Worker

The Worker module provides abstractions for spawning and communicating with worker threads in a type-safe, platform-agnostic way. It supports bidirectional communication with automatic message handling and error propagation.

Overview

Worker features:

Type-safe message passing between main and worker threads
Bidirectional communication (send and receive)
Automatic error handling and propagation
Scope-based lifecycle management
Platform-agnostic abstractions
Support for transferable objects

Basic Concepts

The Worker system consists of:

Spawner: Factory for creating worker instances
Worker: Handle for communicating with a worker thread
WorkerPlatform: Platform-specific worker implementation
Message Protocol: Type-safe message passing

Creating Workers

Define a Spawner

First, define how workers should be spawned:

import { Worker, Layer } from "effect"

// For Node.js
import { Worker as NodeWorker } from "worker_threads"

const SpawnerLive = Worker.layerSpawner(
  (id: number) => new NodeWorker("./worker.js", { workerData: { id } })
)

Spawn a Worker

import { Effect, Worker } from "effect"

const program = Effect.gen(function*() {
  const platform = yield* Worker.WorkerPlatform
  
  // Spawn worker with ID 0
  const worker = yield* platform.spawn(0)
  
  // Use worker...
})

Message Passing

Sending Messages

Send messages from the main thread to the worker:

import { Effect, Worker } from "effect"

interface WorkerInput {
  readonly task: string
  readonly data: unknown
}

const program = Effect.gen(function*() {
  const platform = yield* Worker.WorkerPlatform
  const worker = yield* platform.spawn<unknown, WorkerInput>(0)
  
  // Send a message
  yield* worker.send({
    task: "process",
    data: { value: 42 }
  })
  
  // Send with transferables (for ArrayBuffer, MessagePort, etc.)
  const buffer = new ArrayBuffer(1024)
  yield* worker.send(
    { task: "process-buffer", buffer },
    [buffer] // Transferred, not copied
  )
})

Receiving Messages

Handle incoming messages in the worker using run:

import { Console, Effect, Worker } from "effect"

interface WorkerOutput {
  readonly type: "result" | "progress"
  readonly data: unknown
}

const program = Effect.gen(function*() {
  const platform = yield* Worker.WorkerPlatform
  const worker = yield* platform.spawn<WorkerOutput, unknown>(0)
  
  // Run message handler
  yield* worker.run((message) =>
    Effect.gen(function*() {
      if (message.type === "result") {
        yield* Console.log("Got result:", message.data)
      } else if (message.type === "progress") {
        yield* Console.log("Progress:", message.data)
      }
    })
  )
})

Worker Implementation

Inside the worker file, use WorkerRunner to handle messages:

// worker.ts
import { Console, Effect } from "effect"
import { WorkerRunner } from "effect/unstable/workers"

interface WorkerInput {
  readonly task: string
  readonly data: unknown
}

interface WorkerOutput {
  readonly type: "result" | "progress"
  readonly data: unknown
}

// Define message handler
const handler = (message: WorkerInput) =>
  Effect.gen(function*() {
    yield* Console.log(`Processing task: ${message.task}`)
    
    // Send progress update
    yield* WorkerRunner.send<WorkerOutput>({
      type: "progress",
      data: { percent: 50 }
    })
    
    // Process the task
    const result = yield* processTask(message)
    
    // Send result
    yield* WorkerRunner.send<WorkerOutput>({
      type: "result",
      data: result
    })
  })

// Run the worker
WorkerRunner.run(handler)

Bidirectional Communication

Workers support full bidirectional message passing:

// Main thread
import { Effect, Worker } from "effect"

const mainProgram = Effect.gen(function*() {
  const platform = yield* Worker.WorkerPlatform
  const worker = yield* platform.spawn<WorkerToMain, MainToWorker>(0)
  
  // Start message handler in background
  yield* Effect.forkDaemon(
    worker.run((message) =>
      Effect.gen(function*() {
        yield* Console.log("Worker says:", message)
        // Respond to worker
        yield* worker.send({ response: "acknowledged" })
      })
    )
  )
  
  // Send initial message
  yield* worker.send({ command: "start" })
})

// Worker thread
import { Effect } from "effect"
import { WorkerRunner } from "effect/unstable/workers"

const handler = (message: MainToWorker) =>
  Effect.gen(function*() {
    if (message.command === "start") {
      // Send message to main
      yield* WorkerRunner.send({ status: "started" })
      
      // Wait for response (handled by main's worker.run)
    }
  })

WorkerRunner.run(handler)

Spawn Callbacks

Execute code when the worker spawns:

import { Console, Effect, Worker } from "effect"

const program = Effect.gen(function*() {
  const platform = yield* Worker.WorkerPlatform
  const worker = yield* platform.spawn(0)
  
  yield* worker.run(
    (message) => processMessage(message),
    {
      onSpawn: Console.log("Worker spawned and ready!")
    }
  )
})

Error Handling

Errors in workers are propagated as WorkerError:

import { Effect, Worker } from "effect"

const program = Effect.gen(function*() {
  const platform = yield* Worker.WorkerPlatform
  const worker = yield* platform.spawn(0)
  
  yield* worker.send({ task: "dangerous" }).pipe(
    Effect.catchTag("WorkerError", (error) =>
      Effect.gen(function*() {
        if (error.reason._tag === "WorkerSendError") {
          yield* Effect.log("Failed to send message:", error.reason.message)
        }
      })
    )
  )
})

WorkerError Types

WorkerSendError: Failed to send message to worker
Other platform-specific errors

Complete Example

Main Thread

// main.ts
import { Console, Effect, Layer } from "effect"
import { Worker as NodeWorker } from "worker_threads"
import { Worker, WorkerPlatform } from "effect/unstable/workers"

interface TaskInput {
  readonly task: "compute" | "process"
  readonly value: number
}

interface TaskOutput {
  readonly type: "progress" | "result"
  readonly value: number
}

// Create spawner layer
const SpawnerLive = Worker.layerSpawner(
  (id: number) => new NodeWorker("./worker.js", { workerData: { id } })
)

// Create platform layer
const PlatformLive = Layer.effect(
  WorkerPlatform,
  Effect.map(
    Worker.Spawner,
    (spawner) => Worker.makePlatform(/* ... platform-specific setup ... */)
  )
).pipe(Layer.provide(SpawnerLive))

// Main program
const program = Effect.gen(function*() {
  const platform = yield* WorkerPlatform
  const worker = yield* platform.spawn<TaskOutput, TaskInput>(0)
  
  // Handle worker messages
  yield* Effect.forkDaemon(
    worker.run((message) =>
      Effect.gen(function*() {
        if (message.type === "progress") {
          yield* Console.log(`Progress: ${message.value}%`)
        } else if (message.type === "result") {
          yield* Console.log(`Result: ${message.value}`)
        }
      })
    )
  )
  
  // Send tasks to worker
  yield* worker.send({ task: "compute", value: 42 })
  yield* worker.send({ task: "process", value: 100 })
  
  // Wait for completion
  yield* Effect.sleep("5 seconds")
}).pipe(Effect.provide(PlatformLive))

Effect.runPromise(program)

Worker Thread

// worker.ts
import { Console, Effect } from "effect"
import { WorkerRunner } from "effect/unstable/workers"

interface TaskInput {
  readonly task: "compute" | "process"
  readonly value: number
}

interface TaskOutput {
  readonly type: "progress" | "result"
  readonly value: number
}

const handler = (message: TaskInput) =>
  Effect.gen(function*() {
    yield* Console.log(`Worker received task: ${message.task}`)
    
    // Report progress
    yield* WorkerRunner.send<TaskOutput>({
      type: "progress",
      value: 50
    })
    
    // Simulate work
    yield* Effect.sleep("1 second")
    
    // Calculate result
    const result = message.task === "compute" 
      ? message.value * 2 
      : message.value + 10
    
    // Send result
    yield* WorkerRunner.send<TaskOutput>({
      type: "result",
      value: result
    })
  })

// Start the worker
WorkerRunner.run(handler)

Platform Integration

Node.js Worker Threads

import { Worker as NodeWorker } from "worker_threads"
import { Effect, Layer, Worker } from "effect"
import { NodeWorkerPlatform } from "@effect/platform-node"

const SpawnerLive = Worker.layerSpawner(
  (id: number) => new NodeWorker("./worker.js", {
    workerData: { id },
    env: process.env
  })
)

const PlatformLive = NodeWorkerPlatform.layer.pipe(
  Layer.provide(SpawnerLive)
)

Web Workers (Browser)

import { Effect, Layer, Worker } from "effect"
import { BrowserWorkerPlatform } from "@effect/platform-browser"

const SpawnerLive = Worker.layerSpawner(
  (id: number) => new Worker("./worker.js", { name: `worker-${id}` })
)

const PlatformLive = BrowserWorkerPlatform.layer.pipe(
  Layer.provide(SpawnerLive)
)

Advanced Patterns

Worker Pool

Create a pool of workers for parallel processing:

import { Array, Effect, Worker } from "effect"

const createWorkerPool = (size: number) =>
  Effect.gen(function*() {
    const platform = yield* Worker.WorkerPlatform
    
    // Spawn multiple workers
    const workers = yield* Effect.all(
      Array.range(0, size - 1).map((id) => platform.spawn(id))
    )
    
    return workers
  })

const program = Effect.gen(function*() {
  const pool = yield* createWorkerPool(4)
  
  // Distribute work across pool
  const tasks = Array.range(0, 100)
  
  yield* Effect.all(
    tasks.map((task, index) => {
      const worker = pool[index % pool.length]
      return worker.send({ task })
    }),
    { concurrency: "unbounded" }
  )
})

Worker with Scope

Manage worker lifecycle with scopes:

import { Effect, Scope, Worker } from "effect"

const program = Effect.scoped(
  Effect.gen(function*() {
    const platform = yield* Worker.WorkerPlatform
    const worker = yield* platform.spawn(0)
    
    // Worker automatically cleaned up when scope exits
    yield* worker.send({ task: "process" })
    
    // Process results...
  })
)

Type Reference

interface Worker<O = unknown, I = unknown> {
  readonly send: (
    message: I,
    transfers?: ReadonlyArray<unknown>
  ) => Effect.Effect<void, WorkerError>
  
  readonly run: <A, E, R>(
    handler: (message: O) => Effect.Effect<A, E, R>,
    options?: {
      readonly onSpawn?: Effect.Effect<void> | undefined
    }
  ) => Effect.Effect<never, E | WorkerError, R>
}

WorkerPlatform

class WorkerPlatform extends Service<WorkerPlatform, {
  readonly spawn: <O = unknown, I = unknown>(
    id: number
  ) => Effect.Effect<Worker<O, I>, WorkerError, Spawner>
}> {}

Spawner

interface Spawner {
  readonly _: unique symbol
}

interface SpawnerFn<W = unknown> {
  (id: number): W
}

Message Types

type PlatformMessage = 
  | readonly [ready: 0] 
  | readonly [data: 1, unknown]

Core Modules

Data Structures

Concurrency

Time & Scheduling

Platform

Unstable Modules

Worker

Overview

Basic Concepts

Creating Workers

Define a Spawner

Spawn a Worker

Message Passing

Sending Messages

Receiving Messages

Worker Implementation

Bidirectional Communication

Spawn Callbacks

Error Handling

WorkerError Types

Complete Example

Main Thread

Worker Thread

Platform Integration

Node.js Worker Threads

Web Workers (Browser)

Advanced Patterns

Worker Pool

Worker with Scope

Type Reference

Worker

WorkerPlatform

Spawner

Message Types

See Also

Build docs developers (and LLMs) love

Core Modules

Data Structures

Concurrency

Time & Scheduling

Platform

Unstable Modules

​Overview

​Basic Concepts

​Creating Workers

​Define a Spawner

​Spawn a Worker

​Message Passing

​Sending Messages

​Receiving Messages

​Worker Implementation

​Bidirectional Communication

​Spawn Callbacks

​Error Handling

​WorkerError Types

​Complete Example

​Main Thread

​Worker Thread

​Platform Integration

​Node.js Worker Threads

​Web Workers (Browser)

​Advanced Patterns

​Worker Pool

​Worker with Scope

​Type Reference

​Worker

​WorkerPlatform

​Spawner

​Message Types

​See Also

Build docs developers (and LLMs) love

Overview

Basic Concepts

Creating Workers

Define a Spawner

Spawn a Worker

Message Passing

Sending Messages

Receiving Messages

Worker Implementation

Bidirectional Communication

Spawn Callbacks

Error Handling

WorkerError Types

Complete Example

Main Thread

Worker Thread

Platform Integration

Node.js Worker Threads

Web Workers (Browser)

Advanced Patterns

Worker Pool

Worker with Scope

Type Reference

Worker

WorkerPlatform

Spawner

Message Types

See Also