Concurrency

Concurrency in Effect

Overview

Effect provides lightweight fiber-based concurrency:

• Fibers - Lightweight threads managed by Effect runtime
• Structured concurrency - Parent fibers supervise children
• Safe interruption - Clean cancellation with resource cleanup
• Concurrent primitives - Queue, Deferred, Semaphore, PubSub

Basic Parallel Execution

Effect.all with Concurrency

import { Effect } from "effect"

const results = yield* Effect.all(
  [fetchUser(1), fetchUser(2), fetchUser(3)],
  { concurrency: "unbounded" }
)

const results = yield* Effect.all(tasks, { concurrency: 5 })

const results = yield* Effect.all(tasks)

Effect.forEach with Concurrency

const users = yield* Effect.forEach(
  userIds,
  (id) => fetchUser(id),
  { concurrency: 10 }
)

Fibers

Creating Fibers with fork

const program = Effect.gen(function* () {
  const fiber = yield* Effect.fork(longRunningTask)

  yield* doOtherWork()

  const result = yield* Fiber.join(fiber)
})

Fork Variants

const fiber = yield* Effect.fork(task)

const fiber = yield* Effect.forkDaemon(task)

const fiber = yield* Effect.forkIn(scope)(task)

const fiber = yield* Effect.forkWithErrorHandler(task, onError)

Fiber Operations

import { Fiber } from "effect"

const result = yield* Fiber.join(fiber)

const exit = yield* Fiber.await(fiber)

yield* Fiber.interrupt(fiber)

const maybeResult = yield* Fiber.poll(fiber)

Racing

Effect.race - First to Complete

const fastest = yield* Effect.race(
  fetchFromServer1(),
  fetchFromServer2()
)

Effect.raceAll - Race Many

const fastest = yield* Effect.raceAll([
  fetchFromCDN1(),
  fetchFromCDN2(),
  fetchFromCDN3()
])

Effect.raceFirst - Include Failures

const first = yield* Effect.raceFirst(task1, task2)

Deferred - One-Time Promise

import { Deferred } from "effect"

const program = Effect.gen(function* () {
  const deferred = yield* Deferred.make<string, never>()

  const fiber = yield* Effect.fork(
    Effect.gen(function* () {
      const value = yield* Deferred.await(deferred)
      yield* Effect.log(`Got: ${value}`)
    })
  )

  yield* Deferred.succeed(deferred, "Hello!")

  yield* Fiber.join(fiber)
})

Queue - Concurrent Queue

import { Queue } from "effect"

const program = Effect.gen(function* () {
  const queue = yield* Queue.bounded<number>(100)

  yield* Effect.fork(
    Effect.forEach(
      [1, 2, 3, 4, 5],
      (n) => Queue.offer(queue, n)
    )
  )

  const items = yield* Effect.forEach(
    Array.from({ length: 5 }),
    () => Queue.take(queue)
  )
})

Queue Variants

const bounded = yield* Queue.bounded<number>(100)

const unbounded = yield* Queue.unbounded<number>()

const dropping = yield* Queue.dropping<number>(100)

const sliding = yield* Queue.sliding<number>(100)

PubSub - Publish/​Subscribe

import { PubSub } from "effect"

const program = Effect.gen(function* () {
  const pubsub = yield* PubSub.bounded<string>(100)

  const sub1 = yield* PubSub.subscribe(pubsub)
  const sub2 = yield* PubSub.subscribe(pubsub)

  yield* PubSub.publish(pubsub, "Hello!")

  const msg1 = yield* Queue.take(sub1)
  const msg2 = yield* Queue.take(sub2)
})

Semaphore - Limit Concurrency

import { Effect } from "effect"

const program = Effect.gen(function* () {
  const semaphore = yield* Effect.makeSemaphore(3)

  yield* Effect.forEach(
    tasks,
    (task) => semaphore.withPermits(1)(task),
    { concurrency: "unbounded" }
  )
})

Latch - Coordination Point

import { Latch } from "effect"

const program = Effect.gen(function* () {
  const latch = yield* Latch.make(false)

  yield* Effect.fork(
    Effect.forEach(
      workers,
      (worker) =>
        Effect.gen(function* () {
          yield* Latch.await(latch)
          yield* worker.start()
        }),
      { concurrency: "unbounded" }
    )
  )

  yield* Latch.open(latch)
})

Interruption

Interrupting Fibers

const fiber = yield* Effect.fork(longTask)

yield* Fiber.interrupt(fiber)

Uninterruptible Regions

const critical = Effect.uninterruptible(
  Effect.gen(function* () {
    yield* beginTransaction()
    yield* performOperations()
    yield* commitTransaction()
  })
)

Interruptible Within Uninterruptible

const program = Effect.uninterruptible(
  Effect.gen(function* () {
    yield* criticalSetup()

    // This part can be interrupted
    yield* Effect.interruptible(longOperation)

    yield* criticalTeardown()
  })
)

Supervision

Structured concurrency ensures child fibers are managed:

const parent = Effect.gen(function* () {
  const child1 = yield* Effect.fork(task1)
  const child2 = yield* Effect.fork(task2)

  // If parent fails/​interrupts, children are interrupted
  yield* failingOperation()
})
// child1 and child2 automatically interrupted

Daemon Fibers

Escape supervision with daemon:

const daemon = yield* Effect.forkDaemon(backgroundTask)

Common Patterns

Timeout with Fallback

const withTimeout = task.pipe(
  Effect.timeout("5 seconds"),
  Effect.map(Option.getOrElse(() => defaultValue))
)

Worker Pool

const workerPool = Effect.gen(function* () {
  const semaphore = yield* Effect.makeSemaphore(numWorkers)

  return (task: Effect.Effect<A>) =>
    semaphore.withPermits(1)(task)
})

Parallel with Error Collection

const results = yield* Effect.all(
  tasks,
  {
    concurrency: "unbounded",
    mode: "either" // Collect all results
  }
)

Best Practices

1. Use Effect.all concurrency for simple parallelism
2. Use Semaphore to limit concurrent operations
3. Prefer structured concurrency over daemon fibers
4. Handle interruption in long-running effects
5. Use Queue for producer/​consumer patterns
6. Use Deferred for one-time coordination

Additional Resources

For comprehensive concurrency documentation, consult ${CLAUDE_PLUGIN_ROOT}/​references/​llms-full.txt.

Search for these sections:

• "Fibers" for fiber management
• "Basic Concurrency" for parallel execution
• "Deferred" for synchronization primitives
• "Queue" for concurrent queues
• "PubSub" for publish/​subscribe
• "Semaphore" for concurrency limiting