --  but this only happens during runtime.

   type Child is new KBase (B) with null record;


   --  Var2 is in error and should cause a Constaint_Error
   -- once referenced.
   
   Var  : Base_Access := new Child ;
   Var0 : Base_Access := new Child'(K => B, Dummy => 5);
   Var1 : Base_Access := new Child'Base(K => B);  -- Just for Adam, this works

   Var2 : Base_Access := new Child'Base(K => A);


begin -- t

  Var.Dummy := 2 ;
  Var0.Dummy := 4 ;
  Var1.Dummy := 8 ; -- Just for Adam, this works

  --  Uncommenting the following line will result in a Constraint_Error 
  --  during runtine. But until Var2 is reference the validation of 
  --  type is not performed.

--  Var2.Dummy := 16 ;

end t ;



In <20081115101632.5f98c596@cube.tz.axivion.com>, Stefan Bellon <bellon@software-erosion.org> writes:
>Hi,
>
>I stumbled across some behaviour when extending an unconstrained
>discriminant type while providing the constraint when extending. Assume
>the following code:
>
>   type Kind is (A, B);
>
>   type Base (K : Kind) is abstract tagged record
>      case K is
>         when A => null;
>         when B => Dummy : Natural;
>      end case;
>   end record;
>
>   type Base_Access is access all Base'Class;
>
>   type Child is new Base (B) with null record;
>
>In this case I would have thought that type Child is now constrained to
>Kind B and no object of type Child with Kind A can be created.
>
>However, the following compiles (with GNAT) and does not throw an
>exception at runtime:
>
>   Var : Base_Access := new Child'(K => A);
>
>Am I misunderstanding the Ada semantics? If so, is there a way to
>achieve what I'm trying, i.e. that I can determine the Kind while
>extending and not change it when creating objects?
>
>Greetings,
>Stefan
>