testing-hello-world.mdx

title	Your First Effect Test
id	testing-hello-world
skillLevel	beginner
applicationPatternId	testing
summary	Write your first test for an Effect program using Vitest and Effect's testing utilities.
tags	testing vitest getting-started
rule	description Use Effect.runPromise in tests to run and assert on Effect results.
author	PaulJPhilp
related	testing-with-services testing-mock-dependencies
lessonOrder	2

Guideline

Test Effect programs by running them with Effect.runPromise and using standard test assertions on the results.

---

Rationale

Testing Effect code is straightforward:

1. Effects are values - Build them in tests like any other value
2. Run to get results - Use Effect.runPromise to execute
3. Assert normally - Standard assertions work on the results

---

Good Example

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

// ============================================
// Code to test
// ============================================

const add = (a: number, b: number): Effect.Effect<number> =>
  Effect.succeed(a + b)

const divide = (a: number, b: number): Effect.Effect<number, Error> =>
  b === 0
    ? Effect.fail(new Error("Cannot divide by zero"))
    : Effect.succeed(a / b)

const fetchUser = (id: string): Effect.Effect<{ id: string; name: string }> =>
  Effect.succeed({ id, name: `User ${id}` })

// ============================================
// Tests
// ============================================

describe("Basic Effect Tests", () => {
  it("should add two numbers", async () => {
    const result = await Effect.runPromise(add(2, 3))
    expect(result).toBe(5)
  })

  it("should divide numbers", async () => {
    const result = await Effect.runPromise(divide(10, 2))
    expect(result).toBe(5)
  })

  it("should fail on divide by zero", async () => {
    await expect(Effect.runPromise(divide(10, 0))).rejects.toThrow(
      "Cannot divide by zero"
    )
  })

  it("should fetch a user", async () => {
    const user = await Effect.runPromise(fetchUser("123"))
    
    expect(user).toEqual({
      id: "123",
      name: "User 123",
    })
  })
})

// ============================================
// Testing Effect.gen programs
// ============================================

const calculateDiscount = (price: number, quantity: number) =>
  Effect.gen(function* () {
    if (price <= 0) {
      return yield* Effect.fail(new Error("Invalid price"))
    }
    
    const subtotal = price * quantity
    const discount = quantity >= 10 ? 0.1 : 0
    const total = subtotal * (1 - discount)
    
    return { subtotal, discount, total }
  })

describe("Effect.gen Tests", () => {
  it("should calculate without discount", async () => {
    const result = await Effect.runPromise(calculateDiscount(10, 5))
    
    expect(result.subtotal).toBe(50)
    expect(result.discount).toBe(0)
    expect(result.total).toBe(50)
  })

  it("should apply bulk discount", async () => {
    const result = await Effect.runPromise(calculateDiscount(10, 10))
    
    expect(result.subtotal).toBe(100)
    expect(result.discount).toBe(0.1)
    expect(result.total).toBe(90)
  })

  it("should fail for invalid price", async () => {
    await expect(
      Effect.runPromise(calculateDiscount(-5, 10))
    ).rejects.toThrow("Invalid price")
  })
})

Testing Patterns

Scenario	Approach
Success case	await Effect.runPromise(effect) then assert
Failure case	expect(...).rejects.toThrow()
Multiple effects	Test each independently
Effect.gen	Same as above - it's still an Effect

Best Practices

1. One assertion per test - Clear failure messages
2. Test success and failure - Both paths matter
3. Descriptive names - Explain what's being tested
4. Arrange-Act-Assert - Clear test structure