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Re: isa and hasa in UML

From: Walden Mathews <waldenm@optonline.net>
Date: Fri, 30 May 2003 15:07:06 -0400
To: Martin Chapman <martin.chapman@oracle.com>, www-ws-arch@w3.org
Message-id: <001701c326de$a9f66bc0$1702a8c0@WorkGroup>

+1 (and extra credit for brevity!)

----- Original Message -----
From: "Martin Chapman" <martin.chapman@oracle.com>
To: "Cutler, Roger (RogerCutler)" <RogerCutler@chevrontexaco.com>; "Francis
McCabe" <fgm@fla.fujitsu.com>; <www-ws-arch@w3.org>
Sent: Friday, May 30, 2003 2:33 PM
Subject: RE: isa and hasa in UML


>
> and how is is-a and has-a as defined in our doc any less or more powerful
> than uml.
> its all about set theory and if you choose to model bad sets thats up to
> you.
>
> Martin.
>
> > -----Original Message-----
> > From: www-ws-arch-request@w3.org [mailto:www-ws-arch-request@w3.org]On
> > Behalf Of Cutler, Roger (RogerCutler)
> > Sent: Friday, May 30, 2003 10:43 AM
> > To: Francis McCabe; www-ws-arch@w3.org
> > Subject: RE: isa and hasa in UML
> >
> >
> >
> > This is REALLY discouraging.  I thought that the UML experts in the room
> > at Rennes were saying that "everybody knows" what has-a means in UML,
> > and all you have to do is strip it out of your favorite undergraduate
> > textbook.
> >
> > I have a strong feeling of distaste for ditching the definition of
> > "has-a" currently in the document, which at least has the virtue that I
> > can understand and apply it, in favor of a definition that appears to be
> > like the Indian rope trick -- something that everybody knows exists
> > because somebody else has seen it.
> >
> > -----Original Message-----
> > From: Francis McCabe [mailto:fgm@fla.fujitsu.com]
> > Sent: Friday, May 30, 2003 12:01 PM
> > To: www-ws-arch@w3.org
> > Subject: isa and hasa in UML
> >
> >
> >
> > This is in partial fulfillment of my action item re is-a and has-a
> > w.r.t. UML
> >
> > 0. There is a rather (unintentionally) funny comment in the UML 2.0
> > spec:
> >
> > 2.3.2.3 Semantics
> >    The meanings of the constructs are defined using natural language.
> > ...
> >
> > (This is after a lot of promises of being formal.)
> >
> >    However, UML uses OCL for those cases where natural language is not
> > enough. OCL is similar to a first order predicate calculus. Having said
> > that, the spec does not use OCL very often; including for the
> > definition of relationships such as generalization (is-a) and
> > association (has-a kind of)
> >
> > 1. As I have indicated earlier, UML does not have a precise notion of
> > is-a. The closest is the generalization relationship. This is defined
> > in 3.50:
> >
> > Generalization is the taxonomic relationship between a more general
> > element and a more specific element that is fully consistent and that
> > adds additional information.
> >
> > A couple of comments:
> >
> > 1. Basing is-a on taxonomics raises some serious logical issues. This
> > is analogous to basing everything on sets: every member of the penguin
> > set is also a member of the bird set.
> >
> > The problem is that it becomes really difficult to talk about weird or
> > abstract sets. Basing is-a on this would lead to the following
> > counter-intuitive result: every unicorn is a yeti. (There are no
> > documented instances of either, so the set of unicorns and yetis is
> > indistinguishable.)
> >
> > A more serious issue, sticking with birds for the moment, is that it is
> > similarly hard to talk about properties of birds such as flying: we
> > could not express the fact that all birds except penguins fly.
> >
> > An even more serious issue is that we need to capture the following
> > situation:
> >
> > A service has an identifier
> >
> > A Web service is a service
> > A Web service has a URI
> >
> > The Web service's URI counts_as the service identifier
> >
> > It is that counts_as that is beyond the capabilities of UML's
> > generalization. We *could* extend UML's generalization, and that may be
> > the best overall approach. In fact, we would really need to do that for
> > all our relationships, use <is-a> and <has-a> and *never* rely on UML's
> > built-in relationships. <is-a> and <has-a> could probably be defined in
> > OCL.
> >
> >
> > More to follow....
> > Frank
> >
> >
> >
> >
> >
> >
> >
>
>
Received on Friday, 30 May 2003 15:02:53 GMT

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