elixir-tdd-enforcement

Elixir TDD Enforcement: The Iron Law

THE IRON LAW

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Not sometimes. Not usually. ALWAYS.

If you write production code before a failing test, DELETE IT and start over.

WHEN THIS SKILL APPLIES

• Implementing ANY new function
• Fixing ANY bug
• Adding ANY feature
• Modifying ANY behavior
• Refactoring ANY code

If you're changing .ex files in lib/, this skill is MANDATORY.

THE RED-GREEN-REFACTOR CYCLE

Phase 1: RED (Write Failing Test)

1. Write ONE minimal ExUnit test

   test "creates user with valid attrs" do
     attrs = %{name: "Alice", email: "[email protected]"}
     assert {:ok, %User{} = user} = Accounts.create_user(attrs)
     assert user.name == "Alice"
     assert user.email == "[email protected]"
   end
   

2. Run the test

   mix test test/my_app/accounts_test.exs:42
   

3. VERIFY IT FAILS FOR THE RIGHT REASON

   • Read the error message
   • Confirm it's failing because functionality doesn't exist
   • NOT because of syntax errors or wrong test setup

CHECKPOINT: If test doesn't fail, delete it and write a different test.

Phase 2: GREEN (Minimal Implementation)

1. Write SIMPLEST code to pass the test

   def create_user(attrs) do
     %User{}
     |> User.changeset(attrs)
     |> Repo.insert()
   end
   

2. Run the test again

   mix test test/my_app/accounts_test.exs:42
   

3. VERIFY IT PASSES

   • Read the actual output
   • See the green dot or "1 test, 0 failures"
   • NOT just assume it works

CHECKPOINT: If test doesn't pass, fix implementation (not test).

Phase 3: REFACTOR (Improve While Green)

1. Improve code quality

   • Extract functions
   • Improve names
   • Add pattern matching

2. Run tests after EACH change

   mix test
   

3. Stay GREEN

   • If tests fail during refactor, undo
   • Only refactor when all tests pass

CHECKPOINT: Tests must stay green throughout refactoring.