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Re: a "modest proposal" for PRD

From: Christian de Sainte Marie <csma@ilog.fr>
Date: Wed, 20 Feb 2008 16:33:25 +0100
Message-ID: <47BC4845.2060902@ilog.fr>
To: Gary Hallmark <gary.hallmark@oracle.com>
CC: W3C RIF WG <public-rif-wg@w3.org>

Hi Gary,

We will discuss your proposal at the F2F; here are just a couple 
questions and first thoughts, off the top of my head.

Gary Hallmark wrote:
> 
> Start with exactly the BLD syntax.

Is that feasible? E.g. what about logic functions?

> Add just 1 or 2 "hard" things (e.g. 
> retraction and rule priority) that are common in PR and make a model 
> theory very difficult.

Do you mean that PRD should include a model theory of PR systems? I do 
not know if this is feasible, but, assuming it is, what would be the 
benefit wrt an operational semantics (depends probably on the audience 
we target and the purpose we see for that document, I guess)?

> Do not respecify the syntax, just add the new elements.

Re respecifying the syntax : do we agree that the syntaxes of BLD and 
the current PRD draft are the same (except on logic functions, 
aggregation and actions, and pending some questions about naming and one 
suggestion about stripping - ok, that makes already some differences)?

If so, I understand that you want the syntax of PRD presented exactly 
the same way as the syntax of BLD is: is that what you mean? Benefits, 
drawbacks and sustainability of that approach need be discussed.

> Develop an Operational Semantics for it, as defined by 
> Plotkin in http://homepages.inf.ed.ac.uk/gdp/publications/sos_jlap.pdf.

Any salient reason for that specific choice? (I have some reservation, 
but that is after reading only the first sentence :-)

> Do not spend time on informal semantics that can only diverge from the 
> formal semantics. 

The term "informal semantics" might have been a bad choice (it comes 
from the Boston discussion), but a description of the intended use when 
describing a construct seems rather useful for the reader, if we want to 
separate the specification of the syntax from that of the semantics.

Why can that description of the intended use "only diverge from the 
formal semantics"? Does it in the current PRD draft?

On the other hand, the specification of the normative semantics in the 
current draft of PRD being structured exactly the same way as the 
description of the syntax, it is only a matter of cut and paste to 
replace the current "informal semantics" by the "normative semantics for 
each construct.

Now that you suggested it (indirectly :-), I can see the benefit of 
doing that. A drawback could be the lack of a single place where the 
complete semantics is specified... Hmmm, it is in three different places 
in the current draft, anyway...

Re "formal semantics": when you write "formal", you mean "using a 
mathematical formalism"? If so, what is the benefit wrt the kind of 
specification we have in the current draft (which need certainly lots of 
improvement, but I am talking about the approach, not the current state 
of the spec). Depends on the intended target audience/purpose of the 
spec, again, I guess.

> Only after PRD has caught up with BLD in terms of 
> semantic rigor should it incorporate negation, aggregation, other 
> actions, etc.

That seems to imply that the approach taken in the current draft is not, 
or not easily, amenable to the same level of semantic rigor than BLD: is 
that what you mean?

It also implies that the current draft is in a rather bad state, in 
terms of semantic rigor. Michael wrote that as well. I wonder why: I 
mean, of course, there are probably ambiguities remaining in many 
places, but I thought that it was rather rigorous.

Can you point to specific places where the specification of the 
semantics lacks rigor?

> The task of developing a formal Operational Semantics for PRD is not 
> trivial and would be best attempted by someone who has done something 
> similar before.

Again, I assume that "formal" means, here, "using a mathematical 
formalism". And same interrogation again about the intended benefits...

Not to mention that the first PRD strawman proposed such a "formal" 
approach, based (heavily based, to the point of copy-paste in many 
places) on wrtings by people who had done this before (Michael Kifer, 
Claude Kirchner and Hassan A´t-Kaši, to name them) and that approach was 
rejected in Boston as off target and unlikely to converge to something 
useful in a finite time...

One underlying question I have in mind, of course, is whether the 
current draft is really hopeless to the point that throwing it away is 
the most efficient path to a useful PRD-RIF?

I would not think so, but, of course, I am biased :-)

See you all tomorrow.

Cheers,

Christian
Received on Wednesday, 20 February 2008 15:34:58 GMT

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