Re: state machine programming

[Ralph Lange <Ralph.Lange@bessy.de>]

emmanuel_mayssat@lynceantech.com wrote:
> > Through exec() calls a state machine can call any program or script
> > written in any language on your system. You find that too low level?!
> > Could you explain your definition of high level in that context?
> >     
>
> end_stations_protocol:
>   align sample with beam
>   detector_acquire_image
>   move_grid
>   rotate_sample
>   detector_acquire_image
> [...]
> each command is drag-and-dropped by the experimenter (not a programmer)
> sequence can be changed by the experimenter function of what he wants to do
> He should be able to work independently of programmers
>
> [...]
> Of course, each of those states can be further broken down and later refined
> (I can easily see a state machine with 500+ states)
> interpreted languages are better, no compilation needed 
> + you can program a sequence on the fly (no downtime required)
>   

Ahh... now I can see what you are trying to achieve.
These are not the usual finite state machines, though, and the pluggable 
parts are not states (as the term is used in EPICS context).
The sequencer is not the right tool to do this, neither probably are the 
FSM record or the Qt4 thing you were mentioning.

You are looking for a generic script execution engine for an interpreted 
language, with (possibly) a graphical interface. The trickiest part of 
these things is getting the exception/error handling right, with the 
different options of ignoring vs. instant correcting vs. fallback vs. 
safe state transition. To sucessfully do that, state machine like 
behavior will be part of the solution, of course.
The closest thing within the EPICS context (that I know of) is the PEM 
system you mentioned [1], used at the APS for their scripted operation 
procedures. It almost looks like what you described in your last mail - 
I don't know implementation details or recent developments, though.
Other places to look would be the high level app frameworks: XAL and GDA 
might have interfaces that allow such kind of hierarchical script 
snippeting. Again: I would see the error handling always being the 
hard-to-get-right and crucial part of these engines.

Good luck!
Ralph

[1] 
http://www.aps.anl.gov/Accelerator_Systems_Division/Operations_Analysis/manuals/APSPEM/APSPEM4.html