Re: Operation can be dispatching in only one type

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 19 Nov 2009 02:23:07 +0000 (UTC), tmoran@acm.org wrote:

>> package Documents is
>>
>>     type Document is abstract tagged private;
>>     type Text_Region is abstract tagged private;
>>     type Marker is abstract tagged private;
>>   ...
>>     procedure Move_Text (Doc  : in out Document;
>>                          From : in Text_Region;
>>                          To   : in Marker)
>>        is abstract;
> 
>   There's really no way at compile time to require a particular
> relationship between two (or more) arbitrary parameters to a procedure.
> If that's necessary, you usually combine them into a single object that's
> a single parameter.

Yes, this is why I consider these cases single dispatch on a tuple of
types. It is like

   type Editing is interface (Document, Region, Marker);

And the operations are in fact defined on Editing, which has only one tag.

> Thus if a Document could have up to 10 defined
> Text_Regions and 20 Markers one could say
> 
>       type Document is abstract tagged private;
>       -- Presumably includes an array or list of appropriate Text_Region
>       -- descriptors and of Marker descriptors.
> 
>       type Text_Region_Ids is range 1 .. 10;
>       type Marker_Ids is range 1 .. 20;
> 
>       procedure Move_Text (Doc  : in out Document;
>                            From : in Text_Region_Ids;
>                            To   : in Marker_Ids)
>          is abstract;

It is not ADT. ID does not have finalization and cannot refer to a specific
Document. You will not be able to transparently manage Regions and Markers.

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