Effect-TS Fundamentals

Overview

Effect-TS provides a unified toolkit for building robust applications by treating side effects as values. It enables type-safe error handling, dependency injection, and resource management.

When to Use

• When building complex TypeScript applications requiring strict error tracking.
• When you need to decouple implementation from interface via dependency injection.
• When managing multiple async operations that must be composed safely.

When NOT to use:

• Simple scripts where standard Promise and try/catch suffice.
• Projects where team overhead for learning Effect outweighs architectural benefits.

Core Pattern

Effect replaces manual dependency passing and nested try/catch with a declarative, generator-based flow.

Before (Plain TS):

async function getUserData(userId: string, db: Database) {
  try {
    const user = await db.findUser(userId);
    const posts = await fetchPosts(user.id);
    return { user, posts };
  } catch (e) {
    throw new Error("Failed");
  }
}

After (Effect):

const getUserData = (userId: string) =>
  Effect.gen(function* () {
    const db = yield* DatabaseService;
    const user = yield* db.findUser(userId);
    const posts = yield* fetchPosts(user.id);
    return { user, posts };
  });

Quick Reference

Implementation

1. Modern Service Pattern

Define services with built-in accessors for seamless dependency injection.

export class UserService extends Effect.Service<UserService>()("UserService", {
  accessors: true,
  effect: Effect.gen(function* () {
    const db = yield* DatabaseService;
    return {
      getUser: (id: string) => db.query(`SELECT * FROM users WHERE id = ${id}`)
    };
  }),
  dependencies: [DatabaseService.Default]
}) {}

2. Sequential Logic with Generators

Use Effect.gen and yield* to compose effects without callback hell or manual promise chaining.

const getUserData = (userId: number) =>
  Effect.gen(function* () {
    const user = yield* fetchUser(userId);
    const posts = yield* fetchUserPosts(user.id);
    return { user, posts };
  });

3. Layer-Based Dependency Injection

Compose implementation layers and provide them to your program.

const MainLayer = Layer.merge(DatabaseService.Default, LoggerService.Default);
const program = pipe(myEffect, Effect.provide(MainLayer));

Common Mistakes

• Forgetting yield*: Calling an effect without yield* inside Effect.gen returns the effect itself, not its value.
• Manual try/catch: Use Effect.try or Effect.catchTag instead of standard try/catch to keep errors in the type channel.

REQUIRED SUB-SKILLS:

• effect-ts-errors for advanced error handling.
• effect-ts-resources for safe resource management.