Re: Status of UML 2.0

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From: Hubert Baumeister (
Date: Thu 24 Oct 2002 - 18:19:41 BST

Hi Robert,

Bauer, Robert wrote:
> Heinrich,
> Although I don't read German well, I was able to look at the web pages 
> you referenced below.  One thing bothers me.  As I looked at the 
> website I noticed the goal of being "unambiguous" uml.  
> This does not seem to be possible unless uml was no more expressive than 
> a finite automata - that is, one could represent uml using regular 
> expression (or equivalently a left/right linear grammar).  Now, I 
> haven't thought about this very long or deeply, but it seems to me that 
> while it is relatively easy to check whether two uml "graphs" are 
> equivalent at the syntactical level, is intractable at best and quite 
> possibly impossible - if we were to use 1st order logic to express the 
> semantics, well at best it is only semi-deciable whether two 1st order 
> expressions are equivalent.  Since, ocl cannot yet express all of the 
> meta-model, it cannot possibly express the semantics of every uml model, 
> so while one may be able to decide whether two ocl expression are 
> equivalent (though I suspect even though ocl is not a 1st order logic, 
> it is still undecidable due to the presence of "allInstances"), the 
> "rest" of the semantics of the uml model are either in a natural 
> language (clearly impossible to determine in general whether two 
> sentences are equivalent or equivalently whether a sentence is 
> ambiguous) or anything more expressive than a F.A. (also except for 
> discrete context free languages, clearly impossible to determine 
> equivalence, ambiguity, etc.).
> So, for those reasons, I am not sure why the goal is stated as it is.

My understanding of "unambiguous" UML is that each UML diagram has well defined meaning as opposed 
to several meanings depending on who is looking at the diagram. This does (and should not) exclude 
the possibility that the same meaning can be expressed by different diagrams.


Dr. Hubert Baumeister, Institut für Informatik, Universität München
phone (x49-89)2180-9375  * fax -9175

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