Re: Concrete types: next steps?

From: Dan Connolly (
Date: 01/31/01

First, apologies for missing the last two
teleconferences without notice; by
way of explanation, I was
at a workshop in France last week,

  W3C Rights Management Workshop

and I still haven't managed to reduce
my schedule for tuesdays to less than 6(!)
hours of teleconferences;
yesterday, the meeting right before the DAML
telcon went long, and it was critical to
getting the official W3C Semantic Web stuff going,
so I stayed on.

Mike, again, if you'll get the agenda out
24hours in advance, it'll help me let
you know if I'm unavailable for teleconferences.

OK... enough of that... now onto the
juicy technical bits...

Frank van Harmelen wrote:
> After the teleconf last night, I've tried to summarise the current situation as I perceive it. Perhaps this is useful to find a way forward.
> Frank.
>    ---
> - It is widely accepted that concrete types are a must for any realistic ontology language.


> - Currently the only proposal on the table is 

Er... well.. I don't claim mine is complete, but since
I don't agree that yours is complete either, I suggest
that this is also on the table:

  Using XML Schema Datatypes in RDF and DAML+OIL

Tim and I chatted about it yesterday, and I now think I
know how to fill in the gaps. Oh for an hour or two
to write and think...

(does "the one by Ian & Peter" have a handy address, by the way?
I'm awash in a sea of email.)

> - Its most contentious aspect is the strict separation of abstract and concrete types.

Yes; that is certainly an issue that gives me pause.

> - The main reason for the separation is that this enables efficient sound and complete reasoning.

As I have said, this is not a design goal I subscribe
to. My goal is efficient proof-checking, not theorem

  On the semantic web in general, a party must be
  able to follow a proof of a theorem but is not expected
  to generate one.

  (This fundamental change goals from KR
  systems to the semantic web is loosely analogous with the
  goal change from conventional hypertext systems to the original
  Web design dropping link consistency in favor of expressive
  flexibility and scalability. The latter did not prevent
  individual web sites from having a strict hierarchical order
  or matrix structure, but it did not require it of the
  web as a whole.)

  --        Logic on the Web -- Web architecture
  Sat, 30 Dec 2000 16:53:40 GMT

This point is also made in our DAML proposal somewhere,
but I'm not sure where exactly.

> - This claim is apparantly widely accepted within the DL community.
> Question for Ian: can you provide an example of what goes wrong witout the separation?
> [So far, nothing to disagree with, I guess:-)]
> - a (lesser) complaint (mostly raised by Jeff) was the duplication of syntactic constructs for abstract and concrete types, but this complaint was not as essential, and could also be relieved to a certain extent.
> [Question: is this correct?]

Well... we're doing language design. In my experience,
a critical part of language design is looking at examples
and saying "yeah, that feels right" or "ew... ick."

I'm not sure I've read the most recent iteration
of "the one by Ian & Peter", but the proposal that
I remember reading was very light on concrete
syntax examples.

On the other hand, Hendler proposed a "no aesthetic
arguments" rule back in August, and I've exploited
it extensively in (non-)defense of RDF's syntax. ;-)

So yes, I agree, this is not a critical issue.

> - The separation disallows various constructions:
> [1] construct "mixed classes" which are a combination of abstract classes and concrete types.
> [2] use such mixed classes as the domain of properties
> [3] use such mixed classes as the range of properties
> [4] construct subclasses of concrete types by defining a relation with a non-concrete types (Pat's example of "all salary scales in my company")
> [Question: are these correct? are these all?]
> - Some (Tim, Dan) claim that such expressions are essential to future uses of the language
> - Others (Deb) claim that in their practical modelling experience, they were never significantly hindered by the separation
> [Tim, Dan, Pat: can you come up with convincing examples/experiences?]

None comes to mind just now, but I hope to find time soon.

> - The severity of the restriction also depends on the perceived goal of DAML+OIL: [a] is DAML+OIL simply a small step to extend current web-ontology languages (eg. RDF Schema), and will DAML+OIL itself again be extended?
> [b] or is DAML+OIL itself aiming to be a language for "universal use" that should empose as little restrictions as possible?
> In case [a], all we need worry about is whether the current restriction will hinder future extensions that might lift the restriction (either theoretically or pragmatically (invested effort in tools etc).
> (Exercise: do a headcount,and (unsurprisingly) you will find that people who take view [a] tend to favour the separation, people who take view [b] strongly object to the separation).

Interesting insight.

I would be much happier with "the one by Ian & Peter" if
it were clearly labelled, in the next DAML+ONT release,
as something we're less than 100% satsified with and
that we hope/intend to reconsider.

> - One possible way out of the dillemma I proposed would be to define a language
> [i] which allows mixing of concrete and abstract types, but
> [ii] where the language which does not mix the two is a proper syntactic subset
> [iii] where this subset can be recognised in a computationally feasible manner.
> - An alternative way out (proposed by Ian) would be to do [i], but not insist on [ii] and [iii], provided that reasoners can spot when we are mixing concrete and abstract types, so that users can be warned/computations can be aborted, etc.
> - The difference between these two alternatives is whether the mixing is spotted at parse-time or at reasoning-time.
> - The first alternative is clearly preferable, but we do not know if this will be feasible.
> - Another possible way out is for someone to work out a more liberal proposal, so that we can study the pro's and con's.
> [Dan: you were rumoured to have such a proposal almost done?]

Yes, but my baby brother puked on it, and my dog ate the
other draft, and ... ;-)

Here's hoping I can put something more on the table in the
next couple days...

Dan Connolly, W3C
office: tel:+1-913-491-0501
  (put return phone number in from/subject)

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