Effect Machine

Navigation

What are you building?
├─ Defining states/events           → §Schema-First
├─ Writing transition handlers      → §Transitions
├─ Adding side effects              → §Effects
├─ Testing machines                 → §Testing
├─ Running actors                   → §Actors
├─ Typed ask/reply                  → §Ask / Reply
├─ Recovery/durability              → §Lifecycle
├─ Timeouts / postpone              → §Timeouts, §Postpone
└─ Slots (guards/effects)           → §Slots

Schema-First

States and events ARE schemas. State({}) and Event({}) produce tagged unions with constructors.

import { Schema } from "effect";
import { State, Event } from "effect-machine";

const S = State({
  Idle: {}, // empty → plain value: S.Idle
  Loading: { url: Schema.String }, // non-empty → constructor: S.Loading({ url })
});

const E = Event({
  Start: { url: Schema.String },
  Done: { data: Schema.Unknown },
  GetCount: Event.reply({}, Schema.Number), // reply-bearing event
  GetInfo: Event.reply({ id: Schema.String }, Schema.String), // payload + reply
});

derive — construct from existing state, picks overlapping fields:

S.Active.derive(state); // pick target fields from source
S.Active.derive(state, { count: n + 1 }); // pick + override

Type guards / matching:

S.$is("Loading")(value)                  // boolean type guard
S.$match(value, { Loading: (s) => ..., _: () => ... })  // pattern match

Transitions

const machine = Machine.make({ state: S, event: E, initial: S.Idle })
  // Single state → event → handler
  .on(S.Idle, E.Start, ({ event }) => S.Loading({ url: event.url }))

  // Multi-state source
  .on([S.Loading, S.Retrying], E.Done, ({ event }) => S.Active({ data: event.data }))

  // Wildcard — any state (specific .on wins)
  .onAny(E.Cancel, () => S.Cancelled)

  // Reenter same state (re-triggers spawn effects + timeouts)
  .reenter(S.Active, E.Refresh, ({ state }) => S.Active.derive(state))

  // Mark final states (actor stops, postpone buffer settles)
  .final(S.Done)
  .final(S.Cancelled);

Handler return types:

// Pure: return new state
({ state, event }) => S.Next({ ... })

// Effectful: return Effect<State>
({ state }) => Effect.gen(function* () { ... return S.Next({ ... }) })

// With reply (for actor.ask — event must use Event.reply()):
({ state }) => Machine.reply(S.Same.derive(state), state.count)

Effects

spawn — state-scoped, auto-cancelled on exit

machine.spawn(S.Loading, ({ state, self }) =>
  Effect.gen(function* () {
    const data = yield* fetchData(state.url);
    yield* self.send(E.Done({ data }));
  }),
);

task — spawn + auto-route success/failure

machine.task(S.Loading, ({ state }) => fetchData(state.url), {
  onSuccess: (data) => E.Done({ data }),
  onFailure: () => E.Error,
});

background — machine-lifetime (not state-scoped)

machine.background(({ self }) =>
  Stream.fromSchedule(Schedule.spaced("10 seconds")).pipe(
    Stream.runForEach(() => self.send(E.Heartbeat)),
  ),
);

Slots

Unified parameterized slots via Slot.define + Slot.fn. Handlers take only params:

import { Slot } from "effect-machine";

const MySlots = Slot.define({
  canRetry: Slot.fn({ max: Schema.Number }, Schema.Boolean),
  notify: Slot.fn({ msg: Schema.String }),
});

const machine = Machine.make({
  state: S,
  event: E,
  slots: MySlots,
  initial: S.Idle,
})
  .on(S.Error, E.Retry, ({ slots, state }) =>
    Effect.gen(function* () {
      if (yield* slots.canRetry({ max: 3 })) return S.Loading.derive(state);
      return S.Failed;
    }),
  )
  .spawn(S.Done, ({ slots, state }) => slots.notify({ msg: `Done: ${state.id}` }));

// Provide at spawn time — handlers take only params
const actor =
  yield *
  Machine.spawn(machine, {
    slots: {
      canRetry: ({ max }) => attempts < max,
      notify: ({ msg }) => Effect.log(msg),
    },
  });
yield * actor.start;

Slots are accepted everywhere: Machine.spawn, Machine.replay, simulate, createTestHarness.

Ask / Reply

Events declare reply schemas via Event.reply(fields, schema). Handlers must use Machine.reply():

const E = Event({
  GetCount: Event.reply({}, Schema.Number), // askable
  Reset: {}, // not askable
});

// Handler — Machine.reply() required for reply-bearing events
machine.on(S.Active, E.GetCount, ({ state }) => Machine.reply(S.Active.derive(state), state.count));

// Caller — return type inferred from schema
const count = yield * actor.ask(E.GetCount); // number
// actor.ask(E.Reset) — TYPE ERROR (no reply schema)

Rules:

• Event.reply({}, schema) — empty payload + reply; Event.reply({ id: Schema.String }, schema) — payload + reply
• Handler for reply-bearing event MUST return Machine.reply(state, value) — plain state return is a type error
• Handler for non-reply event CANNOT return Machine.reply() — type error
• .onAny() handlers cannot provide replies — use specific .on() for reply events
• Reply decode mismatch (handler returns wrong type) = defect at runtime
• Cluster: Ask RPC propagates replies through entity boundary

Timeouts

gen_statem-style. Timer starts on state entry, cancels on exit:

machine.timeout(S.Loading, {
  duration: Duration.seconds(30),
  event: E.Timeout,
});

// Dynamic duration from state
machine.timeout(S.Retrying, {
  duration: (state) => Duration.seconds(state.backoff),
  event: E.GiveUp,
});

.reenter() restarts the timer with fresh state values.

Postpone

gen_statem-style. Buffered events drain FIFO on state change, looping until stable:

machine.postpone(S.Connecting, E.Data).postpone(S.Connecting, [E.Data, E.Command]);

Multi-stage: if a drained event causes another state change, postponed events re-drain.

Actors

Machine.spawn — standalone, unstarted actor

Machine.spawn returns a cold actor. Call actor.start to fork the event loop. Events sent before start() are queued.

const actor = yield * Machine.spawn(machine);
yield * actor.start;

// With options
const actor = yield * Machine.spawn(machine, { id: "my-id", hydrate: savedState });
yield * actor.start;

Auto-cleans up if Scope is present. Otherwise call actor.stop manually.

ActorRef API

ActorSystem — registry + lifecycle (auto-starts)

system.spawn auto-starts — no actor.start needed.

const system = yield * ActorSystemService;
const actor = yield * system.spawn("id", machine); // auto-started
const maybe = yield * system.get("id"); // Option<ActorRef>
yield * system.stop("id"); // boolean
system.actors; // ReadonlyMap snapshot
system.events; // Stream<SystemEvent>

Child actors

machine.spawn(S.Active, ({ self }) =>
  Effect.gen(function* () {
    const child = yield* self.spawn("worker", workerMachine).pipe(Effect.orDie);
    yield* child.send(WorkerEvent.Start);
    // auto-stopped when parent exits Active
  }),
);

Lifecycle

Recovery + Durability hooks for persistence. Passed via lifecycle option on Machine.spawn / system.spawn.

const actor =
  yield *
  Machine.spawn(machine, {
    lifecycle: {
      recovery: {
        // Runs during actor.start. Return Some to override initial state, None for cold start.
        resolve: ({ actorId, generation, machineInitial }) =>
          storage.get(actorId).pipe(Effect.map(Option.fromNullable)),
      },
      durability: {
        // Runs after each committed transition
        save: ({ actorId, generation, previousState, nextState, event }) =>
          storage.set(actorId, nextState),
        // Optional sync filter — skip uninteresting transitions
        shouldSave: (state, prev) => state._tag !== prev._tag,
      },
    },
  });
yield * actor.start;

• generation — 0 = cold start, 1+ = supervision restart
• hydrate overrides recovery — Machine.spawn(machine, { hydrate: state }) skips resolve entirely
• Lifecycle<S, E> = { recovery?, durability? } — both optional

Replay + Hydrate

// Restore from snapshot
const actor = yield * Machine.spawn(machine, { hydrate: loadedState });
yield * actor.start;

// Restore from event log
const state = yield * Machine.replay(machine, events);
const actor = yield * Machine.spawn(machine, { hydrate: state });
yield * actor.start;

// Restore from snapshot + tail events
const state = yield * Machine.replay(machine, tailEvents, { from: snapshot });
const actor = yield * Machine.spawn(machine, { hydrate: state });
yield * actor.start;

Machine.replay semantics:

• Folds events through transition handlers (pure or effectful)
• self.send/self.spawn are no-ops (stubbed)
• Spawn effects, background effects, timeouts do NOT run
• Postpone rules respected (loop until stable)
• Final state stops replay
• Unhandled events silently skipped

Testing

import { simulate, assertPath, assertReaches, createTestHarness } from "effect-machine";

// Simulate — run events, get all states
const { states, finalState } = yield * simulate(machine, [E.Start, E.Done]);

// Assertions
yield * assertPath(machine, events, ["Idle", "Loading", "Done"]);
yield * assertReaches(machine, events, "Done");
yield * assertNeverReaches(machine, events, "Error");

// Test harness — step-by-step
const harness = yield * createTestHarness(machine);
yield * harness.send(E.Start);
expect(harness.state._tag).toBe("Loading");

Both simulate and createTestHarness accept Machine directly.

Gotchas

• Machine.spawn returns unstarted actor — must call yield* actor.start. system.spawn auto-starts.
• Slots are provided at spawn time — Machine.spawn(machine, { slots: { ... } })
• Empty state = value, non-empty = constructor — S.Idle vs S.Loading({ url })
• Spawn effects re-run on hydrate — Machine.spawn({ hydrate }) re-runs spawn effects for the hydrated state (timers, scoped resources)
• hydrate overrides recovery — resolve() is never called when hydrate is set
• transitions is observational — PubSub-backed, late subscribers miss edges. Not a durability guarantee.
• Effectful handlers in replay — replay runs handlers but stubs self/system. Side effects through self.send are no-ops.
• ask() requires reply schema — only events with Event.reply() accepted; non-reply events are type errors
• Reply decode failure = defect — if handler returns wrong type, actor dies (broken handler, not business logic)
• v3 compat — import from "effect-machine/v3" for Effect v3 projects

Source: cevr/effect-machine — distributed by TomeVault.