Effect ts test integration with vitest

When to Apply

• You write tests for Effect.ts-based services with side effects (I/O, async, external services)
• You want tests to be deterministic and not flaky
• You're using Vitest as your test runner (not Jest)
• You need to stub external services (git, providers, filesystem)

Steps

1. Define your service layer as Layer.effect(TAG) that yields a { method1, method2 } interface
2. Create test doubles (stubs) that implement the same interface but return fixed/controlled values
3. In test, build a layer graph that includes test doubles instead of real implementations
4. Use Effect.gen to compose the test effect
5. Instead of sleep, use worker.drain() or stream.toArray() to wait for async work
6. Run effect via Effect.runPromise() or Vitest's Effect integration
7. Assert state after all async milestones complete

Example

const mockGitCore = Layer.succeed(GitCore, {
  executeGit: (cmd) => Effect.succeed({ stdout: '...' }),
  getStatus: () => Effect.succeed({ isRepo: true })
})

const testEffect = Effect.gen(function*() {
  const orchestration = yield* OrchestrationEngine
  const worker = yield* checkpointReactor

  yield* orchestration.dispatch({ type: 'thread.turn.start', ... })
  yield* worker.drain()  // wait for checkpoint work to finish

  const checkpoints = yield* orchestration.getCheckpoints(...)
  expect(checkpoints).toHaveLength(1)
})

const layers = Layer.compose(mockGitCore, OrchestrateLayer)
const result = yield* testEffect.pipe(Effect.provide(layers))

Counter / Caveats

• Layer composition order matters; dependencies must be available before use
• Test doubles can diverge from real implementations; keep in sync or use property-based tests
• drain() only works for DrainableWorker; other async patterns need Effect.sleep or Future
• Large test suites with many layers can slow down test startup; consider shared layer fixtures