ivoa: DM type system

[Markus Demleitner]

Hi,

On Tue, Apr 11, 2017 at 06:41:46PM +0200, Laurent Michel wrote:
> I agree with Markus, having LITERAL @dmtypes compliant with VOTable
> datatypes would be valuable.  Unfortunately, LITERAL @dmtypes are
> not necessarily in the ivoa name space (ivoa:xx).
> They can refer to dataTypes defined into the model. This is the
> case for ENUM literals e.g..

Are there any other cases except ENUMs?  If not, I'd say there's no
big incentive to introduce any complication here.  When we just use
VOTable types:

(a) normal clients just mind that they can parse the thing.  For
that, they just need the VOTable datatype (and they already have
parsers for that)

(b) validators can infer the the set of allowed values from the VO-DML
document, because the literal's role is known from where it stands in the
instance document, and the VO-DML states what type that role has
(i.e., in this case the allowed literals).

-- which of course begs the question whether we can't scupper LITERAL
again and just use PARAMs instead of them.

> Finally, I do not think that renaming ivoa types as VOTable
> datatypes will help that much.

Well, it's certainly saving clients the trouble of having to come up
with parsers for all the literals of the VO-DML types when they
already have parsers for the VOTable types.  I'd say that's, if
nothing else, a courtesy to implementors, and given they're the ones
that eventually control takeup, that ain't so bad.

        -- Markus