Agent skill

effect-testing-mocking

Testing patterns with layers, mocks, and deterministic time. Use when preparing testable services and small smoke tests.

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Install this agent skill to your Project

npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/effect-testing-mocking

SKILL.md

Testing & Mocking

When to use

  • You need deterministic tests without external IO
  • You want to inject mock services via layers

Mock Layer

ts
const RepoTest = Layer.succeed(Repo, Repo.of({ find: () => Effect.succeed(mock) }))

Provide Test Impl

ts
const result = yield* Effect.provide(program, RepoTest)

Test Clock

ts
// Provide a deterministic Clock or use platform TestClock layer if available

Guidance

  • Keep tests focused and fast; avoid network by mocking services
  • Provide layers explicitly to match program requirements
  • Keep tests close to real interfaces; do not reshape app code for tests
  • Use .Default layers for quick wiring; add custom mock layers per test
  • Favor small smoke tests that run quickly and fail fast

Pitfalls

  • Leaking real IO into tests → flakiness
  • Over-mocking internals → brittle tests; mock at service boundary

Cross-links

  • Layers & Services for DI patterns
  • Time/Logging for deterministic time and observable output

Local Source Reference

CRITICAL: Search local Effect source before implementing

The full Effect source code is available at docs/effect-source/. Always search the actual implementation before writing Effect code.

Key Source Files

  • Layer: docs/effect-source/effect/src/Layer.ts
  • TestClock: docs/effect-source/effect/src/TestClock.ts
  • TestContext: docs/effect-source/effect/src/TestContext.ts

Example Searches

bash
# Find Layer.succeed patterns for mocks
grep -F "Layer.succeed" docs/effect-source/effect/src/Layer.ts

# Study TestClock operations
grep -F "TestClock" docs/effect-source/effect/src/TestClock.ts

# Find test helpers
grep -F "TestContext" docs/effect-source/effect/src/TestContext.ts

# Look at Layer test examples
grep -F "Layer.succeed" docs/effect-source/effect/test/Layer.test.ts

Workflow

  1. Identify the testing API you need (e.g., Layer.succeed, TestClock)
  2. Search docs/effect-source/effect/src/Layer.ts for the implementation
  3. Study the types and testing patterns
  4. Look at test files for usage examples
  5. Write your code based on real implementations

Real source code > documentation > assumptions

Real-world snippet: Build a comprehensive Test layer

ts
export const TestLayer = (input?: TestLiveInput) =>
  Effect.gen(function* () {
    const tempDir = tempy.temporaryDirectory({ prefix: 'test' })
    const cwd = (yield* setupFixtureFolder({ fixture: input?.fixture, tempDir })) ?? tempDir

    const NodeOsTest = Layer.succeed(NodeOs, new NodeOs({ homedir: cwd, arch: 'arm64', platform: 'darwin' }))
    const NodeProcessTest = Layer.succeed(NodeProcess, new NodeProcess({ cwd, platform: 'darwin', arch: 'arm64' }))

    const ToolkitsRepoTest = Layer.succeed(ComposioToolkitsRepository, new ComposioToolkitsRepository({
      getToolkits: () => Effect.succeed([]),
      getToolsAsEnums: () => Effect.succeed([])
    }))

    const layers = Layer.mergeAll(
      Console.setConsole(yield* MockConsole.effect),
      CliConfig.layer(ComposioCliConfig),
      NodeProcessTest,
      Layer.provideMerge(ComposioUserContextLive, Layer.merge(BunFileSystem.layer, NodeOsTest)),
      ToolkitsRepoTest,
      EnvLangDetector.Default,
      JsPackageManagerDetector.Default,
      BunFileSystem.layer,
      BunContext.layer,
      MockTerminal.layer,
      BunPath.layer
    )

    return layers
  }).pipe(
    Logger.withMinimumLogLevel(LogLevel.Debug),
    Effect.scoped,
    Layer.unwrapEffect,
    Layer.provide(Layer.setConfigProvider(input?.baseConfigProvider ?? ConfigProvider.fromMap(new Map([]))))
  )

References

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