tdd-kotlin

name	tdd-kotlin
description	TDD patterns for Kotlin CA/FP projects using Kotest + MockK + Arrow-kt. Use when writing tests, designing test cases, or following the red-green-refactor cycle. Triggers on: test file creation, writing value object tests, use case tests with mocked repositories, controller tests, property-based testing, coEvery/coVerify patterns.

TDD for Kotlin CA/FP

Test Stack

Library	Purpose
kotlin("test")	@Test annotation
kotest-assertions-core	shouldBe, shouldBeInstanceOf
kotest-assertions-arrow	shouldBeRight(), shouldBeLeft()
kotest-property	checkAll(Arb.xxx()) { }
mockk	mockk<T>(), coEvery, coVerify
kotlinx-coroutines-test	runTest { }

Layer Test Patterns

Layer	Approach
Domain	No mocks. checkAll for boundaries, shouldBeRight/shouldBeLeft for Either results
Application	mockk<Repository>(), coEvery/coVerify, runTest {}
Presentation	mockk<UseCase>(), test each HTTP status code via response.statusCode shouldBe ...
Infrastructure	Integration tests only — requires test containers or H2

Key Assertions

result.shouldBeRight(expectedValue)           // assert Right with specific value
result.shouldBeLeft().shouldBeInstanceOf<E>() // assert Left type
error.message shouldContain "expected text"   // assert error message content

coVerify(exactly = 1) { repo.save(any()) }   // verify call count
coVerify(exactly = 0) { repo.save(any()) }   // verify NOT called

Test Naming

@Test
fun `of returns Left InvalidId when value is zero or negative`() = runTest { ... }

@Test
fun `execute returns Left FetchFailed when repository throws exception`() = runTest { ... }

For complete test skeletons per layer (domain/application/presentation): see references/test-patterns.md.