Re: V4 Data Types: Request for tagged unions

[Ralph Lange <Ralph.Lange@bessy.de>]

Benjamin Franksen wrote:

> On Thursday 16 June 2005 12:55, Benjamin Franksen wrote:
>  
>
> > On Wednesday 15 June 2005 23:12, Andrew Johnson wrote:
> >    
> >
> > > To step back a little: I don't understand why your states have
> > > parameters, and what you could do with them - presumably I can't
> > > put a CA monitor on a member of a tagged union (what happens to the
> > > monitor when the state changes and the member no longer exists?).
> > >      
> > It must disconnect, indicating that the value in question no longer
> > exists. Note that a similar question must be answered for the case of
> > adding/removing views. Here, too, the client will need to receive
> > notification that a certain value is no longer available. We could
> > make this a dedicated event type so that clients can react in a
> > generic way to such conditions.
> >    
>
> Let me state this more precisely:
>
> The very idea of a tagged union implies that independent read or write 
> access to properties subordinate to a certain tag hardly makes sense. I 
> cannot change, nor query, the scanRate of a record, as long as I am not 
> certain that the tag is 'Periodic'. However, subscribing to merely a 
> certain subordinate property might reduce system load.
>
> The problem is that changing the tag independently brings the other 
> subordinate properties to an undefined state and this is something we 
> can't handle because we don't have a representation for "no value 
> here". Thus, at least write access must provide all values every time 
> (i.e. tag + add. properties), while reading and subscribing to either 
> tag only or other properties can be allowed but must fail in case of 
> unavailability.
>  
Hm. I don't see any problem with my original idea to have the write 
traversal for a tagged union (exactly like the read traversal) first 
provide the tag and then only the properties that are valid for the new 
tag value. Valid write operations would succeed, invalid operations 
would fail, no meaningless properties being provided in the process.

Ralph