DAP4: VLens (and Sequences)

"Oh what a tangled web we weave when first we practice to build a type system."

Recently, I made the claim to James that the Sequence construct could serve to represent CDM/HDF5 vlen constructs.

His reply was as follows:

In my opinion we will want vlens to be in the data model, if not as a type, then as a feature of arrays - see the schema and my text on the Data Model page. The reason for that [is that] I have already tried using Sequence for vlen (in hdf4) and it was a failure. Not a failure from the technical POV but because neither server writers nor client writers nor client users ever 'got it.' I think one part of the problem for those people was that vlens in HDF4 do not support relational operators via the API while Sequences are supposed to. So the conceptual mismatch doomed the idea.

This is a very important observation, and has caused me to re-think how sequences (and vlens) should be handled in DAP4.

-Dennis Heimbigner