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Subject: Re: [boost] New library in Vault: Msm (Meta State Machine)
From: Andreas Huber (ahd6974-spamboostorgtrap_at_[hidden])
Date: 2008-10-16 18:26:15

> > What do t & D stand for (they do not appear in Figure 2)? They don't
> > seem to be
> > necessary to implement the chart?
> You should also read the mini doc I included.

Right, sorry I missed that one.

> I added them to make the problem a bit more interesting and show how to
> solve it
> with flags.


>> 1) Is it possible to implement the transitions exactly as shown in the
>> chart?
> It is not, again as explained in the mini doc. I see this as an UML goto
> and decided to implement superior tools instead.

So IIUC then you think that transitions crossing state boundaries are bad
practice and should be avoided at all costs? In general, I agree that you
should avoid them when you can, e.g. in the example chart the transition
triggered by y should rather be a self-transition of C, which is
semantically equivalent. OTOH, the transition triggered by x can only be
made non-state-boundary-crossing by introducing a guard, which is IMO less
concise than allowing for the transition as shown in the chart.

Moreover, things become a bit more messy if you want to allow for multiple
ways of entering a composite state. For example consider Figure 8 in


Here the "alarms-beep" outer state exists primarily because you don't want
to duplicate the "any button pressed" and "30 sec in alarms-beep"
transitions for all three different alarm beep states. The other diagrams in
the document show more uses of state-boundary-crossing transitions. Granted,
Harel sometimes uses them excessively, in places where he could simply have
transitioned to the outer state instead and let the default transition take
care of going to the inner initial state. But in a few places I don't see
any equally concise substitute for such transitions.

As an aside, I don't see much of a connection of state-boundary crossing
transitions with goto. Given your code in the C() function, any transition
could/would be implemented with goto?

> Of course I could implement these entry and exit goto-like pseudo states
> if users want
> them.I just strongly doubt they are needed.

Given the examples in the Harel paper and the fact that the UML standard
expressly allows them, I would encourage you to support them. BTW, for the
same reasons I don't detect value in seeing a composite state as a fully
fledged state machine (as your framework seems to do). I know that the UML
has so-called submachines but they are really just simple composite states
with entry and exit points thrown in.

Just my 2c.


Andreas Huber
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