effect-layer-design

Layer Design Skill

Create layers that construct services while managing their dependencies cleanly.

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:

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

Layer Structure

import { Layer } from 'effect';

// Layer<RequirementsOut, Error, RequirementsIn>
//          ▲                ▲           ▲
//          │                │           └─ What this layer needs
//          │                └─ Errors during construction
//          └─ What this layer produces

Pattern: Simple Layer (No Dependencies)

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

interface ConfigData {
	readonly logLevel: string;
	readonly connection: string;
}

export class Config extends Context.Service<
	Config,
	{
		readonly getConfig: Effect.Effect<ConfigData>;
	}
>()('Config') {}

// Layer<Config, never, never>
//         ▲      ▲      ▲
//         │      │      └─ No dependencies
//         │      └─ Cannot fail
//         └─ Produces Config
export const ConfigLive = Layer.succeed(
	Config,
	Config.of({
		getConfig: Effect.succeed({
			logLevel: 'INFO',
			connection: 'mysql://localhost/db'
		})
	})
);

Pattern: Layer with Dependencies

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

interface ConfigData {
	readonly logLevel: string;
	readonly connection: string;
}

export class Config extends Context.Service<
	Config,
	{
		readonly getConfig: Effect.Effect<ConfigData>;
	}
>()('Config') {}

export class Logger extends Context.Service<
	Logger,
	{
		readonly log: (message: string) => Effect.Effect<void>;
	}
>()('Logger') {}

// Layer<Logger, never, Config>
//         ▲      ▲      ▲
//         │      │      └─ Needs Config
//         │      └─ Cannot fail
//         └─ Produces Logger
export const LoggerLive = Layer.effect(
	Logger,
	Effect.gen(function* () {
		const config = yield* Config; // Access dependency
		return Logger.of({
			log: (message) =>
				Effect.gen(function* () {
					const { logLevel } = yield* config.getConfig;
					yield* Console.log(`[${logLevel}] ${message}`);
				})
		});
	})
);