[xtm-wg] CMS: UCS: ISS: Types (response to discussion in DTD, based on use cases)

["Wrightson, Ann" <Ann.Wrightson@sweetandmaxwell.co.uk>]

A response to the discussion in the DTD comments, concerning class-instance
and subtype-supertype, and based on the Usecases from the UCS survey.
Contains both syntactic and conceptual modelling comments.

Although based on the Usecases, please note this is a personal not a group
contribution.

There were two principal kinds of usecases found in the UCS-group survey,
briefly characterized here as  "topical organization" and "navigation" (see
earlier UCS emails for more details). I believe that these have different
requirements regarding type structures.

1. Topical organization applications 

These have a very wide range of potential structures; this is mainly because
their structures need to reflect accurately the conceptual, informational or
data structures of the subject domain(s) they are aiming to model. The
examples offered in the survey were pretty diverse - and some will require,
for full & faithful modelling, the full power of a type "lattice" (a
structure allowing both multiple subtypes and multiple supertypes, with no
requirement to merge or otherwise organize the supertypes of a type) - in
addition to class-instance (or, more accurately, type - instance). 

Whether we wish to support all that as elementary XTM concepts is to me an
XTM "business decision" - it would be sensible to say that XTM supports a
broad range of topical organization applications, but that the more
"difficult" ones do need 13250, and the most awkward may need to add some
extra HyTime even to 13250 (or even - perish the thought! - use another
technology entirely). I suggest we make sure that XTM can accommodate at
least the complexity of NewsML - a structure based on a well analysed real
Webby requirement.

BTW, I disagree that types are (just) a shortcut to associations - they can
be built that way - i.e. you can implement a subject-domain type structure
in a topic map document using associations - but the concept is different.
Since typing is a simple foundational concept quite well understood in
computer science (i.e. many potential implementers at least had a course on
it at college) then I would like to see it survive as a primitive concept in
XTM. Associations are relations (i.e. like tuples, but  indexed by a set (of
roles) rather than by integers), a different and more complex concept.

2. Navigation Applications

I believe that associations and occurrences alone will support these (I
think in hindsight that a rough estimate of this distinction was probably
the basis for my classification!). I also believe that XTM is only a
technology of choice for these apps by virtue of publicity/availability and
convenience, since they are the "core territory" of HyTime. However, because
of this convenience (especially for hybrids - see below) XTM will be used,
and we need to acknowledge/accommodate that.

Hybrids (between topical organization and navigation)

These will be an interesting challenge to build well. I agree very much with
Steve N's comment that topic maps must not be forced to merge, & I have a
motivating example: 
Several (maybe large/complex) information domains with existing
topical-organization TMs are being cross-referenced, and made available for
a browsing interface, using a navigation-structure TM. I would want to be
free to choose (as the designer of the navigation app) whether the
information-domain TMs were connected into as TM, or used in a more
"standoff" style, as structured resources. In particular, if I was dealing
with a v. complex information-domain, maybe I could make it usable for some
specific purpose, through a channel which had lesser structure-capability,
by careful crafted design of the interconnection between my TM and the
domain TM.

Ann W.

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