DALI-next

[Patrick Dowler]

Last fall (College park) a group of us had a splinter meeting during ADASS
to discuss new xtypes needed for some specific serailisation use cases. I
summarised those in a DALI-1.2 presentation in the spring interop (Paris)
but we did not have time for a discussion.

The use cases and a proposed solution that satisfies them are covered by
that presentation:

https://wiki.ivoa.net/internal/IVOA/InterOpMay2019DAL/DALI-1.1-next.pdf

The last slide (debatable solution) was not presented but included to
initiate discussion.

The "design goal" that led to that solution was to make it as easy as
possible to convert from simple component types (some of which are standard
now) to more general types. In the proposed solution, going from the
current type-safe DALI-1.1 values to polymorphic values only requires
prepending the label (which is the same as the simple xtype). On the
simple-complex axis, simple values are valid complex values so conversion
in that direction is a no-op.

We do not need to standardise all the xtypes that come out of this in
DALI-1.2; the ones with ? in the last slide do not correspond to any use
cases I know of and are there for symmetry/completeness only.

--
Patrick Dowler
Canadian Astronomy Data Centre
Victoria, BC, Canada
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