Re: comments on draft-ietf-sming-reqs-02.txt: Discriminated Unions vs. Real Unions

[David Harrington <dbh@enterasys.com>]

Hi,

In thinking about it, I think the document should be changed - the text
should be improved to eliminate the ambiguity. I believe the response
below is a good description of the difference.

dbh

"Durham, David" wrote:
> 
> > -----Original Message-----
> > From: Juergen Schoenwaelder [mailto:schoenw@ibr.cs.tu-bs.de]
> > Sent: Friday, June 29, 2001 12:08 PM
> >
> <snip>
> >
> > 51: At first glance, it is somewhat confusing that discriminated
> >     unions are required while unions are just nice to have. A
> >     discriminated union is a more precise form of a union and hence
> >     support for unions will fall out when doing discriminated unions.
> >     And since discriminated unions are required, the nice to have
> >     feature of unions will fall out anyway. There is certainly the
> >     issue whether unions without a discriminator are desirable,
> >     although arbitrary unions are classified as nice to have here.
> >
> >     We are confused (and hungry?)
> 
> [Dave] Do you interpret a discriminated union as being able to change the
> underlying base type of an attribute? It was suggested that a "real" union
> (in C for example) can change the type as well as the semantic
> interpretation of an attribute (perhaps even a group of attributes). It was
> mentioned that supporting a real union will require that the SMIng adds ANY
> to the supported base types (which is not currently supported by SMIv2 or
> SPPI). I believe that is why there are two different proposed requirements.
> One for discriminated unions (based on a semantic selector for a statically
> typed attribute or group of attributes) vs. real union which may also
> include a base-type selector.