Runnable at every step
Every lesson ships with an index.test.ts. Run it locally with pnpm test, or read the generated lesson in your browser. The type system and the test suite keep you honest.
Effect 4 · 61 lessons
Learn Effect, step by step.
Effect by Effect is a test-driven tutorial for Effect 4. Each lesson is a runnable test file — read the explanation, fill in the stubs, run the suite. You move from Effect<A, E, R> to services, layers, concurrency, streams, and production patterns used in real Effect applications.
// Effect<A, E, R>
// A = what you get on success
// E = what can go wrong
// R = what needs to be provided
//
// Three dimensions. One type. Everything else is discipline.
import { Effect } from "effect"
const divide = (a: number, b: number): Effect.Effect<number, Error, never> =>
b === 0
? Effect.fail(new Error("Cannot divide by zero"))
: Effect.succeed(a / b)
Effect.runSync(divide(10, 2))
// 5How this tutorial works01 02 03
Dependency order
Topics appear when you need them. Context and layers come after error handling. Streams come after schedules. You build one mental model and extend it — nothing to unlearn later.
Production patterns
Capstones combine retries, dependency injection, schema validation, and observability. The same patterns you will find in Effect's own libraries and documentation.
The curriculum
61 lessons, 10 parts
What you'll build toward
From types to systems
Foundations
Why typed errors beat try/catch, what Effect<A, E, R> describes, and how to create, compose, and run effects with pipe, Effect.gen, and the standard runners.
Services
Context, Layer, and dependency injection — the structure Effect applications use to stay testable.
Testing
Test layers, TestClock, and assertions on Exit — test the program, not the mocks.
Concurrency
Fibers, interruption, Ref, queues, and semaphores — Effect's structured concurrency model in practice.
Production
Config, observability, HTTP clients, CLI parsing, and runtimes — the path from tutorial exercises to deployable code.
Ready?
Clone the repo. Run the tests.
61 lessons, one source tree under steps/. Read here or run locally — same content, same Effect 4 APIs.