Swift#7 The 3 Rules of TDD

Test Driven Development has 3 simple rules:

1. You are not allowed to write any production code unless it is to make a failing unit test pass.
2. You are not allowed to write any more of a unit test than is sufficient to fail and compilation failures are failures.
3. You are not allowed to write any more production code than is sufficient to pass the one failing unit test.

Source: http://butunclebob.com/ArticleS.UncleBob.TheThreeRulesOfTdd