Re: List container strawman

["Nick Roberts" <nickroberts@adaos.worldonline.co.uk>]

May I make a suggestion, which, though I say it myself, I feel is rather
important!

Please make the type List derived from an abstract 'iterator' type. List
would then inherit the iteration operations of this type. An alternative is
for List to provide a function that generates or gives access to an object
derived from this iterator type.

For example:


generic
   type Element_Type is private;

package Containers is

   ...

   type Terminating_Producer is abstract tagged limited private;

   procedure Read (Producer: in out Terminating_Producer; Item: out
Element_Type) is abstract;

   function End_of_Data (Producer: in Terminating_Producer) return Boolean
is abstract;

   ...

   type Sequence_Reproducer is abstract new Terminating_Producer with
private;

   procedure Restart (Reproducer: in out Sequence_Reproducer) is abstract;

   type Sequence_Recorder is abstract new Sequence_Reproducer with private;

   procedure Rewrite (Reproducer: in out Sequence_Reproducer) is abstract;

   procedure Write (Recorder: in out Sequence_Recorder; Item: in
Element_Type) is abstract;

   function Count (Recorder: in Sequence_Recorder) return Natural is
abstract;

   function Is_Recording (Recorder: in Sequence_Recorder) return Boolean is
abstract;

   ...

end Containers;


The Restart procedure tells a sequence reproducer to start producing its
data over again. The Rewrite procedure tells a sequence recorder to start
recording data (at which point Write can be used and Read cannot;
Is_Recording returns True). Restart than tells it to start reading again (at
which point Write cannot be used; Is_Recording returns False). Count returns
the number of items currently recorded.


package Containers.Lists.Unbounded is

   type Linked_List is new Sequence_Recorder with private;

   -- Representing a singly-linked (forward) list type, with typical
operations.

   function "&" (Left, Right: in Linked_List) return Linked_List is
abstract;
   ...

   procedure Push (List: in out Linked_List; Item: in  Element_Type);
   procedure Pop  (List: in out Linked_List; Item: out Element_Type);

   ... -- etc

   type Doubly_Linked_List is new Sequence_Recorder with private;

   function "&" (Left, Right: in Linked_List) return Abstract_List is
abstract;
   ...

   procedure Push (List: in out Linked_List; Item: in  Element_Type);
   procedure Pop  (List: in out Linked_List; Item: out Element_Type);

   procedure Reverse_Push (List: in out Linked_List; Item: in
Element_Type);
   procedure Reverse_Pop  (List: in out Linked_List; Item: out
Element_Type);

   ... -- etc

end Containers.Utility.Lists;


In this way, a piece of software which only needs to iterate over a
container can be passed any of the list types, or other container types.
E.g.:


generic
   with package Specific_Containers is new Containers(<>);

procedure Print_a_List (List: in out
Specific_Containers.Sequence_Producer'Class);

...

   package Thingy_Containers is new Containers(Thingy_Type);

   Thingies_1: Thingy_Containers.Lists.Unbounded.Linked_List;
   Thingies_2: Thingy_Containers.Lists.Unbounded.Doubly_Linked_List;

   ...

   package Print_Thingies is new Print_a_List(Thingy_Containers);

   ...

   Print_Thingies(Thingies_1);
   Print_Thingies(Thingies_2);


We can call instantiations of Print_a_List with objects of type Linked_List,
or Doubly_Linked_List, or anything else (i.e. other containers) derived from
Sequence_Producer.

My choice of identifiers, and the details of my design may not be ideal, but
the basic idea is right.

Please don't get this elementary aspect of the design wrong!

--
Nick Roberts



"Ted Dennison" <dennison@telepath.com> wrote in message
news:kPoE7.9567$xS6.13287@www.newsranger.com...
> Since we do seem to be reaching a small amount of agreement on details, I
went
> ahead a put together a strawman package spec (sans private part) for the
> unbounded list container, for the purposes of keeping the dicussions
rolling in
> a postitive direction. It is attached here, and available on my website at
> http://www.telepath.com/dennison/Ted/Containers-Lists-Unbounded.ads.html .
If it
> is close enough to be worth persuing, we may be able to use it as a
starting
> point.