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Enforces Effect-TS patterns for services with Effect.Service, errors via Schema.TaggedError, layer composition, and effect-atom React components. Use for type-safe Effect codebases.
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This skill enforces opinionated, consistent patterns for Effect-TS codebases. These patterns optimize for type safety, testability, observability, and maintainability.
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This skill enforces opinionated, consistent patterns for Effect-TS codebases. These patterns optimize for type safety, testability, observability, and maintainability.
| 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 |
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:
See references/service-patterns.md for detailed patterns.
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 })),
}),
)
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.
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:
See references/schema-patterns.md for transforms and advanced patterns.
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)
// ...
}
)
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.
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 provides reactive state management for React with Effect integration.
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)
)
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}</>
}
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()
}
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.
For RPC contracts and cluster workflows, see:
references/rpc-cluster-patterns.md - RpcGroup, Workflow.make, Activity patternsThese 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.
// 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.
For detailed patterns, consult these reference files in the references/ directory:
service-patterns.md - Service definition, Effect.fn, Context.Tag exceptionserror-patterns.md - Schema.TaggedError, error remapping, retry patternsschema-patterns.md - Branded types, transforms, Schema.Classlayer-patterns.md - Dependency composition, testing layersrpc-cluster-patterns.md - RpcGroup, Workflow, Activity patternseffect-atom-patterns.md - Atom, families, React hooks, Result handlinganti-patterns.md - Complete list of forbidden patternsobservability-patterns.md - Logging, metrics, config patterns