Effect-TS Best Practices

This skill enforces opinionated, consistent patterns for Effect-TS codebases.

Effect LS diagnostics (agent usage)

Cursor's read_lints does not surface Effect Language Server diagnostics. Use the CLI:

npx effect-language-service diagnostics --file <path>
# or whole project:
npx effect-language-service diagnostics --project tsconfig.json

• Run when editing Effect code; fix reported issues (e.g. unnecessaryFailYieldableError → yield error directly)
• effect-language-service quickfixes shows proposed code changes

Quick Reference: Critical Rules

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; catch only when needed	catchAll; swallowing; catching "just in case"
IDs	Schema.UUID.pipe(Schema.brand("@App/EntityId"))	Plain string for entity IDs
Functions	Effect.fn("Service.method")	Anonymous generators
Params vs deps	Params = runtime data; dependencies = yield from context	Passing Ref/PubSub/service as params
Naming	FooCommand for commands, domain names for helpers	FooEffect suffix (redundant; TS/Effect.fn already convey type)
Logging	Effect.log with structured data	console.log
Config	Config.* with validation	process.env directly (except build-time vars like ESBUILD_*)
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

Params vs Dependencies

• Params = runtime data per call (IDs, user input, per-invocation config)
• Dependencies = shared infrastructure (Ref, PubSub, SubscriptionRef, services) — provide via layer, yield inside the effect
• Build Ref/PubSub/etc in the layer (e.g. buildAllServicesLayer); consumers yield them, don't receive as params

// WRONG - passing shared infra as params
const createStatusBar = (pubsub: PubSub.PubSub<void>, stateRef: SubscriptionRef.SubscriptionRef<State>) =>
  Effect.gen(...)
// Caller must create and pass; wiring scattered at call sites

// CORRECT - yield inside, build in layer
const PubSubTag = Context.GenericTag<PubSub.PubSub<void>>("PubSub")
const createStatusBar = Effect.gen(function* () {
  const pubsub = yield* PubSubTag
  const stateRef = yield* StateRefTag
  // ...
})
// Layer: Layer.effect(PubSubTag, PubSub.sliding<void>(1))

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 }))
    })
  );

When to Catch (and When Not To)

Most errors surface to the user (message/toast at runtime). Only catch when:

• Genuinely ignore – accept failure and continue (e.g. optional pre-create)
• Better message – default vague; map to clearer domain error

Catch sparingly. No catchAll or "swallow to be safe." Use catchTag/catchTags; log or fail with improved error.

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. Span name is required; enforced by local/require-effect-fn-span-name.

// 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);
  // ...
});

// WRONG - params on wrapper arrow, generator has none (closure capture)
// Enforced by local/no-effect-fn-wrapper
const findByIdBad = (id: UserId) =>
  Effect.fn('UserService.findById')(function* () {
    yield* repo.findById(id); // id from closure
  });

// Naming: Don't append Effect. For commands use FooCommand; for helpers/lifecycle use domain names.
// WRONG: logGetEffect, executeAnonymousDocumentEffect, activateEffect
// CORRECT: logGetCommand, executeAnonymousCommand, executeAnonymous (helper), activation (lifecycle)

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 - swallowing errors (most errors surface to user; only catch when ignoring intentionally or providing better message)
yield * effect.pipe(Effect.catchAll(() => Effect.void));

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

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

// EXCEPTION - build-time/bundle-time variables (e.g., ESBUILD_*)
const platform = process.env.ESBUILD_PLATFORM === 'web' ? webImpl : desktopImpl; // OK - build-time conditional

// 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