When do statemachine changes become effective



When do statemachine changes become effective

From: Jörg Pleumann ^lt;joerg@pleumann.de>
Date: Wed 11 May 2005 - 10:53:28 BST
Message-ID: <4281D618.6000302@pleumann.de>
Hello,

I have a question regarding UML statemachines: When does a change caused 
by an action of a triggered transition become effective? My intuition 
says "immediately", that is, in the same run-to-completion step, as soon 
as the action is being executed. Thus, the following transition,

     evt/x=x+1;y=x*5
   ------------------->

with an initial value of x=0, should result in y being 5 afterwards.

In the original Harel statecharts, changes became effective in the next 
step only, if I read the classical semantics paper from '96 correctly. 
Therefore, the above example would have yielded a value of y=0, because 
the second expression relies on the original value of x, not the new 
one. In particular, it should have been possible to use something like this

     evt/x=y;x=y
   ------------------->

to exchange variables (which looks rather strange from a programmer's 
point of view).

I'm pretty sure the UML dropped this delayed visibility of changes, 
because it doesn't seem to make sense in the world of oo/prodedural 
programming, in particular once you allow to invoke methods from a 
transition. I just can't find a corresponding reference in the spec. Can 
someone point me to a statement that says exactly this or allows to 
deduce it?

Regards, Jörg

PS: I aware that the situation is somewhat different when it comes to 
guards and junction/choice points, but that's not what I'm asking for.

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Received on Wed May 11 10:53:45 2005