test-driven-development

name	test-driven-development
description	TDD: enforce RED-GREEN-REFACTOR cycle. Tests before code, always.

Test-Driven Development

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over. No exceptions.

RED-GREEN-REFACTOR Cycle

RED — Write Failing Test

Write one minimal test showing what should happen:

func TestRetriesFailedOperations(t *testing.T) {
    attempts := 0
    operation := func() error {
        attempts++
        if attempts < 3 {
            return errors.New("fail")
        }
        return nil
    }
    err := retryOperation(operation)
    if err != nil {
        t.Fatalf("expected success, got %v", err)
    }
    if attempts != 3 {
        t.Fatalf("expected 3 attempts, got %d", attempts)
    }
}

Use bash to run it:

go test -run TestRetriesFailedOperations ./path/

Expected: FAIL — function not defined.

GREEN — Minimal Implementation

Write the absolute minimum code to make the test pass:

func retryOperation(op func() error) error {
    var err error
    for i := 0; i < 3; i++ {
        err = op()
        if err == nil {
            return nil
        }
    }
    return err
}

Run again: go test -run TestRetriesFailedOperations ./path/ Expected: PASS

REFACTOR — Clean Up

Now that tests pass, improve the code:

• Remove duplication
• Improve names
• Add error wrapping if needed

Run tests after each refactor step.

Red Flags

If you find yourself thinking:

• "This is simple, I can write the code first" → Stop. Write the test.
• "I'll add tests later" → No. Now.
• "Testing this is hard" → The design might be wrong. Consider refactoring for testability.