# Effect Best Practices

> Enforces Effect-TS patterns for services, errors, layers, and atoms. Use when writing code with Effect.Service, Schema.TaggedError, Layer composition, or effect-atom React components.

- Skill: `millionco/effect-best-practices` (Agent Skill, multi-file: 9 files)
- Install (CLI): `npx skillmds@latest add millionco/effect-best-practices`
- Raw SKILL.md: https://api.skillmd.com/api/skills/millionco/effect-best-practices/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: millionco (https://skillmd.com/u/millionco)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/millionco/effect-best-practices

---


# Effect-TS Best Practices

This skill enforces opinionated, consistent patterns for Effect-TS codebases. These patterns optimize for type safety, testability, observability, and maintainability.

## 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`                            | `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.

```typescript
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.

```typescript
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`:**

```typescript
// 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.

```typescript
// 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!
```

```typescript
// 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:

```typescript
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:

```typescript
// 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:

```typescript
// 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:

```typescript
// 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

```typescript
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

```typescript
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`:

```typescript
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

```typescript
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**:

```typescript
// 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

```typescript
// 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