Effect-TS Best Practices

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

Effect Language Server (Required)

The Effect Language Server is essential for Effect development. It catches errors at edit-time that TypeScript alone cannot detect, provides Effect-specific refactors, and improves developer productivity.

Setup

1. Install:

npm install @effect/language-service --save-dev

2. Add to tsconfig.json:

{
  "compilerOptions": {
    "plugins": [{ "name": "@effect/language-service" }]
  }
}

3. Configure your editor to use workspace TypeScript:
   • VSCode: F1 → "TypeScript: Select TypeScript Version" → "Use Workspace Version"
   • JetBrains: Settings → Languages & Frameworks → TypeScript → Use workspace version

Features

• Diagnostics: Detects 30+ Effect-specific issues (floating Effects, missing requirements, incorrect yield patterns)
• Quick Info: Hover to see Effect type parameters (Success, Error, Requirements)
• Completions: Auto-complete Self, Duration strings, Schema brands
• Refactors: Convert async → Effect.gen, auto-compose Layers, transform to Schema

Build-Time Diagnostics

For CI enforcement:

npx effect-language-service patch

See references/language-server.md for configuration options and CLI tools.

Quick Reference: Critical Rules

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,
)

Layer composition patterns:

// Use Layer.mergeAll for flat composition of same-level layers
const RepoLive = Layer.mergeAll(
    UserRepo.Default,
    OrderRepo.Default,
    ProductRepo.Default,
)

// Use Layer.provideMerge for incremental chaining (flatter types than Layer.provide)
const MainLive = DatabaseLive.pipe(
    Layer.provideMerge(ConfigServiceLive),
    Layer.provideMerge(LoggerLive),
    Layer.provideMerge(CacheLive),
)

Why layers over Effect.provide:

• Deduplication: Layers memoize construction - same service instantiated once. Effect.provide creates new instances each call.
• TypeScript performance: Deep Layer.provide nesting creates complex recursive types that slow the LSP. Layer.mergeAll and Layer.provideMerge produce flatter types.
• Resource management: Scoped layers properly share and clean up resources.

See references/layer-patterns.md for testing layers, config-dependent layers, and the layerConfig pattern.

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)

React Mutations

For mutation atoms, derive loading state from result.waiting instead of useState:

const [result, mutate] = useAtom(deleteMutation, { mode: "promise" })
const isLoading = result.waiting // Updates automatically, no useState/finally needed

Dialog ownership: Move mutation logic into dialog components. Dialog owns the mutation hook, loading state, and toasts. Parent provides data props and an onSuccess callback.

Cache invalidation: Use reactivityKeys on both mutation and query atoms to auto-invalidate queries after mutations — replaces manual refresh() calls.

See references/effect-atom-patterns.md for complete patterns including families, localStorage, mutations, 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.redacted("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:

• language-server.md - Effect Language Service setup, diagnostics, refactors, CLI tools
• 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