FSM step by step

Vance Shipley <>
Mon Aug 16 20:38:06 CEST 2004

On Mon, Aug 16, 2004 at 02:55:38PM -0300, Eduardo Figoli wrote:
}  Is there a way to execute an FSM (OTP module FSM behaviour)
}  step by step without modifying the existing FSM implementation code?

You want the debugger!


It's well worth the time to learn.

At a lower level you can use stdlib's sys module.  This is what the 
debugger does.  You can supply debug options (dbg_opt()) in the gen_fsm
start function:

  gen_fsm:start_link(Module, Args, [{debug, Dbgs}])

One of these options can be {install, {Func, FuncState}}}.  This debug
function will be called for every system event.  You would have this
function wait for a "step" message before continuing.  You can install
a debug function in a running process with sys:remove/2.

I have found the debug options to be a handy way of debugging in the
field applications.  I install a handler which formats just the
information I'm interested in and presents it in an application specific
way.  It's easy to turn it on and off and the distibuted code is


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