Writing Effect

Writing Effect

Effect-TS coding guide for this monorepo. Authoritative source: clone Effect v4 source into .repos/effect-v4/ by running pnpm setup:repos (or bash scripts/setup-repos.sh) — matches the convention Effect itself uses (see effect-ts/effect-smol/scripts/worktree-setup.sh and that repo's AGENTS.md, which explicitly points agents at .repos/effect-v4). .repos/ is gitignored, so it stays out of PRs and never appears in pkg.pr.new tarballs.

When you need to verify an API, grep .repos/effect-v4/packages/effect/src/ and .repos/effect-v4/packages/platform-node/src/ directly — docs lag the beta, source doesn't. The cloned repo's own AGENTS.md and LLMS.md are worth a skim. The web docs at https://effect.website/llms-full.txt are useful too but may be v3-flavored in places.

If .repos/effect-v4/ is missing, run pnpm setup:repos and try again. To refresh to upstream main, re-run the same command — it fast-forwards in place. Rules for the cloned repo: read-only, don't import from it in our source (use the effect@beta / @effect/platform-node@beta npm packages instead, both pinned in the workspace catalog).

Version

This monorepo targets Effect v4 beta (pinned in pnpm-workspace.yaml catalog). v4 consolidates many former sub-packages into core effect:

• Schema lives in effect (not @effect/schema). Import: import { Schema } from 'effect'.
• Stream, Sink, Channel, FileSystem, Path, Terminal, Stdio, Schedule, Pool, Semaphore, Cron, DateTime, Match — all in core effect.
• CLI: effect/unstable/cli (unstable).
• Child process: effect/unstable/process + Node binding via @effect/platform-node's NodeChildProcessSpawner (stable).
• HTTP / HttpApi / RPC: effect/unstable/http, effect/unstable/httpapi, effect/unstable/rpc.
• Vitest: @effect/vitest. The v4 API differs from v3: use it.effect (it already includes a Scope — no separate it.scoped). The package re-exports all of vitest, so import describe, expect, it from @effect/vitest. No assert export — use expect. Never use Effect.runSync inside it.

When this guide conflicts with what the cloned .repos/effect-v4/ source shows, trust the source and update this skill.

Repo conventions

• Tabs for indentation. Single quotes. Trailing commas. Width 100. Prettier handles all of it.
• Lint via root pnpm lint (oxlint + prettier). No per-package overrides.
• Prototype repo — break APIs directly, no shims or deprecation cycle. See repo-root AGENTS.md.
• Capture friction in packages/<pkg>/notes/friction.md.

Core idioms

Use Effect.gen for async/await-style flow

import { Effect, Random } from 'effect';

Effect.gen(function* () {
	yield* Effect.sleep('1 second');
	const bool = yield* Random.nextBoolean;
	if (bool) {
		// Always `return yield*` with Effect.fail so TS narrows conditional types
		return yield* Effect.fail('Random boolean was true');
	}
	return 'Returned value';
}).pipe(Effect.withSpan('tracing span'));

Use Effect.fn for named Effect-returning functions

import { Effect, Random } from 'effect';

const myFn = Effect.fn('myFn')(
	function* (x: number, y: number) {
		const bool = yield* Random.nextBoolean;
		if (bool) return yield* Effect.fail('Random boolean was true');
		return x + y;
	},
	Effect.annotateLogs({ some: 'annotation' }),
	(effect, x, y) => Effect.annotateLogs(effect, { x, y }),
);

Avoid try/catch — use Effect.try / Effect.tryPromise

import { Effect, Schema } from 'effect';

class JsonError extends Schema.TaggedError<JsonError>('JsonError')({
	cause: Schema.Defect,
}) {}

Effect.gen(function* () {
	const parsed = yield* Effect.try({
		try: () => JSON.parse('{"invalid": }'),
		catch: (cause) => new JsonError({ cause }),
	});
	const body = yield* Effect.tryPromise({
		try: () => fetch('https://example.com').then((r) => r.json()),
		catch: (cause) => new JsonError({ cause }),
	});
	return { parsed, body };
});

Error handling

• Effect.catchAll — all errors
• Effect.catchAllCause — including defects
• Effect.catchTag / Effect.catchTags — specific tagged errors
• Effect.catchIf — conditional

someEffect.pipe(
	Effect.catchTag('ErrorA', (e) => Effect.log('Caught ErrorA', e)),
	Effect.catchTags({
		ErrorA: (e) => Effect.log('A', e),
		ErrorB: (e) => Effect.log('B', e),
	}),
);

Services — the most important pattern

Most Effect code should be written as services. They bundle related Effect functions and let Effect's DI wire them up.

import { Effect, Schema } from 'effect';

export class Database extends Effect.Service<Database>()('Database', {
	dependencies: [],
	// ESSENTIAL: always use `scoped:` — gives the service a Scope for finalizers
	scoped: Effect.gen(function* () {
		const query = Effect.fn('Database.query')(function* (sql: string) {
			yield* Effect.annotateCurrentSpan({ sql });
			return { rows: [] };
		});
		return { query } as const;
	}),
}) {}

export class UserServiceError extends Schema.TaggedError<UserServiceError>('UserServiceError')({
	cause: Schema.optional(Schema.Defect),
}) {}

export class UserService extends Effect.Service<UserService>()('UserService', {
	dependencies: [Database.Default], // Database.Default is the auto-generated Layer
	scoped: Effect.gen(function* () {
		const database = yield* Database;
		const getAll = database.query('SELECT * FROM users').pipe(
			Effect.map((r) => r.rows),
			Effect.mapError((cause) => new UserServiceError({ cause })),
		);
		return { getAll } as const;
	}),
}) {}

Essential: there should be exactly one Effect.provide in an application, at the top level. Compose Layers via the Layer module instead.

Domain entities via Schema

import { Schema } from 'effect';

export const UserId = Schema.String.pipe(Schema.brand('UserId'));
export type UserId = (typeof UserId).Type;

export class User extends Schema.Class<User>('User')({
	id: UserId,
	name: Schema.String,
	email: Schema.String,
	createdAt: Schema.DateTimeUtc, // prefer DateTimeUtc
}) {}

export class UserError extends Schema.TaggedError<UserError>('UserError')({
	cause: Schema.optional(Schema.Defect),
	message: Schema.String,
}) {}

Observability

• Effect.withSpan('name') — attach a span
• Effect.fn('name')(fn) — function with span
• Effect.annotateCurrentSpan({ k: v }) — add attributes
• Effect.log / logInfo / logWarning / logError / logFatal / logDebug / logTrace

Testing with @effect/vitest

In v4 the primary runner is it.effect — it implicitly provides a Scope. it.live runs against the live clock; it.layer(layer)('group', (it) => {...}) shares a Layer across nested tests.

import { Effect } from 'effect';
import { describe, expect, it } from '@effect/vitest';

describe('My Effect tests', () => {
	it.effect('runs an Effect', () =>
		Effect.gen(function* () {
			const result = yield* Effect.succeed('Hello');
			expect(result).toBe('Hello');
		}),
	);

	it.effect('handles errors with Effect.flip', () =>
		Effect.gen(function* () {
			const error = yield* Effect.fail('boom').pipe(Effect.flip);
			expect(error).toBe('boom');
		}),
	);
});

• @effect/vitest re-exports all of vitest, so describe, expect, it, etc. come from it. There is no assert export — use expect.
• Never use Effect.runSync inside it; use it.effect.

When in doubt

Grep .repos/effect-v4/packages/effect/src/ (and .repos/effect-v4/packages/platform-node/src/ for the Node bindings). The cloned source is the source of truth — if missing, run pnpm setup:repos.