test-driven-development

name	test-driven-development
description	Use when implementing any feature or bug fix. Defines the red-green-refactor cycle and the rule that no production code may be written without a failing test first. Load before starting any implementation work.

Test-Driven Development

Write the test first. Watch it fail. Write minimal code to pass. Refactor.

The cycle

RED   → write one failing test
         ↓ confirm it fails for the right reason
GREEN → write minimal code to pass
         ↓ confirm all tests pass
REFACTOR → clean up, no new behaviour
         ↓ confirm still green
         → next test

Red

Write one test covering one behaviour. The test must fail before any implementation exists. If it passes immediately, it is not testing anything new.

class CreateHandlerTest : FunSpec({
    test("returns Left(AuthorizationError) when party does not match model") {
        val repo = mockk<RequestRepository>()
        val partyService = mockk<PartyService>()

        coEvery { partyService.resolve(any()) } returns mismatchedParty.right()

        val result = CreateHandler(repo, partyService).invoke(validModel)

        result.shouldBeLeft(CreateError.AuthorizationError)
    }
})

Run the test. Confirm it fails because the feature doesn't exist, not because of a compilation error or wrong setup.

Green

Write the minimum code to make the test pass. Do not add behaviour the test doesn't require.

Refactor

Clean up duplication and naming. Do not change behaviour. All tests must remain green throughout.

Hard rules

• No production code without a failing test first. If you wrote code before the test, delete it and start from the test.
• One test per behaviour. If the test name contains "and", split it.
• suspend functions are mocked with coEvery, not every.
• Mocks are reset in beforeTest — see the Testing skill.

Bug fixes

Before fixing a bug, write a test that reproduces it. The test must fail. Then fix. The test is the regression guard.

What counts as a complete implementation

• Every new function or method has a test that was written first
• Each test was seen to fail before the implementation existed
• All tests pass
• No test-only methods exist on production classes