Re: tagged type as generic parameter

[Adam Beneschan <adam@irvine.com>]

On Jan 3, 7:51 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> 3. Aggregation instead of inheritance:
>
>    type Item is record
>       Next  : Item_Ptr;
>       Value : Data;
>    end record;
>
> No inheritance, no problem.

Actually, I think the OP really needs a darn good reason why he would
want Item to be inherited from his generic formal type, rather than
simply a record containing a value and a link as in your above
suggestion.  Conceptually, at least to me, a type extension is
supposed to define a type that is a "kind of" the parent type.  In
this case, Item, as the OP originally defined it, doesn't really fit
the concept.  It's not really a "kind of" Data with additional
properties.  I'm sorry that my "pedagogical" understanding of object-
oriented programming isn't solid enough that I understand what terms
to use, but hopefully I'm getting some sort of notion across.

Or, to put it another way, don't use type extensions just because
they're there.

To me, #3 stands out as the *correct* solution to the problem, in most
cases.  Plus, if you do that, Data no longer needs to be a tagged
formal type, so that you can use your generic on both tagged and
untagged types.

                           -- Adam