Testing

Testing is one of the main reasons to keep Services and Layers separate. The Service contains behavior; the test Layer supplies controlled implementations.

Use the memory backend

import { MemoryLayerBackend } from 'better-effect/testing'

const backend = new MemoryLayerBackend()
const runtime = await Runtime.make(AppTest, backend)

const result = await runtime.run(() => applicationProgram)

await runtime.dispose()

The backend is lazy, caches by Service tag, and clears its state during disposeAll.

Override one provider

const DatabaseTest = Layer.succeed(
  Database,
  Database.of({
    findById: async () => Result.ok({ id: 'test-user' })
  })
)

const AppTest = Layer.override(AppLive, DatabaseTest)

Only the explicit replacement changes; other production providers remain in the environment. If the fake's contract is incompatible with the production Service, TypeScript reports an override collision instead of letting an invalid provider cross the Runtime boundary.

Resolver context isolation

ServiceRuntime uses AsyncLocalStorage, so tests should enter context through ServiceRuntime.run or a Runtime boundary:

const result = await runtime.run(() =>
  Effect.gen(async function* () {
    const repository = yield* UserRepository
    return yield* Result.await(repository.findById('test-user'))
  })
)

Concurrent Runtime executions remain isolated. Avoid global “reset the container” helpers that create serial-test coupling.

Type-contract tests

Type inference is part of the API. Use expectTypeOf in tests/types:

const program = Effect.gen(function* () {
  const service = yield* AuthService
  return Result.ok(service)
})

expectTypeOf(service).toEqualTypeOf<AuthService>()
expectTypeOf<EffectRequirements<typeof program>>().toEqualTypeOf<AuthService>()

Protect contracts such as:

• yield* AuthService infers exactly AuthService;
• ServiceRuntime.resolve(AuthService) resolves to AuthService;
• Layer.merge rejects duplicate tags;
• Runtime.run rejects missing program requirements;
• Layer.override rejects incompatible same-tag contracts;
• Effect.add accepts only disposable values.

Test cleanup behavior

Test both the happy path and the boundary guarantees:

Recommended test layout

Runtime tests belong in tests/*.test.ts; compile-time contracts belong in tests/types/. Use Bun's bun:test runner.