empty param values in votable

[Markus Demleitner]

Hi Carlos,

On Fri, Jul 17, 2020 at 09:44:47AM +0200, Carlos Rodrigo wrote:
> the astropy people have found an issue in one SVO service votable and I am not sure if I understand
> the VOTable standard about this.
> 
> The main point is:
> 
> <PARAM name="INPUT:Instrument" datatype="char" arraysize="*">
> <VALUES>
> <OPTION name="All" value=""/>
> <OPTION value="90prime"/>
> <OPTION value="AcqCam-N"/>
> </VALUES>
> </PARAM>
> 
> is correct? Is it valid to specify value="" for an OPTION element?

The schema says that @value is xs:string, and, by XML Schema, the empty
string is a valid literal for xs:string.  In the document, I cannot
find prose that would further constrain @value, so yes, that's
perfectly fine.

As usual in VOTable, we have a NULL problem; in VOTable TABLEDATA,
which is (or so I like to claim) what governs the serialisation of
PARAM/@value and friends, we do not discriminate between the empty
string and the NULL.  However, here we might, as what we're
describing is an input parameter, and NULL might mean "leave it out",
whereas an empty string would be "pass Instrument=" (which,
incidentally, would be treated identically by anything building on
the default behaviour of Python's cgi.FieldStorage...).

For PARAM, I'd say a NULL can be represented without excessive
nerve-wracking using VALUES/@null, whereas there are probably many
cases where we want an empty string.

Hence, I'd say we should make it clear somehow that an empty string
in a PARAM/@value is an empty string and not NULL.

> A related question:
> 
> Is it valid to write:
> 
> <PARAM name="Instrument" datatype="char" arraysize="*" value=""/>
> 
> that is, specifying an empty value for a PARAM element?

Again, a clear yes.  The use of this in Datalink, however, may
suggest that we have it in dangerous vicinity of NULL there.  Ah
well.

> And, a last question, having a VALUES sub-element replaces the need
> of having a VALUE attribute for params?

No, at least not yet.  A PARAM in VOTable must have a @value
attribute (which, admittedly, makes it a clumsy tool for the
declaration of input parameters, as happens here).  

We could have said that a missing @value means "NULL", and, enemy of
VALUES/@null that I am, I personally think we should.  But we
haven't, and the current schema is clear about that: @value has
use="required".

          -- Markus