SKILL.md - AiToEarn

SKILL.md

1	---
2	name: tdd
3	description: Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.
4	---
5
6	# Test-Driven Development
7
8	## Philosophy
9
10	Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.
11
12	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 checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.
13
14	Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.
15
16	See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines.
17
18	## Anti-Pattern: Horizontal Slices
19
20	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."
21
22	This produces crap tests:
23
24	- Tests written in bulk test _imagined_ behavior, not _actual_ behavior
25	- You end up testing the _shape_ of things (data structures, function signatures) rather than user-facing behavior
26	- Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
27	- You outrun your headlights, committing to test structure before understanding the implementation
28
29	Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.
30
31	```
32	WRONG (horizontal):
33	RED: test1, test2, test3, test4, test5
34	GREEN: impl1, impl2, impl3, impl4, impl5
35
36	RIGHT (vertical):
37	RED→GREEN: test1→impl1
38	RED→GREEN: test2→impl2
39	RED→GREEN: test3→impl3
40	...
41	```
42
43	## Workflow
44
45	### 1. Planning
46
47	When exploring the codebase, 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 touching.
48
49	Before writing any code:
50
51	- [ ] Confirm with user what interface changes are needed
52	- [ ] Confirm with user which behaviors to test (prioritize)
53	- [ ] Identify opportunities for [deep modules](deep-modules.md) (small interface, deep implementation)
54	- [ ] Design interfaces for [testability](interface-design.md)
55	- [ ] List the behaviors to test (not implementation steps)
56	- [ ] Get user approval on the plan
57
58	Ask: "What should the public interface look like? Which behaviors are most important to test?"
59
60	You can't 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.
61
62	### 2. Tracer Bullet
63
64	Write ONE test that confirms ONE thing about the system:
65
66	```
67	RED: Write test for first behavior → test fails
68	GREEN: Write minimal code to pass → test passes
69	```
70
71	This is your tracer bullet - proves the path works end-to-end.
72
73	### 3. Incremental Loop
74
75	For each remaining behavior:
76
77	```
78	RED: Write next test → fails
79	GREEN: Minimal code to pass → passes
80	```
81
82	Rules:
83
84	- One test at a time
85	- Only enough code to pass current test
86	- Don't anticipate future tests
87	- Keep tests focused on observable behavior
88
89	### 4. Refactor
90
91	After all tests pass, look for [refactor candidates](refactoring.md):
92
93	- [ ] Extract duplication
94	- [ ] Deepen modules (move complexity behind simple interfaces)
95	- [ ] Apply SOLID principles where natural
96	- [ ] Consider what new code reveals about existing code
97	- [ ] Run tests after each refactor step
98
99	Never refactor while RED. Get to GREEN first.
100
101	## Checklist Per Cycle
102
103	```
104	[ ] Test describes behavior, not implementation
105	[ ] Test uses public interface only
106	[ ] Test would survive internal refactor
107	[ ] Code is minimal for this test
108	[ ] No speculative features added
109	```
110

110 lines MARKDOWN