A Supervisor<A> is a tool that manages the creation and termination of fibers, producing some visible value of type A based on its supervision.



To create a supervisor, you can use the Supervisor.track function. It generates a new supervisor that keeps track of its child fibers in a set.



Whenever you need to supervise an effect, you can employ the Effect.supervised function. This function takes a supervisor and returns an effect that behaves the same as the original effect. However, all child fibers forked within this effect are reported to the specified supervisor.

By doing this, you associate the behavior of child fibers with the provided supervisor, giving you access to all the information about these child fibers through the supervisor.


In the following example, we will periodically monitor the number of fibers throughout our application's lifecycle:

import { Effect, Supervisor, Schedule, Fiber, FiberStatus } from "effect"
const program = Effect.gen(function* (_) {
  const supervisor = yield* _(Supervisor.track)
  const fibFiber = yield* _(
    fib(20).pipe(Effect.supervised(supervisor), Effect.fork)
  const policy = Schedule.spaced("500 millis").pipe(
    Schedule.whileInputEffect((_) =>
      Fiber.status(fibFiber).pipe( => status !== FiberStatus.done)
  const monitorFiber = yield* _(
    monitorFibers(supervisor).pipe(Effect.repeat(policy), Effect.fork)
  yield* _(Fiber.join(monitorFiber))
  const result = yield* _(Fiber.join(fibFiber))
  console.log(`fibonacci result: ${result}`)
const monitorFibers = (
  supervisor: Supervisor.Supervisor<Array<Fiber.RuntimeFiber<any, any>>>
): Effect.Effect<never, never, void> =>
  Effect.gen(function* (_) {
    const fibers = yield* _(supervisor.value)
    console.log(`number of fibers: ${fibers.length}`)
const fib = (n: number): Effect.Effect<never, never, number> =>
  Effect.gen(function* (_) {
    if (n <= 1) {
      return 1
    yield* _(Effect.sleep("500 millis"))
    const fiber1 = yield* _(Effect.fork(fib(n - 2)))
    const fiber2 = yield* _(Effect.fork(fib(n - 1)))
    const v1 = yield* _(Fiber.join(fiber1))
    const v2 = yield* _(Fiber.join(fiber2))
    return v1 + v2
number of fibers: 0
number of fibers: 2
number of fibers: 6
number of fibers: 14
number of fibers: 30
number of fibers: 62
number of fibers: 126
number of fibers: 254
number of fibers: 510
number of fibers: 1022
number of fibers: 2034
number of fibers: 3795
number of fibers: 5810
number of fibers: 6474
number of fibers: 4942
number of fibers: 2515
number of fibers: 832
number of fibers: 170
number of fibers: 18
number of fibers: 0
fibonacci result: 10946