using an Ada.Container as a private type

using an Ada.Container as a private type

[Graham]

Hi,
    I'm writing a little financial application. I want to model a
stream of payments made at arbitrary periods (the periods are positive
numbers; they might be actual dates later on).

I think I want to declare this:


type Payment_Stream is private;

procedure Add_To_Payment_Stream(
            stream : in out Payment_Stream;
            period :  Positive;
            amount : Money );

......

and then privately declare the payment stream as an Ordered Map, like:

private

   package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
      Element_Type => Money,
      Key_Type => Positive );

   type Payment_Stream is new Payment_Stream_Package.Map;

but I can't do this. Gnat complains that "type derived from tagged
type must have extension".
I'm fairly new to Ada but thought I was getting on top of it! What is
the principal that stops me doing this? What should I do instead?

thanks,

Graham Stark

Re: using an Ada.Container as a private type

[Robert A Duff]

Graham <graham.stark@virtual-worlds.biz> writes:

>    type Payment_Stream is new Payment_Stream_Package.Map;
>
> but I can't do this. Gnat complains that "type derived from tagged
> type must have extension".
> I'm fairly new to Ada but thought I was getting on top of it! What is
> the principal that stops me doing this? What should I do instead?

Not really a "principle" -- more like an arbitrary restriction.
Map is tagged, and tagged types have to obey that rule.

You should say:

    type Payment_Stream is new Payment_Stream_Package.Map
        with null record;

to create an empty extension.

- Bob

Re: using an Ada.Container as a private type

[Dmitry A. Kazakov]

On Thu, 3 Apr 2008 08:29:28 -0700 (PDT), Graham wrote:

> Hi,
>     I'm writing a little financial application. I want to model a
> stream of payments made at arbitrary periods (the periods are positive
> numbers; they might be actual dates later on).
> 
> I think I want to declare this:
> 
> 
> type Payment_Stream is private;
> 
> procedure Add_To_Payment_Stream(
>             stream : in out Payment_Stream;
>             period :  Positive;
>             amount : Money );
> 
> ......
> 
> and then privately declare the payment stream as an Ordered Map, like:
> 
> private
> 
>    package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
>       Element_Type => Money,
>       Key_Type => Positive );
> 
>    type Payment_Stream is new Payment_Stream_Package.Map;

   type Payment_Stream is new Payment_Stream_Package.Map with null record;

> but I can't do this. Gnat complains that "type derived from tagged
> type must have extension".

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: using an Ada.Container as a private type

[Graham]

On Apr 3, 5:02 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Thu, 3 Apr 2008 08:29:28 -0700 (PDT), Graham wrote:
> > Hi,
> >     I'm writing a little financial application. I want to model a
> > stream of payments made at arbitrary periods (the periods are positive
> > numbers; they might be actual dates later on).
>
> > I think I want to declare this:
>
> > type Payment_Stream is private;
>
> > procedure Add_To_Payment_Stream(
> >             stream : in out Payment_Stream;
> >             period :  Positive;
> >             amount : Money );
>
> > ......
>
> > and then privately declare the payment stream as an Ordered Map, like:
>
> > private
>
> >    package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
> >       Element_Type => Money,
> >       Key_Type => Positive );
>
> >    type Payment_Stream is new Payment_Stream_Package.Map;
>
>    type Payment_Stream is new Payment_Stream_Package.Map with null record;
>
> > but I can't do this. Gnat complains that "type derived from tagged
> > type must have extension".
>
> --
> Regards,
> Dmitry A. Kazakovhttp://www.dmitry-kazakov.de

Oh, I see (I think..) Thanks very much to both of you.

Graham

Re: using an Ada.Container as a private type

[Maciej Sobczak]

On 3 Kwi, 17:29, Graham <graham.st...@virtual-worlds.biz> wrote:

>    package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
>       Element_Type => Money,
>       Key_Type => Positive );
>
>    type Payment_Stream is new Payment_Stream_Package.Map;

Instead of a solution, I have a question:

Why do you derive from Map? Which operations of Map do you plan to
override?

Do you ever plan to pass Payment_Stream where
Payment_Stream_Package.Map'Class is expected? What effect would you
like to achieved this way?

Why derivation and not composition?

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Re: using an Ada.Container as a private type

[Graham]

On Apr 3, 9:41 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> On 3 Kwi, 17:29, Graham <graham.st...@virtual-worlds.biz> wrote:
>
> >    package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
> >       Element_Type => Money,
> >       Key_Type => Positive );
>
> >    type Payment_Stream is new Payment_Stream_Package.Map;
>
> Instead of a solution, I have a question:
>
> Why do you derive from Map? Which operations of Map do you plan to
> override?
>
> Do you ever plan to pass Payment_Stream where
> Payment_Stream_Package.Map'Class is expected? What effect would you
> like to achieved this way?
>
> Why derivation and not composition?
>
> --
> Maciej Sobczak *www.msobczak.com*www.inspirel.com

Good question. The intention was not to derive from Map at all: I
simply wanted to keep the fact that it was implemented as a Map
private. Is there a better way of doing that, or is keeping it private
overdesign?

Graham

Re: using an Ada.Container as a private type

[Adam Beneschan]

On Apr 4, 1:02 am, Graham <graham.st...@virtual-worlds.biz> wrote:
> On Apr 3, 9:41 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>
>
>
> > On 3 Kwi, 17:29, Graham <graham.st...@virtual-worlds.biz> wrote:
>
> > >    package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
> > >       Element_Type => Money,
> > >       Key_Type => Positive );
>
> > >    type Payment_Stream is new Payment_Stream_Package.Map;
>
> > Instead of a solution, I have a question:
>
> > Why do you derive from Map? Which operations of Map do you plan to
> > override?
>
> > Do you ever plan to pass Payment_Stream where
> > Payment_Stream_Package.Map'Class is expected? What effect would you
> > like to achieved this way?
>
> > Why derivation and not composition?
>
> > --
> > Maciej Sobczak *www.msobczak.com*www.inspirel.com
>
> Good question. The intention was not to derive from Map at all: I
> simply wanted to keep the fact that it was implemented as a Map
> private. Is there a better way of doing that, or is keeping it private
> overdesign?

No, that's the sort of thing you're supposed to do.  If you had
derived it in the visible (public) part, you're telling all users of
the package that all the operations available on
Payment_Stream_Package.Map are available for a Payment_Stream, and
that's apparently not what you want.  So doing what you did is the
right thing.

I suspect Maciej just missed the fact that the derived type
declaration was in the private part.

                                 -- Adam

Re: using an Ada.Container as a private type

[Robert A Duff]

Graham <graham.stark@virtual-worlds.biz> writes:

> Good question. The intention was not to derive from Map at all: I
> simply wanted to keep the fact that it was implemented as a Map
> private. Is there a better way of doing that,...

You could say:

    type Payment_Stream is
        record
            M : Payment_Stream_Package.Map;
        end record;

which might be better, depending on what you want.

>... or is keeping it private
> overdesign?

No.

- Bob

Re: using an Ada.Container as a private type

[Simon Wright]

Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> Graham <graham.stark@virtual-worlds.biz> writes:
>
>> Good question. The intention was not to derive from Map at all: I
>> simply wanted to keep the fact that it was implemented as a Map
>> private. Is there a better way of doing that,...
>
> You could say:
>
>     type Payment_Stream is
>         record
>             M : Payment_Stream_Package.Map;
>         end record;
>
> which might be better, depending on what you want.

You could also, for example, include a mutex in the record for locking
in the presence of concurrency.