Effect TS

Effect TypeScript Best Practices

Effect is a TypeScript library for building complex, type-safe applications with structured error handling, dependency injection via services/layers, fiber-based concurrency, and resource safety.

When to Apply

• Writing or reviewing TypeScript code that imports from effect, @effect/schema, or @effect/platform
• Implementing typed error handling with Effect<Success, Error, Requirements>
• Building services and layers for dependency injection
• Working with Schema for data validation, decoding, and transformation
• Using fiber-based concurrency (queues, semaphores, PubSub, deferred)
• Processing data with Stream and Sink
• Migrating from Promises, fp-ts, neverthrow, or ZIO to Effect

How to Use

This skill is organized by domain. Read the relevant reference file for the area you're working in.

Read First: The Paradigm

Always read this before diving into API references, especially when refactoring existing code to use Effect or writing new Effect services:

Core Foundations

Data & Validation

Architecture & Dependencies

Concurrency & Streaming

Platform & Observability

Style, AI & Migration

Quick Reference — Common Patterns

The Effect Type

//         ┌─── Success type
//         │        ┌─── Error type
//         │        │      ┌─── Required dependencies
//         ▼        ▼      ▼
Effect<Success, Error, Requirements>

Creating Effects

import { Effect } from "effect"

// From sync values
const succeed = Effect.succeed(42)
const fail = Effect.fail(new Error("oops"))

// From sync code that may throw
const sync = Effect.try(() => JSON.parse(data))

// From promises
const async = Effect.tryPromise(() => fetch(url))

// From generators (recommended for complex flows)
const program = Effect.gen(function* () {
  const user = yield* getUser(id)
  const todos = yield* getTodos(user.id)
  return { user, todos }
})

Running Effects

// Async (returns Promise)
Effect.runPromise(program)

// With full Exit information
Effect.runPromiseExit(program)

// Sync (throws on async)
Effect.runSync(program)

Typed Errors

import { Data, Effect } from "effect"

class NotFound extends Data.TaggedError("NotFound")<{
  readonly id: string
}> {}

class Unauthorized extends Data.TaggedError("Unauthorized")<{}> {}

// Error type is tracked: Effect<User, NotFound | Unauthorized>
const getUser = (id: string) =>
  Effect.gen(function* () {
    // ...
  })

Services and Layers

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

// Define a service
class UserRepo extends Context.Tag("UserRepo")<
  UserRepo,
  { readonly findById: (id: string) => Effect.Effect<User, NotFound> }
>() {}

// Use in effects — adds to Requirements channel
const program = Effect.gen(function* () {
  const repo = yield* UserRepo
  return yield* repo.findById("1")
})

// Implement with a Layer
const UserRepoLive = Layer.succeed(UserRepo, {
  findById: (id) => Effect.succeed({ id, name: "Alice" })
})

// Provide and run
program.pipe(Effect.provide(UserRepoLive), Effect.runPromise)

Schema Validation

import { Schema } from "effect"

const User = Schema.Struct({
  id: Schema.Number,
  name: Schema.String,
  email: Schema.String.pipe(Schema.pattern(/@/))
})

type User = typeof User.Type

// Decode (parse + validate)
const decode = Schema.decodeUnknownSync(User)
const user = decode({ id: 1, name: "Alice", email: "a@b.com" })

Pipelines

import { Effect, pipe } from "effect"

// Data-last (pipe style)
const result = pipe(
  getTodos,
  Effect.map((todos) => todos.filter((t) => !t.done)),
  Effect.flatMap((active) => sendNotification(active.length)),
  Effect.catchTag("NetworkError", () => Effect.succeed("offline"))
)

// Fluent (method style)
const result2 = getTodos.pipe(
  Effect.map((todos) => todos.filter((t) => !t.done)),
  Effect.flatMap((active) => sendNotification(active.length))
)

Gotchas

See gotchas.md for known failure points.