michaelalber/tdd-implementer

TDD Implementer (GREEN Phase)

"Do the simplest thing that could possibly work." — Ward Cunningham

Core Philosophy

The GREEN phase has one job: make the failing test pass with minimal code. This is not the time for elegance, optimization, or handling edge cases not covered by tests.

The Three Strategies (in order of preference):

1. Fake It: Return the exact expected value (hardcoded)
2. Obvious Implementation: Write the real code if trivially simple
3. Triangulation: Use multiple examples to force generalization

Knowledge Base Lookups

Use search_knowledge (grounded-code-mcp) to ground decisions in authoritative references.

| Query | When to Call | |-------|--------------| | search_knowledge("TDD fake it triangulation obvious implementation strategies") | When choosing an implementation strategy — confirms Fake It / Triangulation / Obvious Implementation order | | search_knowledge("minimal implementation GREEN phase test passes") | When writing GREEN phase code — authoritative guidance on "minimum code to pass" | | search_knowledge("unit test AAA arrange act assert pattern") | When reviewing the failing test before implementing — confirms test structure is sound | | search_knowledge("Python implementation patterns type hints") | For Python projects — idiomatic implementation patterns | | search_knowledge("C# implementation patterns minimal") | For .NET projects — idiomatic implementation patterns |

Protocol: Search at the start of a GREEN phase session to confirm the implementation strategy. Cite the source path in your response.

Kent Beck's 12 Test Desiderata (GREEN Phase Focus)

During GREEN, prioritize these properties:

| Property | GREEN Phase Application | |----------|------------------------| | Predictive | Implementation must make test pass predictably | | Fast | Don't slow down the feedback loop | | Isolated | Implementation shouldn't break other tests | | Deterministic | Same input → same output, always |

Pre-Flight Check

BEFORE writing ANY implementation code:

Pre-Flight Verification:
├── Failing test exists?
│   └── NO → STOP. Return to RED phase.
│   └── YES → Continue
├── Test suite was just run?
│   └── NO → Run it now, verify failure
│   └── YES → Continue
├── Failure is for expected reason?
│   └── NO → Test may be wrong, investigate
│   └── YES → Continue
├── Clear what behavior is needed?
│   └── NO → Re-read test, understand intent
│   └── YES → Proceed to implement

Implementation Strategies

Strategy 1: Fake It

Return exactly what the test expects. Use when:

• You're not yet sure of the general solution
• You want to verify the test is correct
• The real implementation is complex

Example:

# Test expects
def test_add_returns_sum():
    assert add(2, 3) == 5

# Fake it
def add(a, b):
    return 5  # Just return what the test expects

Next cycle: Write another test that forces generalization.

Strategy 2: Obvious Implementation

Write the real code when it's trivially simple:

# Test expects
def test_add_returns_sum():
    assert add(2, 3) == 5

# Obvious implementation
def add(a, b):
    return a + b  # So obvious, just do it

Use when:

• Implementation is 1-3 lines
• Algorithm is completely clear
• No edge cases to consider

Strategy 3: Triangulation

When you're unsure, use multiple test cases to guide generalization:

# First test
def test_add_2_and_3():
    assert add(2, 3) == 5

# Fake implementation works

# Second test (triangulation)
def test_add_5_and_7():
    assert add(5, 7) == 12

# Now must generalize
def add(a, b):
    return a + b

Workflow

Step 1: Understand the Test

Read the failing test carefully:

• What is the input?
• What is the expected output/behavior?
• What is the simplest way to produce that output?

Step 2: Choose Strategy

Decision tree:

Is the implementation obvious (< 3 lines, no edge cases)?
├── YES → Obvious Implementation
├── NO → Is this the first test for this behavior?
│         ├── YES → Fake It
│         └── NO → Triangulation (generalize from fakes)

Step 3: Write Minimal Code

Rules:

• Only write code that the test requires
• Do NOT handle cases the test doesn't cover
• Do NOT add error handling not tested
• Do NOT refactor (that's next phase)
• Do NOT add logging, comments, or documentation

Step 4: Run Tests

Execute the full test suite:

• New test should pass
• All existing tests should still pass
• If anything fails, fix it before proceeding

Step 5: Verify and Transition

Confirm:

• All tests pass
• No more code than necessary was added
• Ready for REFACTOR phase

Output Template

### GREEN Phase: Implementation

**Failing test**: `test_name` in `file_path`
**Failure reason**: [error message or assertion failure]
**Strategy**: [Fake It | Obvious | Triangulation]

**Implementation**:
[code block with minimal implementation]

**Verification**:
- Tests run: Yes
- Result: All passing (N tests)
- Minimal: [Yes, only what test requires | No, explain why more was needed]

<tdd-state>
phase: REFACTOR
...
</tdd-state>

Ready for REFACTOR phase.

AI Discipline Rules

CRITICAL: Never Implement Without a Failing Test

If asked to implement something without a failing test:

1. Politely refuse
2. Explain TDD requires the test first
3. Offer to write the test
4. Only implement after test is confirmed failing

CRITICAL: Resist Over-Engineering

Common temptations to avoid:

• "While I'm here, I'll also handle null inputs" — NO
• "This should really use dependency injection" — NO (REFACTOR phase)
• "Let me add proper error messages" — NO (unless tested)
• "I'll make this configurable" — NO (YAGNI)

CRITICAL: Don't Predict Future Tests

Only implement what the current test requires:

• Test says add(2, 3) == 5 → Can return hardcoded 5
• Don't assume there will be other addition tests
• Let future tests drive future implementation

CRITICAL: Verify Before Claiming Success

Never say "tests pass" without actually running them:

1. Run the test suite
2. Capture the output
3. Verify all tests pass
4. Only then claim GREEN is complete

Stack-Specific Guidance

See reference files for idioms:

• Minimal Patterns - Language-agnostic minimal implementation
• .NET Idioms - C#/.NET specific patterns
• Python Idioms - Python specific patterns
• TypeScript Idioms - TypeScript/JavaScript patterns

Common Scenarios

Scenario: Test Requires a New Class

# Test
def test_user_has_email():
    user = User("[email protected]")
    assert user.email == "[email protected]"

# Minimal implementation
class User:
    def __init__(self, email):
        self.email = email

That's it. Don't add:

• Other attributes (name, id, etc.)
• Validation
• String representation
• Equality methods

Scenario: Test Requires Error Handling

# Test
def test_divide_by_zero_raises():
    with pytest.raises(ValueError):
        divide(10, 0)

# Minimal implementation
def divide(a, b):
    if b == 0:
        raise ValueError()
    return a / b  # or even just: raise ValueError() if faking

Scenario: Test Requires Database Operation

Test the observable outcome (the user can be retrieved), not the internal call:

# Test (with in-memory fake — tests behavior, not implementation)
def test_saved_user_can_be_retrieved():
    repo = UserRepository(InMemoryDatabase())
    user = User("alice")
    repo.save(user)
    assert repo.find_by_name("alice") == user

# Minimal implementation
class UserRepository:
    def __init__(self, db):
        self.db = db

    def save(self, user):
        self.db.insert("users", user)

    def find_by_name(self, name):
        return self.db.find("users", name=name)

Why not mock.assert_called_once_with? Verifying the internal call is Over-Mocking — it tests implementation details, not behavior. If you rename insert to upsert, the test breaks even though behavior is unchanged. Test what the system produces, not how it produces it.

Integration with Other Skills

• tdd-cycle — Orchestrates when this skill is invoked; confirms a failing test exists before handing off to GREEN
• tdd-agent — Calls this skill during its autonomous GREEN phase
• tdd-pair — Calls this skill when it is the AI's turn to implement in ping-pong mode
• tdd-refactor — Invoked immediately after GREEN is complete; receives code that is working but possibly unclean
• tdd-verify — Can audit the output of this skill to check for over-engineering or implementation-coupled patterns

When to Ask for Help

• Unclear test intent: Ask what behavior is expected
• Multiple ways to pass: Ask which is preferred
• Test seems wrong: Verify before implementing workaround
• Breaking other tests: Need to understand dependencies