Re: List Container Straw Man

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

"Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org> wrote in
message news:9s99tt$pdb$1@nh.pace.co.uk...
> Doing what Ted did and throwing out a package spec (or two or three...)
> would be a good idea. Its hard to see the merits of something like this
> without some actual code to criticise. It need not include all details or
> all possible operations - just enough of the basics to indicate how the
idea
> would be realized.

Actually I did precisely that 3 days ago, but I think must have missed
again, so here's a repeat.

-----

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!

-----

This is all based on a detailed (if only fundamental) proposal I put to the
ASCL (Ada Standard Component Library) group in 1999. I was puzzled by the
stunned silence it seemed to be met with. I would be happy to e-mail that
proposal to anyone who would like to see it.

nickroberts@adaos.worldonline.co.uk

And then I'd be happy to field any and all questions, suggestions, etc.

--
Nick Roberts