getting started with Spark; generic queue package

[Stephen Leake <stephen_leake@stephe-leake.org>]

I'm finally getting around to trying Spark, mostly because it finally has some support for access types. For now, I'm using GNAT Community 2018; I'll upgrade to 2019 soon.

To start, I'm trying to add contracts to an existing bounded queue package:

generic
   type Item_Type is private;
package SAL.Gen_Bounded_Definite_Queues
  with Spark_Mode
is
   pragma Pure;

   type Queue_Type (Size : Positive) is tagged private;
   --  Size is maximum number of items in the queue.
   --  Tagged to allow Object.Method syntax.

   function Get_Overflow_Handling (Queue : in Queue_Type) return Overflow_Action_Type;
   procedure Set_Overflow_Handling (Queue : in out Queue_Type; Handling : in Overflow_Action_Type);
   --  See Add for meaning of Overflow_Handling. Default is Error.

   procedure Clear (Queue : in out Queue_Type)
   with Post => Queue.Count = 0;
   --  Empty Queue of all items.

   function Count (Queue : in Queue_Type) return Natural;
   --  Returns count of items in the Queue

   function Is_Empty (Queue : in Queue_Type) return Boolean
   with Post => (if Is_Empty'Result then Queue.Count = 0);
   --  Returns true if no items are in Queue.

   function Is_Full (Queue : in Queue_Type) return Boolean
   with Post => (if Is_Full'Result then Queue.Count = Queue.Size);
   --  Returns true if Queue is full.

   function Remove (Queue : in out Queue_Type) return Item_Type with
     Pre => Queue.Count > 0,
     Post => Queue.Count = Queue.Count'Old - 1;
   --  Remove head item from Queue, return it.
   --
   --  Raises Container_Empty if Is_Empty.

   --  other stuff deleted
private

   type Item_Array_Type is array (Positive range <>) of Item_Type;

   type Queue_Type (Size : Positive) is tagged record
      Overflow_Handling : Overflow_Action_Type := Error;

      Head  : Natural := 0;
      Tail  : Natural := 0;
      Count : Natural := 0;
      Data  : Item_Array_Type (1 .. Size);
      --  Add at Tail + 1, remove at Head. Count is current count;
      --  easier to keep track of that than to compute Is_Empty for
      --  each Add and Remove.
   end record;

end SAL.Gen_Bounded_Definite_Queues;


with SAL.Gen_Bounded_Definite_Queues;
package Prove_Integer_Queues is new SAL.Gen_Bounded_Definite_Queues (Integer);


Running gnatprove on this gives info messages like:

prove_integer_queues.ads:2:01: info: in instantiation at sal-gen_bounded_definite_queues.ads:36
prove_integer_queues.ads:2:01: info: implicit function contract not available for proof ("Count" might not return)

Is there any way to turn those off? I don't see such an option in 'gnatprove -help', nor in the Spark 2014 user guide (not I have read every page yet; I did go thru the tutorial).

Worse, in the gnatprove.out file, it says:

  Prove_Integer_Queues.Add at sal-gen_bounded_definite_queues.ads:66, instantiated at prove_integer_queues.ads:2 skipped

and so on, for every subprogram.

Is there a way to find out why it was skipped? And what do I need to do to fix this?

The body file has 'with Spark_Mode' on the package.

In general, is it useful to use Spark on generic packages? Section 4.3 in the Spark user guide says:

   Note that a generic package instance is considered to be declared at its   
   instantiation point.  For example, a generic package cannot be both 
   marked SPARK_Mode and instantiated in a subprogram body.

But I think that only applies if I run gnatprove on that instantiation; if I only run it on Prove_Integer_Queues, it should be ok. On the other hand, does that mean other instantiations of Gen_Bounded_Definite_Queues are "proved" in any sense?

Most of my packages rely on Ada.Finalization, which is not currently supported in Spark. Is there any plan to add that?

-- Stephe