Re: [boost] FSM Review Announcement

From: Chris Knight (cknite_at_[hidden])
Date: 2008-08-05 18:02:02

On Tuesday 05 August 2008 4:34:30 pm David Abrahams wrote:

> I don't know if you've seen the FSM examples from "C++ Template
> Metaprogramming" or not.  It would be interesting for this library's
> documentation to explain its advantages over that approach (I can see
> some from here already).
>
> One other thing I'd like to see is an implementation of nested states.
> I once promised some people who needed that functionality that I'd
> extend our framework to support them, but was never able to figure out
> what their semantics are ;-)

Nested states would be nice. I looked through the "C++ Template 
Metaprogramming" book as another reference when developing our internal 
implementation. I didn't get too fancy in our own but some protocol's like 
FIX for instance has many, many dark corners. I would see nested states as 
simply a further restriction on the matching of the from state. This could 
provide further optimizations as a well designed transition table could 
reduce the set of states to match against. However, I'm not quite sure how 
that plays out in a design with the enter/exit state handlers, transition 
handlers, et al.

The finite state machine I came up with for the FIX protocol was well over 50 
transitions. I implemented ours so that the from state could be boost::any 
which acts acts like '.' in regexps and I suppose means our implementation 
should be renamed kmt::nfsm. It was a zero-cost implentation as it simply 
adds one last transition to each from state's action linked-list.I was able 
to get the transition list down to 22 with that one feature. Mostly all the 
different error conditions that you want to handle gracefully in a text 
protocol originally designed in the 1980's and still used by finance 
exchanges today. <sigh>

The other feature that I'm not sure whether FSM supports or not is static 
registering of posted event types so as to skip around run-time double 
dispatching. I suspect any implementation that seeks to provide a 
simplified/speedier interface over StateChart should support that as well.

Chris