[calsify] iCalendar Series draft

[Michael Douglass <[e-mail]>]

At the last joint call there were a few questions and proposals...

*** Why standardize? ***

One was something like "why standardize this -couldn't it just be a 
reimplementation of recurrences".

I guess the flip reply is why standardize anything? More specifically 
though - how would a client store a series definition or transport it 
from one service to another?

*** Why the new SRULE etc properties ***

There's still the ongoing question of whether a series is just another 
form of a recurrence - i.e. why do we need to replicate the recurrence 
properties. I still feel the rules governing series should be distinct 
from those for recurrences. I have had cases where a series of 
recurrences makes sense (university tours comes to mind) and I don't 
want to prevent that possibility.

To enlarge on the university tour (for prospective students). As I 
recall the tours were identical on each day they occurred - something 
like every hour. There was some sort of pattern to the actual days 
allocated for the tours. At the time I felt something like recurring 
recurrences would work.

I don't see this is a significant complication. Simply duplicating the 
relevant properties with mostly the same semantics and value types.

*** Template - not master ***

This is a good idea I'd like to extend. I'm inclined to see this as 
something distinct from series - that is a template could be used for 
anything.

Of course - for a template to be useful it needs some variable parts adn 
it may be enough to just list the properties that MUST be defined

If we define a new TYPE and REQUIRED property we could have a template 
that looked like:

BEGIN:VTEMPLATE
TYPE:EVENT
REQUIRED:DTSTART,DTEND|DURATION
<a bunch of properties>
END:TEMPLATE

Of course a template could be provided completely filled in and that 
could take the place of the series master, e.g.

BEGIN:VTEMPLATE
TYPE:EVENT
DTSTART:20210315
DURATION:T1H
SRULE:...
<a bunch of properties>
END:TEMPLATE

I see templates being useful for events that occur irregularly but with 
a consistent structure - e.g. we always have a group meeting for 1 hour 
with a known set of attendees but there's no particular pattern to when.

*** Self-generation ***

One part of the series draft is how many instances are generated ahead 
of time. This came from a discussion long ago. With many recurring 
events it doesn't make much sense to create an actual instance more than 
a few weeks in advance. Many room booking systems have a booking window 
of a few weeks and there's not much point in having attendees agree to 
attend more than a few weeks out anyway.

I think this feature is really separate from series - it could just as 
well be applied to recurrences.

*** Proposed changes ***

1. Come up with a separate draft defining templates.

2. Change the series draft to use a template rather than a master.

3. create a separate draft for the parts which define how to limit the 
generation of instances in advance. This would show how to apply it to 
recurrences and to series.