Effect Best Practices

For diff/plan review against these patterns, invoke the effect-advocate subagent (.claude/agents/effect-advocate.md).

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

• The PostToolUse verify-on-edit.sh hook auto-runs --file <edited> on every .ts Edit/Write and surfaces output as followup_message. Address what it reports.
• Address warnings AND messages, not just errors. references/diagnostics-findings.md maps each common finding to its fix; config/effect-diagnostics.json enforcedRules is the build gate.
• Enforcing a rule takes two edits, not just an enforcedRules entry — see references/diagnostics-findings.md.
• After a batch of edits, run --project tsconfig.json for the affected package to catch cross-file issues.
• effect-language-service quickfixes shows proposed code changes.

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
• Interfaces with caller-provided implementations — no single canonical one to bundle as .Default (e.g. SoqlBuilderService, implemented once by the VS Code host and once by a test fake); see references/service-patterns.md

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 with rich context (userId, channelId, expiredAt), not one generic NotFoundError everything maps to. A generic { _tag: 'NotFoundError', message: 'Not found' } can't tell the frontend which resource failed or how to recover; explicit tags drive specific UI. See references/error-patterns.md for the WRONG/CORRECT contrast and naming conventions.

Accumulating Errors Across a Collection

To continue past failures instead of short-circuiting on the first, don't hand-roll Either + catchTag + a re-loop. Use Effect.partition (both buckets), Effect.validateAll (all-or-nothing), or Effect.validateFirst. These recover the typed error channel per item but do NOT capture interruption — so a Cancel still aborts the whole loop.

See references/error-patterns.md for the accumulation/interruption nuance, 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: Prefer Effect.fn over Effect.gen

Prefer Effect.fn for effectful code. Provides automatic tracing with proper span names. Span name required; enforced by local/require-effect-fn-span-name.

Use Effect.gen only when you need a shared effect with common .pipe attached so multiple consumers don't each pipe the same things — e.g. provided dependencies, common error handlers, retries. (Less common with Runtimes.) Service definition bodies are a valid use (shared wiring).

// 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
  });

// WRONG - Effect.fn invoked immediately (config-enforced effectFnIife). Effect.fn builds a reusable
// function; for one-shot use write Effect.gen and keep the span with a piped withSpan.
const opened = Effect.fn('FsService.open')(function* () {
  yield* fs.showTextDocument(uri);
})();
// CORRECT
const openedOk = Effect.gen(function* () {
  yield* fs.showTextDocument(uri);
}).pipe(Effect.withSpan('FsService.open'));

// 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)

See references/composition-style.md for how to compose these: flat build-then-run pipes, terminal runner, point-free safety, Match dispatch, guard clauses.

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.

Effect-Owned Resources

Resources created inside an Effect service/layer belong to its scope. Prefer scoped plus Effect.addFinalizer or Effect.acquireRelease; don't expose dispose or delegate cleanup to a host lifecycle.

export class StatusService extends Effect.Service<StatusService>()('StatusService', {
  accessors: true,
  scoped: Effect.gen(function* () {
    const item = vscode.window.createStatusBarItem();
    yield* Effect.addFinalizer(() => Effect.sync(() => item.dispose()));
    return { show: Effect.sync(() => item.show()) };
  })
}) {}

The layer owner must close its scope. A ManagedRuntime owns its layers' scopes; dispose it during extension deactivation. Use context.subscriptions only for resources created outside Effect ownership.

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)

Reactive React state via @effect-atom/atom-react. Define atoms OUTSIDE components; keepAlive for state that must persist; useAtomSet to write; Result.builder to render effectful results; get.addFinalizer to clean up listeners.

import { Atom, Result, useAtomValue, useAtomSet } from '@effect-atom/atom-react';

const countAtom = Atom.make(0); // outside the component
const prefsAtom = Atom.make({ theme: 'dark' }).pipe(Atom.keepAlive); // persistent

function Counter() {
  const count = useAtomValue(countAtom);
  const setCount = useAtomSet(countAtom);
  return <button => setCount(c => c + 1)}>{count}</button>;
}

// Effectful atom → Result; handle loading/error/success
function UserProfile() {
  return Result.builder(useAtomValue(userAtom))
    .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();
}

SubscriptionRef

SubscriptionRef<A> is a mutable ref whose .changes stream always emits the current value as element 0, then all future mutations.

Implemented as (from effect/src/internal/subscriptionRef.ts):

stream.concat(stream.make(currentValue), stream.fromPubSub(pubsub))

The Ref.get + pubsub subscription happen atomically under a semaphore — no events are missed.

// WRONG — prepended get is always redundant
Stream.concat(Stream.fromEffect(SubscriptionRef.get(ref)), ref.changes)
Stream.concat(Stream.make(yield* SubscriptionRef.get(ref)), ref.changes)
Stream.merge(Stream.fromEffect(SubscriptionRef.get(ref)), ref.changes)

// CORRECT — .changes already provides the snapshot
ref.changes.pipe(...)

To skip the initial snapshot (e.g. avoid a spurious refresh on activation), use Stream.drop(1).

Grouping

Object.groupBy is typed Partial<Record<K, V[]>>, so every consumer filters or defaults the undefined. Arr.groupBy returns Record<K, NonEmptyArray<V>> — total, so Object.entries needs no guard.

import * as Arr from 'effect/Array';

const grouped = Arr.groupBy(ids, id => id.slice(0, 3)); // Record<string, NonEmptyArray<string>>
Object.entries(grouped).map(([prefix, group]) => query(prefix, group));

// vs Object.groupBy, where the same line needs:
//   .filter((entry): entry is [string, string[]] => isNotUndefined(entry[1]))

Keep Object.groupBy only when the Partial is load-bearing — e.g. destructuring absent keys with defaults, const { deploys = [], deleted = [] } = ....

Anti-Patterns (Forbidden)

The DON'T column above names each forbidden pattern. references/anti-patterns.md has the complete list — each with rationale and the correct alternative.

Emptiness Guards: match the declared type

== null / != null are banned in effect packages (noNullCompare in eslint.config.mjs). Pick the effect/Predicate guard by what the declared type actually admits — one guard per union shape, no exceptions:

import { isNotNull, isNotUndefined, isNullable, isUndefined } from 'effect/Predicate';

// T | undefined — the common case (Optional<T>, optional props, ?? sources)
if (isUndefined(maybeValue)) return;
Effect.filterOrFail(isNotUndefined, () => new NotFoundError({ message: '...' }));

// T | null — e.g. RegExp.exec, JSON payload fields
const match = scriptRegex.exec(html);
if (isNotNull(match)) { … }

// T | null | undefined — e.g. `exclude?: vscode.GlobPattern | null` (optional AND nullable)
const arr = isNullable(exclude) ? undefined : [exclude];

Runtime semantics: isUndefined is x === undefined, isNull is x === null, isNullable is either (node_modules/effect/src/Predicate.ts:611,649,925). A wider guard than the type needs still compiles — it just implies a case the type can't produce, so it reads as a lie about the value. Note typeof x === 'object' && x !== null object-narrowing stays as-is; that's a structural check, not an emptiness check.

Point-free Predicates in Filters

Use Predicate.not() for filter negations; combines point-free and readability.

import { not } from 'effect/Predicate';

// CORRECT — point-free negation
arr.filter(not(isFlowTest))

// AVOID — arrow wrapper around negation
arr.filter(x => !isFlowTest(x))

Works with any predicate — built-in (isString, isError, isNotUndefined, isNotNullable) or custom. Only the wrapped predicate can be receiver-sensitive: not(obj.method) detaches the receiver, so verify the impl uses no this first (see composition-style).

Keep ! where the predicate isn't element-level (!isCollectionType(decl.type)) unless a named element predicate already exists or is worth adding.

Imports: Prefer Deep Imports from @effect/platform

Barrel imports from @effect/platform bundle HttpApiSwagger (Swagger UI), which esbuild cannot tree-shake. This bloats web/desktop bundles by ~5.5MB per output and can trigger security scanner false positives (e.g., ClamAV signatures).

Always import the submodule as a namespace (the submodule is the namespace — it has no self-named export):

// CORRECT — deep namespace import, tree-shakes unused
import * as FetchHttpClient from '@effect/platform/FetchHttpClient';
// use: FetchHttpClient.layer

// WRONG — barrel import drags in HttpApiSwagger
import { FetchHttpClient } from '@effect/platform';

Matches the repo's import * as Effect from 'effect/Effect' style. Applies to all @effect/* packages. Check import source before committing.

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:

• composition-style.md - Effects as flat build-then-run pipes: terminal runner, point-free safety, keep side effects (even terminal) in tap, Match dispatch, guard clauses, linear body as point-free pipe vs generator
• 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
• anti-patterns.md - Complete list of forbidden patterns
• diagnostics-findings.md - Effect LS finding → fix, per rule
• observability-patterns.md - Logging, metrics, config patterns