Agent skill

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.

View SKILL.md on GitHub Repository
Stars 45
Forks 2

Install this agent skill to your Project

npx add-skill https://github.com/Makisuo/skills/tree/main/effect-best-practices

SKILL.md

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:
bash
npm install @effect/language-service --save-dev
  1. Add to tsconfig.json:
json
{
  "compilerOptions": {
    "plugins": [{ "name": "@effect/language-service" }]
  }
}
  1. 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:

bash
npx effect-language-service patch

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

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
Layers Layer.mergeAll for flat composition Deeply nested Layer.provide chains
Layer Chaining Layer.provideMerge for incremental composition Multiple Layer.provide (creates nested types)
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,
)

Layer composition patterns:

typescript
// 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:

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)

React Mutations

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

typescript
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:

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.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

Recommended Agent Skills

Expand your agent's capabilities with these related and highly-rated skills.

Makisuo/skills

effect-review

This skill should be used when the user asks to "review code", "review my changes", "check effect patterns", "run effect review", "effect review", "review for effect best practices", or wants a comprehensive code review against Effect-TS conventions, branded types, observability, error handling, test coverage, and UI quality.

45 2
Explore
mattpocock/skills

edit-article

Edit and improve articles by restructuring sections, improving clarity, and tightening prose. Use when user wants to edit, revise, or improve an article draft.

111,310 9,758
Explore
mattpocock/skills

handoff

Compact the current conversation into a handoff document for another agent to pick up.

111,310 9,758
Explore
mattpocock/skills

git-guardrails-claude-code

Set up Claude Code hooks to block dangerous git commands (push, reset --hard, clean, branch -D, etc.) before they execute. Use when user wants to prevent destructive git operations, add git safety hooks, or block git push/reset in Claude Code.

111,310 9,758
Explore
mattpocock/skills

obsidian-vault

Search, create, and manage notes in the Obsidian vault with wikilinks and index notes. Use when user wants to find, create, or organize notes in Obsidian.

111,310 9,758
Explore
mattpocock/skills

setup-pre-commit

Set up Husky pre-commit hooks with lint-staged (Prettier), type checking, and tests in the current repo. Use when user wants to add pre-commit hooks, set up Husky, configure lint-staged, or add commit-time formatting/typechecking/testing.

111,310 9,758
Explore

Didn't find tool you were looking for?

Be as detailed as possible for better results