xtype: similarity to VODataService's externalType

[Arnold Rots]

I probably should have reminded people, for the record, of something I
mentioned a long time ago: that the whole business of ISO-8601 time
stamps could be avoided if VOTable were to recognize the datatype
xs:datetime.

  - Arnold

Douglas Tody wrote:
> Hi -
> 
> As I mentioned in the VOTable session, we need to be very careful
> to distinguish XTYPE from UTYPE.  So XTYPE is used to define a
> "custom type".  But an "abstract type" sounds very similar, but can
> be an object or data model, which is where UTYPE is already used.
> 
> The distinction should be that anything which has an XTYPE is an
> atomic type which could be used anywhere (not defined as an element of
> some data model), and which can be mapped to a primary type such as
> any VOTable datatype.  Basically it is used to extend the "machine"
> level typing system to say things such as a primitive type of string
> contains a timestamp or STC-S region.
> 
> We can probably ensure against semantic confusion by insisting that an
> XTYPE be storable in a single item of primitive type, and manipulated
> atomically as a primitive type would be, at the layer where the type
> is defined.  It would be possible to abuse this simple concept by
> serializing an object as a string or blob, but if it can nonetheless
> be manipulated as an atomic object at the level where the datatype
> is defined (as in storing an object in a field of a table) this might
> still be ok.
> 
> Hence a FIELD or PARAM (and maybe INFO for consistency although I
> would be happy with only having string there) can have an XTYPE,
> but GROUP should not, nor any higher level non-atomic construct
> which has some internal structure.   If it can't have a "datatype",
> it can't have an XTYTPE.  This is for VOTable, but if VODataService
> deals with this as well it should be consistent.
> 
>  	- Doug
> 
> 
> On Thu, 28 May 2009, Ray Plante wrote:
> 
> > Hi VOTablers,
> > 
> > As discussed in the last VOTable session in Strasbourg, we are considering 
> > adding a new FIELD attribute called xtype (or exttype or externalType). 
> > As I mentioned, this is very similar to an attribute availble in 
> > VODataService for describing a table column.
> > 
> > For reference, here's how the VODataService Document 
> > (http://www.ivoa.net/Documents/latest/VODataService.html) describes 
> > them...
> >
> >    More descriptive information about the type can be provided via the
> >    extendedType and extendedSchema, which provide an alternate data type
> >    name. It's expected that this name will only be understood by a special
> >    subset of applications. The name given in the element content, then,
> >    represents a more commonly understood "fall-back" type.
> >
> >      extendedType  Value type: 	     string: xs:string.
> >                    Semantic Meaning:  the data value represented by this
> >                                       type can be interpreted as of a
> >                                       custom type identified by the value
> >                                       of this attribute.
> >                    Occurrences: 	     optional
> >                    Comments:          The name implies a particular
> >                                       expected format for the data
> >                                       value that can be parsed into a
> >                                       value in memory.
> >
> >                                       If an application does not recognize
> >                                       this extendedType, it should attempt
> >                                       to handle value assuming the type
> >                                       given by the element's value.
> >                                       "string" (or its equivalent) is a
> >                                       recommended default type.
> >
> >                                       This element may make use of the
> >                                       extendedSchema attribute and/or any
> >                                       arbitrary (qualified) attribute to
> >                                       refine the identification of the
> >                                       type.
> >
> >    extendedSchema  Value type:        URI: xs:anyURI.
> >                    Semantic Meaning:  An identifier for the schema that
> >                                       the value given by the extended
> >                                       attribute is drawn from.
> >                    Occurrences:       optional
> >                    Comments:          This attribute is normally ignored if
> >                                       the extended element is not present.
> > 
> > hope this is helpful for reference.
> > 
> > cheers,
> > Ray
> >
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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