Effect TypeScript Library

Effect is a powerful TypeScript library for building type-safe, composable, synchronous and asynchronous programs with:

• Typed errors – Track expected errors at the type level
• Dependency injection – Services, Context, and Layers
• Resource management – Safe acquisition and release
• Concurrency – Fiber-based lightweight threads
• Composability – Pipe-based functional programming

Quick Reference

For detailed patterns and examples, see:

• CORE.md – Effect basics, creating and running effects
• ERRORS.md – Error handling patterns
• SERVICES.md – Dependency injection with Services and Layers
• SCHEMA.md – Data validation with Schema
• PATTERNS.md – Common patterns and best practices

The Effect Type

Effect<Success, Error, Requirements>

• Success (A) – The value produced on success
• Error (E) – Expected errors that can occur
• Requirements (R) – Services/dependencies needed to run

Essential Imports

import { Effect, pipe } from "effect"
import { Schema } from "effect"
import { Context, Layer } from "effect"
import { Option, Either } from "effect"

Creating Effects

// Success values
const success = Effect.succeed(42)
const lazy = Effect.sync(() => computeValue())

// Failures
const fail = Effect.fail(new MyError())
const die = Effect.die("unexpected error")

// From promises
const fromPromise = Effect.tryPromise({
  try: () => fetch(url),
  catch: (e) => new FetchError(e)
})

// From sync that may throw
const fromSync = Effect.try({
  try: () => JSON.parse(input),
  catch: (e) => new ParseError(e)
})

Running Effects

// Async execution
await Effect.runPromise(effect)
await Effect.runPromiseExit(effect)

// Sync execution (only for sync effects)
Effect.runSync(effect)

// Fork as fiber
const fiber = Effect.runFork(effect)

Composing Effects (Generators – Recommended)

const program = Effect.gen(function* () {
  const a = yield* getUser(id)
  const b = yield* getProfile(a.profileId)
  return { user: a, profile: b }
})

Composing Effects (Pipe Style)

const program = pipe(
  getUser(id),
  Effect.flatMap(user => getProfile(user.profileId)),
  Effect.map(profile => profile.name)
)

// Or using .pipe() method
const program = getUser(id).pipe(
  Effect.flatMap(user => getProfile(user.profileId)),
  Effect.map(profile => profile.name)
)

Error Handling

// Catch specific error
effect.pipe(
  Effect.catchTag("NotFound", (e) => Effect.succeed(defaultValue))
)

// Catch all errors
effect.pipe(
  Effect.catchAll((e) => Effect.succeed(fallback))
)

// Map errors
effect.pipe(
  Effect.mapError((e) => new WrappedError(e))
)

Defining Services

// Define service interface
class UserService extends Context.Tag("UserService")<
  UserService,
  {
    readonly getUser: (id: string) => Effect.Effect<User, NotFoundError>
    readonly saveUser: (user: User) => Effect.Effect<void, DatabaseError>
  }
>() {}

// Use service
const program = Effect.gen(function* () {
  const userService = yield* UserService
  const user = yield* userService.getUser("123")
  return user
})

// Implement as Layer
const UserServiceLive = Layer.succeed(UserService, {
  getUser: (id) => Effect.succeed({ id, name: "John" }),
  saveUser: (user) => Effect.succeed(undefined)
})

// Provide layer
Effect.runPromise(program.pipe(Effect.provide(UserServiceLive)))

Schema Validation

import { Schema } from "effect"

// Define schema
const User = Schema.Struct({
  id: Schema.String,
  name: Schema.String,
  age: Schema.Number.pipe(Schema.positive())
})

// Infer type
type User = Schema.Schema.Type<typeof User>

// Decode
const decode = Schema.decodeUnknown(User)
const result = decode({ id: "1", name: "John", age: 30 })

// Use in Effect
const parseUser = Schema.decodeUnknown(User)
const user = yield* parseUser(rawData)

Concurrency

// Run in parallel
const [a, b, c] = yield* Effect.all([effectA, effectB, effectC], {
  concurrency: "unbounded"
})

// Race - first to complete wins
const result = yield* Effect.race(effectA, effectB)

// Timeout
const result = yield* effect.pipe(Effect.timeout("5 seconds"))

Resource Management

// Scoped resource
const file = Effect.acquireRelease(
  Effect.sync(() => openFile(path)),
  (file) => Effect.sync(() => file.close())
)

// Use with scoped
const program = Effect.scoped(
  Effect.gen(function* () {
    const f = yield* file
    return yield* f.read()
  })
)

API Reference

For complete API documentation: https://tim-smart.github.io/effect-io-ai/

Official Documentation

• Getting Started: https://effect.website/docs/getting-started/introduction/
• Schema: https://effect.website/docs/schema/introduction/
• Error Management: https://effect.website/docs/error-management/two-error-types/
• Requirements (DI): https://effect.website/docs/requirements-management/services/
• Concurrency: https://effect.website/docs/concurrency/fibers/