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Re: Preconditions /effects vs Preconditions/Postconditions

From: Sheila McIlraith <sam@ksl.Stanford.EDU>
Date: Mon, 15 Sep 2003 12:02:56 -0700 (PDT)
To: Walden Mathews <waldenm@optonline.net>
Cc: Charlie Abela <abcharl@keyworld.net>, "Li, Yinsheng" <Yinsheng.Li@nrc-cnrc.gc.ca>, <www-ws@w3.org>
Message-ID: <Pine.GSO.4.44.0309151151490.27852-100000@ksl.Stanford.EDU>


Walden, Charlie, all,

The terms "postcondition" and "effect" are not used consistently
within the computer science literature.  (Neither is the term
"condition", for that matter, which appears to be conflated with
"well-formed formula" in some of the email exchangtes.)

While the distinction you make below may be a reasonable one, I don't
think it reflects the use of *DAML-S* "effects" and *WS Choreography*
"postconditions".  The DAML-S Coalition will (I hope) discuss
this offline on Wednsday and come up with a response to Monika Solaki's
original question, which was to distinguish between these two uses of the
terms.

Regards,
Sheila McIlraith

On Mon, 15 Sep 2003, Walden Mathews wrote:

>
> How about this:
>
> The post-condition of a (successful) credit card transaction
> is that your loan balance is increased and your available credit
> is decreased, both by the transaction amount.
>
> (An invariant on the account state is that current balance
> plus available credit always = credit limit, but that's a little
> outside the scope here.)
>
> The effect of a (successful) credit card transaction is the
> existence of a new transaction identifier, which is handed to
> you as an outcome of the operation.
>
> Failure states are not part of the post-condition or effect
> landscape.  In other words pre- and post-states are part of
> the model of successful operation.
>
> How does that fly?
>
> Walden Mathews
>
>
> ----- Original Message -----
> From: "Charlie Abela" <abcharl@keyworld.net>
> To: "Li, Yinsheng" <Yinsheng.Li@nrc-cnrc.gc.ca>
> Cc: <www-ws@w3.org>
> Sent: Monday, September 15, 2003 1:04 PM
> Subject: RE: Preconditions /effects vs Preconditions/Postconditions
>
>
> > IMHO, the distinction between the post-condition and effect is not a
> > clear-cut distinctionme definitions that I found for "post-condition" over
> the web:
> >
> > i. The post condition is a statement of what the world should look like
> > after an operation. For instance if we define the operation square on a
> > number the post-condition would be of the form result = this * this (where
> > result is the output and this is the object on which the operation was
> > invoked). The post condition is a useful way of saying what we do, without
> > saying how we do it, separating interface from implementation.
> > ii. Post Condition: State of the system after executing the operation.
> > iii. A post-condition specifies some facts about the world which can be
> > expected to be valid after the service operation has finished its
> execution
> > regularly.
> >
> > As for the defined example I'd say that the distinction could become
> clearer
> > if:
> >
> > Post-condition: credit card billed/not billed
> > Effect: transaction successful/failed
> >
> > Charlie
> >
> > -------------------------------------------------
> > Charlie Abela
> > Research Student,
> > Dept. of Computer Science and AI
> > University of Malta,
> > MSD06. Malta
> > Web: http://www.semantech.org
> > Email: abcharl@keyworld.net
> >
> > -----Original Message-----
> >
> >
> >
> > All email is scanned by Keyworld against known Viruses. This service is
> offered to all Keyworld subscribers and hosted domains and does not carry
> any warranty. You are advised to protect your PC with updated antivirus
> software at all times.
> >
>
>
>

==============================================================================

Sheila McIlraith, PhD                 Phone: 650-723-7932
Senior Research Scientist             Fax:  650-725-5850
Knowledge Systems Lab
Department of Computer Science
Gates Sciences Building, 2A-248       http://www.ksl.stanford.edu/people/sam
Stanford University                   E-mail: sam-at-ksl-dot-stanford-dot-edu
Stanford, CA 94305-9020
Received on Monday, 15 September 2003 15:03:34 GMT

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