Examples

Runnable programs live in the engine repo under examples/, and the package's testable Example functions double as documentation on pkg.go.dev.

You can open any of these live in the studio.

Counter — context that mutates

One state, context updated on each event. The smallest useful machine.

m, _ := fate.CreateMachine(fate.MachineConfig[Ctx, Evt]{
	ID: "counter", Initial: "active",
	States: map[string]fate.StateNodeConfig[Ctx, Evt]{
		"active": {On: map[string][]fate.TransitionConfig[Ctx, Evt]{
			"Inc":   {{Actions: []fate.Action[Ctx, Evt]{fate.Assign(func(c Ctx, _ Evt) Ctx { c.Count++; return c })}}},
			"Reset": {{Target: "active", Actions: []fate.Action[Ctx, Evt]{fate.Assign(func(c Ctx, _ Evt) Ctx { c.Count = 0; return c })}}},
		}},
	},
})

Full source: examples/quickstart.

Traffic light — compound (hierarchical) states

red → green → yellow → red, with a pedestrian walk sub-state nested inside red. The NEXT transition is declared once on the parent red and applies whichever child is active.

m, _ := fate.CreateMachine(fate.MachineConfig[struct{}, string]{
	ID: "traffic", Initial: "red",
	States: map[string]fate.StateNodeConfig[struct{}, string]{
		"red": {
			Initial: "stop",
			States: map[string]fate.StateNodeConfig[struct{}, string]{
				"stop": {On: map[string][]fate.TransitionConfig[struct{}, string]{"WALK": {{Target: "walk"}}}},
				"walk": {On: map[string][]fate.TransitionConfig[struct{}, string]{"WAIT": {{Target: "stop"}}}},
			},
			On: map[string][]fate.TransitionConfig[struct{}, string]{"NEXT": {{Target: "green"}}},
		},
		"green":  {On: map[string][]fate.TransitionConfig[struct{}, string]{"NEXT": {{Target: "yellow"}}}},
		"yellow": {On: map[string][]fate.TransitionConfig[struct{}, string]{"NEXT": {{Target: "red"}}}},
	},
})

a := fate.NewActor(m)
_ = a.Start(context.Background())
fmt.Println(a.Snapshot().Value.Path()) // red.stop

Then render it:

fmt.Println(fate.RenderMermaid(m.Describe(), fate.MermaidOptions{}))

Full source: examples/trafficlight.

Real-time timer — delayed (after) transitions

A machine with an after transition, driven by a real-time adapter that reads PendingTimers() and calls FireTimer(...) when the OS timer elapses — the engine itself never touches the clock.

Full source: examples/realtime-timer.

Feature matrix

Want to see…	Look at
Context mutation with Assign	quickstart
Compound / hierarchical states	trafficlight
Delayed after transitions + an adapter	realtime-timer
Parallel regions, history, guards, invoked actors	pkg.go.dev examples

Next

• How it works — the engine/adapter split behind the timer example.
• Concepts — the API names for each feature above.