Support if-else in State Transition Specifier

[etiennecollin]

Hi!
In state machines, the FPP specification currently only supports the following pattern for state transitions:

on identifier [ if identifier ] transition-or-do

See: State Transition Specifier

It would be great, and I don't think too hard, to enable the use of an else in that transition specifier. Here is a pattern that we would like to use in our flight sequencer:

guard guard_sensors_read_nominal_values
signal tick
action current_state_action

state CURRENT_STATE {
	on tick if guard_sensors_read_nominal_values do { current_state_action } else enter ABORT_STATE
}

state ABORT_STATE {}

In essence, we want something like the choice, but that supports both the transition-expression and the do-expression. This would allow a more solid and intuitive logic flow in the state machine.

Let me know if you believe it would be possible and acceptable for you to add that syntax to FPP.

Thank you for your work and have a great day,
Etienne

[etiennecollin]

To add to my request:

It might currently be possible to do something like:

guard guard_sensors_read_nominal_values
signal tick
action current_state_action

state CURRENT_STATE {
	on tick choice {
		if guard_sensors_read_nominal_values do { current_state_action} enter CURRENT_STATE else enter ABORT_STATE
	}
}

state ABORT_STATE {}

but re-entering the current state like that feels wrong and might introduce unnecessary function calls to re-enter the state.

Let me know what you think!

[bocchino]

This sounds like a useful feature. I think we could also do this with a choice and a local transition: https://statecharts.dev/glossary/local-transition.html. Currently FPP state machines do not support local transitions, except in the special case of an internal (or local) transition to self. We should consider adding them.

[etiennecollin]

Hey! Thanks for answering! I also think local-transitions could work. We currently settled on the following pattern using sub-states (which, in a sense, is similar to local transitions):

signal tick
guard guard_sensors_read_nominal_values
action current_state_action

state CURRENT_STATE {
  initial enter LOCAL_IDLE
  state LOCAL_IDLE {}

  on tick enter TICK_CHECK
  choice TICK_CHECK {
    if guard_sensors_read_nominal_values do { current_state_action } enter LOCAL_IDLE else enter ABORT_STATE
  }
}

state ABORT_STATE {}

It's not the most efficient, but for now it's enough and it's relatively easy to expand with other sub-states when needed.

[bocchino]

Yes, that sounds like a good solution for now.