Experimental Physics and Industrial Control System

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.

Ben

Replies:

Re: V4 Data Types: Request for tagged unions Ralph Lange

References:

RE: V4 Data Types: Request for tagged unions Jeff Hill

Re: V4 Data Types: Request for tagged unions Benjamin Franksen

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