gen_fsm learnings, part 3

12 Nov 2013 13:51 erlang

Some odds and ends.

handle_info information

The last of the “I don’t know what this is for” functions is handle_info. This is documented as being called for messages that aren’t events or system messages.

Your implementation of this should take a look at the event (in Info), and then it can return {next_state, NextStateName, NewStateData} to move to the next state.

But this isn’t particularly state-machine-like, since you’re not looking at the current state when working out what to do with the event.

One possible answer (from here) is to simply write it as follows:

handle_info(Info, StateName, StateData) ->
    ?MODULE:StateName(Info, StateData).

But, if you have a choice, you should try to avoid sending messages directly to your FSM process.

Entering a state

I can’t find any way to be notified when the FSM enters a particular state. That is, there’s nothing like Module:State_enter(StateData) or Module:handle_enter(StateName, StateData). So, you have to handle this case when transitioning from the previous state.

Something like the following might suit:

enter_gamma(StateData) ->
    %% Code goes here
    {next_state, gamma, StateData}.

alpha(gamma, StateData) ->

beta(gamma, StateData) ->


You can have timeouts in your FSM, by returning {next_state, NextStateName, NewStateData, Timeout}. If, while in the given state, no events are received, you’ll receive a call to Module:State(timeout, StateData).

You can choose to do something and then remain in this state, or you could transition to another state.

If you return {next_state, SameState, NewStateData}, the timeout is discarded. If you return {next_state, SameState, NewStateData, Timeout}, the timeout is restarted. If you want the timeout to continue, you’ll need to keep track of the time remaining yourself. See, for example, my answer on Stack Overflow about this.

Note, however, that I’m not sure how the timeouts are calculated if you turn handle_info into a direct call to the Module:State/2 function, as above.

How do I detect current state?

You can detect the current state of your FSM by using sys:get_state/1, as follows:

{StateName, StateData} = sys:get_state(Pid).

Note that this won’t work reliably for asynchronous events (race condition).