Re: Defining types in private packages for use in parent spec

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

On Wed, 27 Aug 2003 07:51:02 +0000 (UTC), Lutz Donnerhacke
<lutz@iks-jena.de> wrote:

>How to solve the following problem? It's necessary to define the type Base
>in the implementation defined private subpackage, because it varies on
>various implementations.

The first design question is when implementations should be selected?
At compile time or possibly at run time? For the latter case you have
only two options: tagged types vs. discriminated types (with a variant
part or not). Otherwise there is a third opton the generics. To
summarize, the ways you could come to different implementations of a
polymorphic object:

1. Tagged types: tag --- dispatch ---> implementation
2. Discriminated types: discriminant(s) --- case ----> implementation
3. "Generic" types: parameter(s) --- instantiation ---> implementation

Private vs. public, child vs. unrelated package is the second and less
important question. Usually when you solve the first and define the
public interface of your objects, you will have little choice to
decide.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de