utypes: a proposal

[Paul Harrison]

On 2008-10 -30, at 20:45, Norman Gray wrote:

>
> On 2008 Oct 30, at 16:19, Paul Harrison wrote:
>
>> one of Norman's earlier suggestions http://www.ivoa.net/Documents/latest/utype-uri.html 
>> .
>
> Just to be clear, what I'm suggesting here is independent of this  
> earlier one.  I think that some of the advantges of the earlier one  
> would naturally attach to this proposal, but in particular the extra  
> explanatory structure is not part of what I proposed here.  The  
> utype-as-xpath suggestion is purely syntactical.
>>

I have to admit, I am not really keen on the implication that the  
UType has anything to do with xml - it should be related to a "purer"  
representation of the data model, as championed by the Theory IG.  
Having the XML representation of the data model as the "canonical"  
representation causes problems because of the design decisions that  
the vagaries of the XML schema language force on the data model author.

Perhaps the problem is that there is  a notion that there should be  
some way of "reasoning" about the UType from its form, and I think I  
agree with Doug that the name might allow a human to make some  
deductions, but in general I think that a machine will not be able to  
make any - this is why I like your http://www.ivoa.net/Documents/latest/utype-uri.html 
  proposal as it does allow a way to add back the ability to do some  
machine reasoning.

To take a concrete example - from the using STC in VOtable document http://www.ivoa.net/Documents/latest/VOTableSTC.html

<FIELD name="RAJ2000" ucd="pos.eq.ra;meta.main" ref="Coo1"  
ID="RAJ2000" utype="stc:AstroCoords.Position2D.Value2.C1"  
datatype="float" precision="4" unit="deg" />
When writing software to read this, what are you going to do with the  
UType here? You could mechanically decompose it and build an STC  
document that you then parse, but that would seem a little perverse -  
more likely you will want to directly parse the values into an  
internal representation of the STC model, an in which case the most  
likely implementation will be a direct mapping of the UType as a whole  
to the internal model. This example also shows that the amount of  
semantic information that a human can read from the UType is also  
highly dependent on the data model - in this case the UCD is more  
semantically precise than the UType (which only says that it is the  
first coordinate of a 2d position), making the UType somewhat  
redundant, and the software is already going to know the  
correspondence between the UCD and the position in the STC data model.  
This makes me wonder whether UTypes and UCDs are really fulfilling  
distinct roles.....

Cheers,
	Paul.