Effect Has a Free TypeScript Standard Library — Here's How to Use It

#typescript #functional #errors #webdev

TypeScript has no standard library for errors, retries, concurrency, or scheduling. Effect fills that gap — typed errors, resource management, dependency injection, and more.

What Is Effect?

Effect is a comprehensive TypeScript library that provides tools for managing errors, async operations, concurrency, and application architecture. Think of it as "what TypeScript's standard library should be."

Quick Start

npm install effect

import { Effect, Console } from 'effect';

const program = Effect.gen(function* () {
  yield* Console.log('Hello from Effect!');
  const result = yield* Effect.succeed(42);
  yield* Console.log(`Result: ${result}`);
  return result;
});

Effect.runPromise(program);

Typed Errors (The Killer Feature)

class UserNotFoundError {
  readonly _tag = 'UserNotFoundError';
  constructor(readonly userId: string) {}
}

class DatabaseError {
  readonly _tag = 'DatabaseError';
  constructor(readonly message: string) {}
}

// TypeScript KNOWS which errors this can throw
const getUser = (id: string): Effect.Effect<User, UserNotFoundError | DatabaseError> =>
  Effect.gen(function* () {
    const db = yield* connectToDb(); // Can fail with DatabaseError
    const user = yield* findUser(db, id); // Can fail with UserNotFoundError
    return user;
  });

// Handle specific errors
const program = getUser('123').pipe(
  Effect.catchTag('UserNotFoundError', (err) =>
    Effect.succeed({ id: err.userId, name: 'Unknown' })
  ),
  // DatabaseError still propagates — TypeScript enforces this!
);

Retries

const fetchData = pipe(
  Effect.tryPromise(() => fetch('/api/data')),
  Effect.retry({
    times: 3,
    schedule: Schedule.exponential('1 second'),
  }),
);

Concurrency

// Run 3 tasks in parallel, max 5 concurrent
const results = yield* Effect.forEach(
  urls,
  (url) => fetchUrl(url),
  { concurrency: 5 }
);

// Race — first to complete wins
const fastest = yield* Effect.race([
  fetchFromServer1(query),
  fetchFromServer2(query),
]);

// All must succeed
const [users, posts, comments] = yield* Effect.all([
  getUsers(),
  getPosts(),
  getComments(),
]);

Resource Management

const dbPool = Effect.acquireRelease(
  // Acquire
  Effect.tryPromise(() => createPool(config)),
  // Release (guaranteed to run)
  (pool) => Effect.promise(() => pool.end()),
);

// Pool is automatically cleaned up, even on errors
const program = Effect.scoped(
  Effect.gen(function* () {
    const pool = yield* dbPool;
    return yield* queryDb(pool, 'SELECT * FROM users');
  })
);

Why Effect

Problem	Without Effect	With Effect
Error handling	try/catch, unknown type	Typed errors, exhaustive
Retries	Manual loops	`Effect.retry()`
Concurrency	Promise.all/race	Structured concurrency
Resource cleanup	try/finally	`acquireRelease`
Dependency injection	Frameworks	Built-in `Layer` system
Interruption	Not possible	Cancel running effects