Category	DO	DON'T
Services	`Effect.Service` with `accessors: true`	`Context.Tag` for business logic
Dependencies	`dependencies: [Dep.Default]` in service	Manual `Layer.provide` at usage sites
Errors	`Schema.TaggedError` with `message` field	Plain classes or generic Error
Error Specificity	`UserNotFoundError`, `SessionExpiredError`	Generic `NotFoundError`, `BadRequestError`
Error Handling	`catchTag`/`catchTags`	`catchAll` or `mapError`
IDs	`Schema.UUID.pipe(Schema.brand("@App/EntityId"))`	Plain `string` for entity IDs
Functions	`Effect.fn("Service.method")`	Anonymous generators
Logging	`Effect.log` with structured data	`console.log`
Config	`Config.*` with validation	`process.env` directly
Options	`Option.match` with both cases	`Option.getOrThrow`
Nullability	`Option<T>` in domain types	`null`/`undefined`
Atoms	`Atom.make` outside components	Creating atoms inside render
Atom State	`Atom.keepAlive` for global state	Forgetting keepAlive for persistent state
Atom Updates	`useAtomSet` in React components	`Atom.update` imperatively from React
Atom Cleanup	`get.addFinalizer()` for side effects	Missing cleanup for event listeners
Atom Results	`Result.builder` with `onErrorTag`	Ignoring loading/error states

Service Definition Pattern

Always use Effect.Service for business logic services. This provides automatic accessors, built-in Default layer, and proper dependency declaration.

import { Effect } from "effect";

export class UserService extends Effect.Service<UserService>()("UserService", {
  accessors: true,
  dependencies: [UserRepo.Default, CacheService.Default],
  effect: Effect.gen(function* () {
    const repo = yield* UserRepo;
    const cache = yield* CacheService;

    const findById = Effect.fn("UserService.findById")(function* (id: UserId) {
      const cached = yield* cache.get(id);
      if (Option.isSome(cached)) return cached.value;

      const user = yield* repo.findById(id);
      yield* cache.set(id, user);
      return user;
    });

    const create = Effect.fn("UserService.create")(function* (data: CreateUserInput) {
      const user = yield* repo.create(data);
      yield* Effect.log("User created", { userId: user.id });
      return user;
    });

    return { findById, create };
  }),
}) {}

// Usage - dependencies are already wired
const program = Effect.gen(function* () {
  const user = yield* UserService.findById(userId);
  return user;
});

// At app root
const MainLive = Layer.mergeAll(UserService.Default, OtherService.Default);

When Context.Tag is acceptable:

Infrastructure with runtime injection (Cloudflare KV, worker bindings)
Factory patterns where resources are provided externally

See references/service-patterns.md for detailed patterns.

Error Definition Pattern

Always use Schema.TaggedError for errors. This makes them serializable (required for RPC) and provides consistent structure.

import { Schema } from "effect";
import { HttpApiSchema } from "@effect/platform";

export class UserNotFoundError extends Schema.TaggedError<UserNotFoundError>()(
  "UserNotFoundError",
  {
    userId: UserId,
    message: Schema.String,
  },
  HttpApiSchema.annotations({ status: 404 }),
) {}

export class UserCreateError extends Schema.TaggedError<UserCreateError>()(
  "UserCreateError",
  {
    message: Schema.String,
    cause: Schema.optional(Schema.String),
  },
  HttpApiSchema.annotations({ status: 400 }),
) {}

Error handling - use catchTag/catchTags:

// CORRECT - preserves type information
yield *
  repo.findById(id).pipe(
    Effect.catchTag("DatabaseError", (err) =>
      Effect.fail(new UserNotFoundError({ userId: id, message: "Lookup failed" })),
    ),
    Effect.catchTag("ConnectionError", (err) =>
      Effect.fail(new ServiceUnavailableError({ message: "Database unreachable" })),
    ),
  );

// CORRECT - multiple tags at once
yield *
  effect.pipe(
    Effect.catchTags({
      DatabaseError: (err) =>
        Effect.fail(new UserNotFoundError({ userId: id, message: err.message })),
      ValidationError: (err) =>
        Effect.fail(new InvalidEmailError({ email: input.email, message: err.message })),
    }),
  );

Prefer Explicit Over Generic Errors

Every distinct failure reason deserves its own error type. Don't collapse multiple failure modes into generic HTTP errors.

// WRONG - Generic errors lose information
export class NotFoundError extends Schema.TaggedError<NotFoundError>()(
  "NotFoundError",
  { message: Schema.String },
  HttpApiSchema.annotations({ status: 404 }),
) {}

// Then mapping everything to it:
Effect.catchTags({
  UserNotFoundError: (err) => Effect.fail(new NotFoundError({ message: "Not found" })),
  ChannelNotFoundError: (err) => Effect.fail(new NotFoundError({ message: "Not found" })),
  MessageNotFoundError: (err) => Effect.fail(new NotFoundError({ message: "Not found" })),
});
// Frontend gets useless: { _tag: "NotFoundError", message: "Not found" }
// Which resource? User? Channel? Message? Can't tell!

// CORRECT - Explicit domain errors with rich context
export class UserNotFoundError extends Schema.TaggedError<UserNotFoundError>()(
  "UserNotFoundError",
  { userId: UserId, message: Schema.String },
  HttpApiSchema.annotations({ status: 404 }),
) {}

export class ChannelNotFoundError extends Schema.TaggedError<ChannelNotFoundError>()(
  "ChannelNotFoundError",
  { channelId: ChannelId, message: Schema.String },
  HttpApiSchema.annotations({ status: 404 }),
) {}

export class SessionExpiredError extends Schema.TaggedError<SessionExpiredError>()(
  "SessionExpiredError",
  {
    sessionId: SessionId,
    expiredAt: Schema.DateTimeUtc,
    message: Schema.String,
  },
  HttpApiSchema.annotations({ status: 401 }),
) {}

// Frontend can now show specific UI:
// - UserNotFoundError → "User doesn't exist"
// - ChannelNotFoundError → "Channel was deleted"
// - SessionExpiredError → "Your session expired. Please log in again."

See references/error-patterns.md for error remapping and retry patterns.

Schema & Branded Types Pattern

Brand all entity IDs for type safety across service boundaries:

import { Schema } from "effect";

// Entity IDs - always branded
export const UserId = Schema.UUID.pipe(Schema.brand("@App/UserId"));
export type UserId = Schema.Schema.Type<typeof UserId>;

export const OrganizationId = Schema.UUID.pipe(Schema.brand("@App/OrganizationId"));
export type OrganizationId = Schema.Schema.Type<typeof OrganizationId>;

// Domain types - use Schema.Struct
export const User = Schema.Struct({
  id: UserId,
  email: Schema.String,
  name: Schema.String,
  organizationId: OrganizationId,
  createdAt: Schema.DateTimeUtc,
});
export type User = Schema.Schema.Type<typeof User>;

// Input types for mutations
export const CreateUserInput = Schema.Struct({
  email: Schema.String.pipe(Schema.pattern(/^[^\s@]+@[^\s@]+\.[^\s@]+$/)),
  name: Schema.String.pipe(Schema.minLength(1)),
  organizationId: OrganizationId,
});
export type CreateUserInput = Schema.Schema.Type<typeof CreateUserInput>;

When NOT to brand:

Simple strings that don't cross service boundaries (URLs, file paths)
Primitive config values

See references/schema-patterns.md for transforms and advanced patterns.

Function Pattern with Effect.fn

Always use Effect.fn for service methods. This provides automatic tracing with proper span names:

// CORRECT - Effect.fn with descriptive name
const findById = Effect.fn("UserService.findById")(function* (id: UserId) {
  yield* Effect.annotateCurrentSpan("userId", id);
  const user = yield* repo.findById(id);
  return user;
});

// CORRECT - Effect.fn with multiple parameters
const transfer = Effect.fn("AccountService.transfer")(function* (
  fromId: AccountId,
  toId: AccountId,
  amount: number,
) {
  yield* Effect.annotateCurrentSpan("fromId", fromId);
  yield* Effect.annotateCurrentSpan("toId", toId);
  yield* Effect.annotateCurrentSpan("amount", amount);
  // ...
});

Layer Composition

Declare dependencies in the service, not at usage sites:

// CORRECT - dependencies in service definition
export class OrderService extends Effect.Service<OrderService>()("OrderService", {
  accessors: true,
  dependencies: [UserService.Default, ProductService.Default, PaymentService.Default],
  effect: Effect.gen(function* () {
    const users = yield* UserService;
    const products = yield* ProductService;
    const payments = yield* PaymentService;
    // ...
  }),
}) {}

// At app root - simple merge
const AppLive = Layer.mergeAll(
  OrderService.Default,
  // Infrastructure layers (intentionally not in dependencies)
  DatabaseLive,
  RedisLive,
);

See references/layer-patterns.md for testing layers and config-dependent layers.

Option Handling

Never use Option.getOrThrow. Always handle both cases explicitly:

// CORRECT - explicit handling
yield *
  Option.match(maybeUser, {
    onNone: () => Effect.fail(new UserNotFoundError({ userId, message: "Not found" })),
    onSome: (user) => Effect.succeed(user),
  });

// CORRECT - with getOrElse for defaults
const name = Option.getOrElse(maybeName, () => "Anonymous");

// CORRECT - Option.map for transformations
const upperName = Option.map(maybeName, (n) => n.toUpperCase());

Effect Atom (Frontend State)

Effect Atom provides reactive state management for React with Effect integration.

Basic Atoms

import { Atom } from "@effect-atom/atom-react";

// Define atoms OUTSIDE components
const countAtom = Atom.make(0);

// Use keepAlive for global state that should persist
const userPrefsAtom = Atom.make({ theme: "dark" }).pipe(Atom.keepAlive);

// Atom families for per-entity state
const modalAtomFamily = Atom.family((type: string) =>
  Atom.make({ isOpen: false }).pipe(Atom.keepAlive),
);

React Integration

import { useAtomValue, useAtomSet, useAtom, useAtomMount } from "@effect-atom/atom-react"

function Counter() {
    const count = useAtomValue(countAtom)           // Read only
    const setCount = useAtomSet(countAtom)          // Write only
    const [value, setValue] = useAtom(countAtom)    // Read + write

    return <button onClick={() => setCount((c) => c + 1)}>{count}</button>
}

// Mount side-effect atoms without reading value
function App() {
    useAtomMount(keyboardShortcutsAtom)
    return <>{children}</>
}

Handling Results with Result.builder

Use Result.builder for rendering effectful atom results. It provides chainable error handling with onErrorTag:

import { Result } from "@effect-atom/atom-react"

function UserProfile() {
    const userResult = useAtomValue(userAtom) // Result<User, Error>

    return Result.builder(userResult)
        .onInitial(() => <div>Loading...</div>)
        .onErrorTag("NotFoundError", () => <div>User not found</div>)
        .onError((error) => <div>Error: {error.message}</div>)
        .onSuccess((user) => <div>Hello, {user.name}</div>)
        .render()
}

Atoms with Side Effects

const scrollYAtom = Atom.make((get) => {
  const onScroll = () => get.setSelf(window.scrollY);

  window.addEventListener("scroll", onScroll);
  get.addFinalizer(() => window.removeEventListener("scroll", onScroll)); // REQUIRED

  return window.scrollY;
}).pipe(Atom.keepAlive);

See references/effect-atom-patterns.md for complete patterns including families, localStorage, and anti-patterns.

RPC & Cluster Patterns

For RPC contracts and cluster workflows, see:

references/rpc-cluster-patterns.md - RpcGroup, Workflow.make, Activity patterns

Anti-Patterns (Forbidden)

These patterns are never acceptable:

// FORBIDDEN - runSync/runPromise inside services
const result = Effect.runSync(someEffect); // Never do this

// FORBIDDEN - throw inside Effect.gen
yield *
  Effect.gen(function* () {
    if (bad) throw new Error("No!"); // Use Effect.fail instead
  });

// FORBIDDEN - catchAll losing type info
yield * effect.pipe(Effect.catchAll(() => Effect.fail(new GenericError())));

// FORBIDDEN - console.log
console.log("debug"); // Use Effect.log

// FORBIDDEN - process.env directly
const key = process.env.API_KEY; // Use Config.string("API_KEY")

// FORBIDDEN - null/undefined in domain types
type User = { name: string | null }; // Use Option<string>

See references/anti-patterns.md for the complete list with rationale.

Observability

// Structured logging
yield * Effect.log("Processing order", { orderId, userId, amount });

// Metrics
const orderCounter = Metric.counter("orders_processed");
yield * Metric.increment(orderCounter);

// Config with validation
const config = Config.all({
  port: Config.integer("PORT").pipe(Config.withDefault(3000)),
  apiKey: Config.secret("API_KEY"),
  maxRetries: Config.integer("MAX_RETRIES").pipe(
    Config.validate({ message: "Must be positive", validation: (n) => n > 0 }),
  ),
});

See references/observability-patterns.md for metrics and tracing patterns.

Reference Files

For detailed patterns, consult these reference files in the references/ directory:

service-patterns.md - Service definition, Effect.fn, Context.Tag exceptions
error-patterns.md - Schema.TaggedError, error remapping, retry patterns
schema-patterns.md - Branded types, transforms, Schema.Class
layer-patterns.md - Dependency composition, testing layers
rpc-cluster-patterns.md - RpcGroup, Workflow, Activity patterns
effect-atom-patterns.md - Atom, families, React hooks, Result handling
anti-patterns.md - Complete list of forbidden patterns
observability-patterns.md - Logging, metrics, config patterns

effect-best-practices

Effect-TS Best Practices

Quick Reference: Critical Rules