[boost] Re: [fsm] Why exit actions must not fail

From: David Abrahams (dave_at_[hidden])
Date: 2004-05-30 07:27:53

"Andreas Huber" <ah2003_at_[hidden]> writes:

> David Abrahams wrote:
>>> I'm reading it like a bible only when it helps to argue my case ;-).
>>
>> I know there's a smiley, but you're not doing your credibility any
>> favors with that statement.  It makes me wonder how many other
>> decisions in the design have such a flimsy foundation.
>
> I think a foundation for a decision is not flimsy when the behavior
> in question has proved to be useful in practice. So far nobody, who
> seems to have experience with non-trivial FSMs, has doubted that it
> is useful to terminate a state machine when it is destructed (my
> assumption A1).  

I don't doubt that it's often useful.  I also think it is surely
sometimes highly undesirable.  If you remove A1, the "useful" behavior
is trivial to achieve without transforming the FSM, so it seems that a
design without A1 is both more flexible and more orthogonal.

> BTW, as pointed out by someone else the user *does*
> have a choice. If she happens to have an exit action that she'd
> rather not have executed upon destruction then she can always
> transform the machine part in question such that her action is
> executed as part of a transition action instead (I would consider it
> a design error not to do so).

Why?

> Every Moore machine (an FSM that has only entry and exit actions)
> can be transformed to a Mealy machine (an FSM that has only
> transition actions) and vice versa.

Why require a transformation when it's not neccessary?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com