Elixir TDD Enforcement: The Iron Law

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.

VERIFICATION CHECKLIST

Before claiming you're done, verify:

• I wrote the test BEFORE any implementation code
• I watched the test FAIL for the right reason
• I read the actual failure message
• I implemented only enough code to pass the test
• I ran the test again and saw it PASS
• I read the actual success message
• All other tests still pass
• I refactored only while tests were green

If you can't check ALL boxes, you didn't follow TDD.

COMMON VIOLATIONS AND RESPONSES

Violation: "I'll just write the code, then write the test"

Response: NO. Delete the code. Write test first.

Violation: "The function is simple, I don't need to see it fail"

Response: WRONG. Even simple code needs failing tests. Write test, watch fail.

Violation: "I already know what the test will look like"

Response: Irrelevant. Write it first anyway.

Violation: "I wrote the test and implementation together"

Response: Delete both. Write test, watch fail, then implement.

Violation: "The test passed on first run"

Response: RED FLAG. Test might not be testing anything. Review test.

Violation: "I'm just refactoring, I don't need new tests"

Response: Correct - but ALL existing tests must stay GREEN.

ELIXIR-SPECIFIC TEST PATTERNS

Testing Context Functions

# RED: Write test first
test "list_users/​0 returns all users" do
  user1 = fixture(:user)
  user2 = fixture(:user)
  users = Accounts.list_users()
  assert length(users) == 2
  assert user1 in users
  assert user2 in users
end

# Run test → watch it fail (function doesn't exist)

# GREEN: Implement
def list_users do
  Repo.all(User)
end

# Run test → watch it pass

Testing Changesets

# RED: Write test for validation
test "changeset with invalid email" do
  changeset = User.changeset(%User{}, %{email: "invalid"})
  refute changeset.valid?
  assert %{email: ["invalid format"]} = errors_on(changeset)
end

# Run test → watch it fail

# GREEN: Add validation
def changeset(user, attrs) do
  user
  |> cast(attrs, [:email])
  |> validate_format(:email, ~r/​@/)
end

Testing Phoenix Controllers

# RED: Write test
test "GET /​users returns 200", %{conn: conn} do
  conn = get(conn, ~p"/​users")
  assert html_response(conn, 200)
end

# Run test → watch it fail (route doesn't exist)

# GREEN: Add route and controller action

DIALYZER ERRORS: SPECIAL CASE

If Dialyzer reports an error:

1. Write a test that exercises the problematic code
2. Make sure test passes (proving code works)
3. Add @spec to guide Dialyzer
4. Run mix dialyzer to verify

NEVER:

• Add to dialyzer.ignore
• Modify dialyzer PLT to suppress
• Comment out the code

The test proves it works. The spec helps Dialyzer understand.

CREDO WARNINGS: SPECIAL CASE

If Credo reports a warning:

1. Understand WHY it's warning
2. Fix the actual issue (complexity, style, etc.)
3. Run mix credo to verify

NEVER:

• Add to .credo.exs disabled list
• Use inline # credo:disable-for-this-file
• Ignore the warning

Credo is helping you write better code. Listen to it.

THE DISCIPLINE

TDD feels slow at first. That's because you're used to:

• Writing code fast (then debugging for hours)
• Skipping tests (then breaking things in production)
• Guessing if it works (then finding out it doesn't)

TDD is actually faster because:

• Tests catch bugs immediately
• You know exactly what to implement
• Refactoring is safe
• Code works the first time

ENFORCEMENT

Before writing ANY Elixir production code, ask:

1. "Have I written a failing test for this?"
2. "Have I actually RUN the test and seen it fail?"
3. "Do I know WHY it's failing?"

If any answer is NO → write the test first.

REMEMBER

"Tests that pass on the first run might not be testing anything."

"Code without a failing test first is guess-driven development."

"TDD is slow. Debugging untested code is slower."

THE RULE

RED → GREEN → REFACTOR

Not GREEN → RED → "oops"

Not WRITE → PRAY → DEBUG

RED → GREEN → REFACTOR

Every. Single. Time.