effect-testing

# Effect Testing

Master testing Effect applications with test utilities, mock layers, and
patterns for testing effectful code. This skill covers unit testing, integration
testing, and testing concurrent and resource-managed code.

## Basic Effect Testing

### Testing with Effect.gen

```typescript
import { Effect } from "effect"
import { describe, it, expect } from "vitest"

describe("User Service", () => {
  it("should fetch user by ID", async () => {
    const program = Effect.gen(function* () {
      const user = yield* fetchUser("123")
      return user
    })

    const result = await Effect.runPromise(program.pipe(
      Effect.provide(TestLayer)
    ))

    expect(result.id).toBe("123")
    expect(result.name).toBe("Alice")
  })
})
```

### Testing Success and Failure

```typescript
import { Effect, Exit } from "effect"
import { describe, it, expect } from "vitest"

describe("Validation", () => {
  it("should succeed with valid email", async () => {
    const program = validateEmail("alice@example.com")

    const result = await Effect.runPromise(program)

    expect(result).toBe("alice@example.com")
  })

  it("should fail with invalid email", async () => {
    const program = validateEmail("invalid")

    const exit = await Effect.runPromiseExit(program)

    expect(Exit.isFailure(exit)).toBe(true)

    if (Exit.isFailure(exit)) {
      const error = Cause.failureOption(exit.cause)
      expect(error._tag).toBe("ValidationError")
    }
  })
})
```

## Mock Layers for Testing

### Creating Test Layers

```typescript
import { Context, Effect, Layer } from "effect"

interface UserRepository {
  findById: (id: string) => Effect.Effect<Option<User>, DbError, never>
  save: (user: User) => Effect.Effect<User, DbError, never>
}

const UserRepository = Context.GenericTag<UserRepository>("UserRepository")

// In-memory test implementation
const UserRepositoryTest = Layer.succeed(
  UserRepository,
  {
    findById: (id: string) =>
      Effect.succeed(
        id =