Effect-TS

Effect is a powerful TypeScript library for building robust, type-safe applications. It provides a functional effect system with typed errors, dependency injection, resource management, and concurrency primitives.

Why Effect?

Type-safe errors: Unlike Promise which loses error type information, Effect tracks errors in the type system with Effect<Success, Error, Requirements>.

Dependency injection: Services are declared explicitly using Context.Tag and provided via Layer, making dependencies visible in types.

Resource safety: Effect.acquireRelease ensures resources are always cleaned up, even on failure or interruption.

Composability: Effects compose naturally with pipe, Effect.gen generators, and combinators like Effect.all, Effect.flatMap.

Quick Reference

For detailed patterns and examples, see:

• Core Concepts - Effect type, creating and running effects, generators
• Error Handling - Typed errors, catchAll, catchTag, Either
• Services & Layers - Dependency injection with Context.Tag and Layer
• Schema - Data validation with encode/decode
• Resources - Resource management with acquireRelease and Scope

The Effect Type

The core type Effect<Success, Error, Requirements> represents a computation that:

• Produces a value of type Success on success
• May fail with an error of type Error
• Requires a context of type Requirements to run

import { Effect } from "effect"

// Effect<number, never, never> - succeeds with number, cannot fail, no requirements
const succeed = Effect.succeed(42)

// Effect<never, string, never> - always fails with string error
const fail = Effect.fail("Something went wrong")

// Effect<string, Error, never> - may succeed with string or fail with Error
const parse = (input: string): Effect.Effect<number, Error> =>
  Effect.try({
    try: () => JSON.parse(input),
    catch: (e) => new Error(`Parse failed: ${e}`)
  })

Creating Effects

import { Effect } from "effect"

// From synchronous values
const fromValue = Effect.succeed(42)
const fromThunk = Effect.sync(() => Date.now())

// From failures
const fromError = Effect.fail(new Error("Failed"))
const fromDie = Effect.die("Unexpected error") // Defect, not typed

// From async operations
const fromPromise = Effect.tryPromise({
  try: () => fetch("/api/data").then(r => r.json()),
  catch: (e) => new Error(`Fetch failed: ${e}`)
})

// From nullable values
const fromNullable = Effect.fromNullable(maybeValue)

Running Effects

import { Effect } from "effect"

const program = Effect.succeed(42)

// Synchronous (throws if effect is async or fails)
const result = Effect.runSync(program) // 42

// Promise-based
const promise = Effect.runPromise(program) // Promise<42>

// With exit status
const exit = Effect.runSyncExit(program)
const exitPromise = Effect.runPromiseExit(program)

Effect.gen Generators

Write async-looking code with full type safety using generators:

import { Effect } from "effect"

const program = Effect.gen(function* () {
  const user = yield* fetchUser(userId)
  const posts = yield* fetchPosts(user.id)
  const enriched = yield* enrichPosts(posts)
  return { user, posts: enriched }
})

// Equivalent to:
const programPipe = fetchUser(userId).pipe(
  Effect.flatMap(user =>
    fetchPosts(user.id).pipe(
      Effect.flatMap(posts =>
        enrichPosts(posts).pipe(
          Effect.map(enriched => ({ user, posts: enriched }))
        )
      )
    )
  )
)

Error Handling

Effect tracks errors in the type system, making error handling explicit:

import { Effect } from "effect"

class UserNotFound extends Error {
  readonly _tag = "UserNotFound"
  constructor(readonly userId: string) {
    super(`User not found: ${userId}`)
  }
}

class DatabaseError extends Error {
  readonly _tag = "DatabaseError"
  constructor(readonly cause: unknown) {
    super("Database error")
  }
}

// Effect<User, UserNotFound | DatabaseError, never>
const getUser = (id: string) => Effect.gen(function* () {
  const record = yield* queryDatabase(id)
  if (!record) {
    return yield* Effect.fail(new UserNotFound(id))
  }
  return record
})

// Handle specific errors
const handled = getUser("123").pipe(
  Effect.catchTag("UserNotFound", (e) =>
    Effect.succeed({ id: e.userId, name: "Anonymous" })
  )
)

// Handle all errors
const recovered = getUser("123").pipe(
  Effect.catchAll((error) =>
    Effect.succeed({ id: "unknown", name: "Fallback" })
  )
)

Services and Dependency Injection

Define services with Context.Tag and provide them with Layer:

import { Context, Effect, Layer } from "effect"

// Define a service interface and tag
class Database extends Context.Tag("Database")<
  Database,
  {
    readonly query: (sql: string) => Effect.Effect<unknown[]>
    readonly execute: (sql: string) => Effect.Effect<void>
  }
>() {}

// Use the service in effects
const getUsers = Effect.gen(function* () {
  const db = yield* Database
  return yield* db.query("SELECT * FROM users")
})

// Create a layer that provides the service
const DatabaseLive = Layer.succeed(Database, {
  query: (sql) => Effect.sync(() => {
    console.log(`Executing: ${sql}`)
    return []
  }),
  execute: (sql) => Effect.sync(() => {
    console.log(`Executing: ${sql}`)
  })
})

// Provide the layer to run the effect
const runnable = Effect.provide(getUsers, DatabaseLive)
Effect.runPromise(runnable)

Schema Validation

Use Effect Schema for type-safe data validation:

import { Schema } from "effect"

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

// Infer types from schema
type User = Schema.Schema.Type<typeof User>

// Decode unknown data
const decoded = Schema.decodeUnknownSync(User)({
  id: "123",
  name: "Alice",
  email: "alice@example.com",
  age: 30
})

// Decode with Effect (for better error handling)
const decodeEffect = Schema.decodeUnknown(User)
const result = decodeEffect({ id: "123", name: "Alice", email: "a@b.com", age: 30 })
// Effect<User, ParseError, never>

Resource Management

Safely manage resources that need cleanup:

import { Effect, Scope } from "effect"

// Define a resource with acquisition and release
const withConnection = Effect.acquireRelease(
  // Acquire
  Effect.sync(() => {
    console.log("Opening connection")
    return { query: (sql: string) => sql }
  }),
  // Release
  (conn) => Effect.sync(() => {
    console.log("Closing connection")
  })
)

// Use the resource in a scoped effect
const program = Effect.scoped(
  Effect.gen(function* () {
    const conn = yield* withConnection
    return conn.query("SELECT * FROM users")
  })
)

// Connection is automatically closed after use
Effect.runPromise(program)

Common Patterns

Sequential vs Parallel Execution

import { Effect } from "effect"

const tasks = [task1, task2, task3]

// Sequential execution
const sequential = Effect.all(tasks, { concurrency: 1 })

// Parallel execution (all at once)
const parallel = Effect.all(tasks, { concurrency: "unbounded" })

// Parallel with limit
const limited = Effect.all(tasks, { concurrency: 5 })

Retry with Backoff

import { Effect, Schedule } from "effect"

const retried = fetchData.pipe(
  Effect.retry(
    Schedule.exponential("100 millis").pipe(
      Schedule.jittered,
      Schedule.compose(Schedule.recurs(5))
    )
  )
)

Timeouts

import { Effect, Duration } from "effect"

const withTimeout = longRunningTask.pipe(
  Effect.timeout(Duration.seconds(30))
)

Guidelines

1. Use Effect.gen for complex flows - Generators make sequential operations readable
2. Tag errors with _tag - Enables catchTag for precise error handling
3. Define services with Context.Tag - Makes dependencies explicit and testable
4. Use Layer for service composition - Layers compose and manage lifecycle
5. Prefer Effect.try over manual try/catch - Keeps errors in the Effect channel
6. Use Schema for external data - Validates and transforms API/DB data
7. Scope resources with acquireRelease - Guarantees cleanup on success or failure
8. Run effects at the edge - Keep runPromise/runSync at application boundaries

Integration with Other Skills

Effect-TS integrates well with:

• Functional Patterns - Effect builds on Option/Either concepts
• Drizzle ORM - Wrap Drizzle queries in Effect for typed errors
• Fastify - Use Effect for request handling with typed errors

When This Skill Loads

This skill automatically loads when discussing:

• Effect-TS library usage
• Typed error handling in TypeScript
• Dependency injection with Context and Layer
• Effect.gen generators
• Schema validation with Effect
• Resource management and Scope
• Functional effect systems