effect-service-implementation

Service Implementation Skill

Design and implement Effect services as focused capabilities that compose into complete solutions.

Effect Source Reference

The Effect v4 source is available at ~/.cache/effect-v4/. Browse and read files there directly to look up APIs, types, and implementations.

Reference this for:

• Context source: packages/effect/src/Context.ts
• Layer source: packages/effect/src/Layer.ts
• Migration guide: MIGRATION.md
• Effect source: packages/effect/src/

Service Declaration: Namespace-Module Pattern

The recommended pattern organizes each service as a TypeScript namespace containing: an Interface type, a Service class (pure tag/identifier), and layer / defaultLayer exports. The class body is always empty — construction logic lives in Layer.effect, not in make: or class statics.

The main payoff is a visible service graph. Prefer patterns that make dependencies obvious at yield* call sites and in defaultLayer composition over patterns that keep compatibility shims or helper modules in the foreground.

import { Effect, Layer, Schema, Context } from 'effect';

class UserNotFound extends Schema.TaggedErrorClass<UserNotFound>()(
	'UserNotFound',
	{
		userId: Schema.String,
		message: Schema.String
	}
) {}

export namespace UserRepository {
	export interface Interface {
		readonly findById: (id: string) => Effect.Effect<User, UserNotFound>;
		readonly create: (data: CreateUserData) => Effect.Effect<User>;
	}

	export class Service extends Context.Service<Service, Interface>()(
		'@app/UserRepository'
	) {}

	export const layer = Layer.effect(
		Service,
		Effect.gen(function* () {
			const db = yield* DatabaseClient.Service;

			const findById = Effect.fn('UserRepository.findById')(function* (
				id: string
			) {
				const row = yield* db.query(
					'SELECT * FROM users WHERE id = ?',
					id
				);
				if (!row)
					return yield* new UserNotFound({
						userId: id,
						message: `User ${id} not found`
					});
				return row as User;
			});

			const create = Effect.fn('UserRepository.create')(function* (
				data: CreateUserData
			) {
				return yield* db.insert('users', data);
			});

			return Service.of({ findById, create });
		})
	);

	export const defaultLayer = layer.pipe(
		Layer.provide(DatabaseClient.defaultLayer)
	);
}