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Re: Confused by recurring-duration

From: Ashok Malhotra/Watson/IBM <petsa@us.ibm.com>
Date: Fri, 15 Sep 2000 09:11:30 -0400
To: James Clark <jjc@jclark.com>
Cc: Paul.V.Biron@kp.org, XML Schema Comments <www-xml-schema-comments@w3.org>
Message-ID: <OFFA8219F5.C20F5E39-ON8525695B.004793E9@pok.ibm.com>

James:
A few comments embedded in your note below.

All the best, Ashok


James Clark <jjc@jclark.com> on 09/15/2000 01:09:34 AM

To:   Ashok Malhotra/Watson/IBM@IBMUS
cc:   Paul.V.Biron@kp.org, XML Schema Comments
      <www-xml-schema-comments@w3.org>
Subject:  Re: Confused by recurring-duration



Ashok Malhotra/Watson/IBM wrote:
>
> Oh boy!  I just looked at the draft and realized there is no discussion
> of a value space for recurringDuration.  Thank you, James, for catching
> this.  My apologies for the omission.  Paul, we need to fix this.
>
> Let me take a stab at defining the value space for recurringDuration:
> "The value space for recurringDuration is a series of timeDurations
> that recur with some frequency.  The frequency of recurrence is defined
> as the timeDuration between two occurrences."

Each member of the value space is a series.  Thus the value space is
itself a set of series.  The members of each series in the value are
surely timePeriods not timeDurations (they start at a particular instant
of time, and persist for a particular duration).  Only a series that
satisfies the following constraints can be members of the value space:

- fixed duration: the duration of each member of the series must be the
same

- fixed period: the duration from the start of one member to the start
of another member must be the same for any two consecutive members of
the series

- infinite: every member has a preceding and a following member

In other words it is a single duration of time that recurs starting at a
series of equally-separated instants of times.

A single period of time is modelled as an infinite series with a zero
period.

AM>> Yes, the above is correct.

The lexical space represents the value space by representing the start
of one the time periods that is a member of the series.  The choice of
which time period to represent is arbitrary.  Thus

<recurringDuration duration="P0Y1M"
period="P1Y">2000-01-01T00:00:00</recurringDuration>

represents the same value as

<recurringDuration duration="P0Y1M"
period="P1Y">2001-01-01T00:00:00</recurringDuration>

AM>> The above two look the same to me.

I find it hard to see how you can pick a single canonical lexical
representation.  Perhaps recurringDuration should be abstract.

AM>> In the abstract, recuringDuration has three variables:
AM>> the fixed duration, the fixed period and the start instant.
AM>> Since a value in an XML instance document can specify only
AM>> one of these three values, two have to be specified when the
AM>> datatype is defined.  We, somewhat arbitrarily, picked the
AM>> duration and period to be specified when the datatype is defined
AM>> and the start instant as the value to appear in the instance.

James
Received on Friday, 15 September 2000 09:11:35 UTC

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