Test-Driven Development

Philosophy

Tests verify behavior through public interfaces, not implementation. Good tests survive refactors.

See principles.md for testing philosophy and anti-patterns.

Workflow

1. Planning

• Confirm interface design with user
• List behaviors to test (prioritize critical paths)
• Get approval before writing code

2. Tracer Bullet

RED:   Write first test → fails
GREEN: Minimal code to pass → passes

3. Incremental Loop

For each remaining behavior:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

• One test at a time
• Minimal code to pass
• No refactoring while RED

4. Refactor

Once all tests GREEN:

• Remove duplication
• Improve structure
• Tests must stay GREEN

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation.
DO use vertical slices: one test → one implementation → repeat.

See examples.md for workflow demonstrations.