Declaration and Transitions

On this page

A machine block lists states with state, marks the initial state with initial, and declares each transition as A -> B on event. From that, the compiler automatically generates the constructor and control methods — zero boilerplate.

.send("event") attempts to advance the machine: it returns true if the transition existed from the current state, or false if the event does not apply. The state never becomes inconsistent.

Traffic light with three states; send returns true/false depending on the event.

01-traffic-light.zolo

Playground

// Feature: state machines — `machine` keyword, declarative transitions

// Syntax:

//   machine Name {

//       state A, B, C

//       initial A

//       A -> B on event

//   }

// `Name.new()` constructs an instance; `.state` returns the current

// state; `.send(event)` advances and returns true/false depending on

// whether the event was applicable.

// When to use: protocols, UI flows, parsers, devices with a small

// set of well-defined states. Cleaner than ad-hoc enums + match.


machine TrafficLight {
  state Red, Yellow, Green
  initial Red
  Red -> Green on go
  Green -> Yellow on caution
  Yellow -> Red on stop
}

let light = TrafficLight.new()
print(light.state)

// expected: Red


let ok = light.send("go")
print(ok)  // expected: true

print(light.state)  // expected: Green


light.send("caution")
print(light.state)  // expected: Yellow


light.send("stop")
print(light.state)  // expected: Red


// An unknown event returns false and the state stays put.

let bad = light.send("unknown")
print(bad)  // expected: false

print(light.state)  // expected: Red

Challenge

Add a fourth state Blink and a transition Red -> Blink on blink. Fire light.send("blink") from Red and confirm that .state changes. Then try firing "blink" from Green and observe the false return value.