Feature: event data matching

[x37v]

Following on to my question in #23, I'm realizing that guards won't solve my problems, specifically, I'm implementing a series of menus that are driven by a pad of 16 buttons.. I'm trying to avoid creating unique events per button press/release while also allowing for using constants to identify details of the event matching so that I might easily use that same data to identify where to display something and potentially change the value of the constant (button that represents the transition/display area) in the future.

I'm wondering if it would make sense to expand the Event syntax to allow for pattern matching, so you can reuse the same event but with different patterns to specify different transitions:

const FOO_INDEX: usize = 2;
const BAR_INDEX: usize = 3;

pub struct ButtonData {
  pub index: usize,
  pub down: bool
}

statemachine! {
    transitions: {
        *State1 + Button(ButtonData { index: FOO_INDEX, down : true }) = State2,
        State1 + Button(ButtonData { index: BAR_INDEX, .. }) = State3,
    }
}

which I assume would generate code somewhat like:

pub fn process_event(&mut self, mut event: Events) -> Result<&States, Error> {
    match self.state {
        States::State1 => match event {
            Events::Button(ButtonData { index: FOO_INDEX, down: true })  => {
                self.state = States::State2;
                Ok(&self.state)
            }
            Events::Button(ButtonData { index: BAR_INDEX, .. })  => {
                self.state = States::State3;
                Ok(&self.state)
            }
            _ => Err(Error::InvalidEvent),
        },
        States::State2 => match event {
            _ => Err(Error::InvalidEvent),
        },
        States::State3 => match event {
            _ => Err(Error::InvalidEvent),
        },
        _ => Err(Error::InvalidEvent),
    }
}

[x37v]

I could also see allowing for the match guard style syntax:
*State1 + Button(ButtonData { index, down : true } if index < 20) = State2

[x37v]

BTW.. i ended up forking and implementing an approach that allows for patterns in events and uses match guards to implement the guards.. its a bit more verbose and less "tied together" (you have to implement the Events enum yourself).. but has some other benefits..

https://github.com/x37v/smlang-rs

I figure its a relatively big departure so I'm not sure if a PR to get it back here would make sense.

here is an example of the DSL:

statemachine! {
   transitions: {
        *State1 + ButtonEvent(Button { down: true, .. }) / ctx.action(event); = State2,
        State1 + ButtonEvent(_) [!event.down] / {ctx.action(event)} = State3(2),
        State1 + FooEvent("blah") = State3(30),
        State3(usize) + ButtonEvent(Button { down: true, ..}) [event.index == 20 && *state < 20]
            / { ctx.action(event); println!("foo {}", state) } = State3(ctx.action2(*state, event)),
        State3(usize) + ButtonEvent(Button { down: false, ..}) = State3(*state + 1),

        //can't express State3(0) + FooEvent = State1 but can use a guard
        State3(usize) + FooEvent("blah") [state == &0] = State1,
        State5(StateData) + FooEvent("blah") [state.num == 0] = State1,
        State5(StateData) + BarEvent(0) = State1,
   }
}