tdd

Files (11)

• SKILL.md
• deep-modules.md
• design-de-interface.md
• interface-design.md
• mocking.md
• mocks.md
• modulos-profundos.md
• refactoring.md
• refatoracao.md
• testes.md
• tests.md

SKILL.md (first lines)

# Test-Driven Development

## Philosophy

**Core principle**: Tests must verify behavior through public interfaces, not implementation details. Code can change completely; tests should not.

**Good tests** are integration-style: they exercise real code paths through public APIs. They describe *what* the system does, not *how* it does it. A good test reads like a specification — "user can check out with a valid cart" says exactly what capability exists. These tests survive refactoring because they don't care about internal structure.

**Bad tests** are coupled to implementation. They mock internal collaborators, test private methods, or verify via external means (like querying a database directly instead of using the interface). The red flag: your test breaks when you refactor, but the behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.

See [tests.md](tests.md) for examples and [mocks.md](mocks.md) for mocking guidelines.

## Anti-Pattern: Horizontal Slicing

**DO NOT write all tests first, then all implementation.** This is "horizontal slicing" — treating RED as "write all tests" and GREEN as "write all code."

This produces **bad tests**:

- Tests written in batch test *imagined* behavior, not *actual* behavior
- You end up testing the *shape* of things (data structures, function signatures) instead of user-facing behavior
- Tests become insensitive to real changes — they pass when behavior breaks, fail when behavior is correct
- You outrun your headlights, committing to test structure before understanding implementation

**Correct approach**: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Since you just wrote the code, you know exactly which behavior matters and how to verify it.

```
WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...
```

## Workflow

### 1. Planning

When exploring code, use the project's domain glossary so that test names and interface vocabulary match the project's language, and respect ADRs in the area you're working in.

Before writing any code:

- [ ] Confirm with the user which interface changes are needed
- [ ] Confirm with the user which behaviors to test (prioritize)
- [ ] Identify opportunities for [deep modules](modulos-profundos.md) (small interface, deep implementation)
- [ ] Design interfaces for [testability](design-de-interface.md)
- [ ] List the behaviors to test (not implementation steps)
- [ ] Get user approval on the plan

Ask: "What should the public interface look like? Which behaviors are most important to test?"

**You cannot test everything.** Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.

### 2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

```
RED:   Write test for the first behavior → test fails
GREEN: Write minimal code to pass → test passes
```

This is your tracer bullet — it proves the path works end-to-end.

### 3. Incremental Loop

For each remaining behavior:

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

Rules:

- One test at a time
- Only enough code to pass the current test
- Do not anticipate future tests
- Keep tests focused on observable behavior

### 4. Refactor

After all tests pass, look for [refactoring candidates](refatoracao.md):

- [ ] Extract duplication
- [ ] Deepen modules (move complexity behind simple interfaces)
- [ ] Apply SOLID principles where natural
- [ ] Consider what the new code reveals about existing code
- [ ] Run tests after each refactoring step

**Never refactor during RED.** Get to GREEN first.

## Per-Cycle Checklist

```
[ ] Test describes behavior, not implementation
[ ] Test uses only public interface
[ ] Test would survive internal refactoring
[ ] Code is minimal for this test
[ ] No speculative functionality added
```

From hcb2019/claude-code-skills, MIT.

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