A traffic light on three pins, written as a state machine

# signal.zeno: a traffic light on three pins, written as a state machine.
#
#     zeno run examples/signal.zeno      run it on the simulated board
#     zeno build --firmware --no-collector examples/signal.zeno
#                                        the build for a microcontroller:
#                                        no setjmp, no garbage collector
#
# On a computer the board is simulated. A line such as `pin 2 on` is what
# a logic analyser clipped to that pin would show. The times are short so
# that the whole run takes a fifth of a second.

from board import pin, led
from time import sleep, now

enum Light {
    Red
    Green
    Amber
}

let red = pin(2)
let amber = pin(3)
let green = pin(4)

# Switch the lamps over: the old one off, then the new one on.
function show(light: Light) {
    match light {
        case Red {
            amber.off()
            red.on()
        }
        case Green {
            red.off()
            green.on()
        }
        case Amber {
            green.off()
            amber.on()
        }
    }
}

function after(light: Light) -> Light {
    match light {
        case Red { return Green }
        case Green { return Amber }
        case Amber { return Red }
    }
}

# How long each light stays. The unit is part of the type: a function
# that returned `40` here would not compile.
function hold(light: Light) -> ms {
    match light {
        case Red { return 40 ms }
        case Green { return 30 ms }
        case Amber { return 10 ms }
    }
}

let start = now()
var light = Red
for step in range(7) {
    show(light)
    led.toggle()                 # a heartbeat: the program is alive
    sleep(hold(light))
    light = after(light)
}

# Seven steps hold for 200 ms in all. `now` only ever goes forward.
print("took at least 200 ms:", now() - start >= 200 ms)

pin 2 on
led on
pin 2 off
pin 4 on
led off
pin 4 off
pin 3 on
led on
pin 3 off
pin 2 on
led off
pin 2 off
pin 4 on
led on
pin 4 off
pin 3 on
led off
pin 3 off
pin 2 on
led on
took at least 200 ms: true