From 2011bff9f215a72065b72ce3a730006d19fc6105 Mon Sep 17 00:00:00 2001 From: bdbch Date: Thu, 22 Oct 2026 20:55:30 +0200 Subject: [PATCH] Add /tdd and /codebase-design --- skills/codebase-design/SKILL.md | 114 ++++++++++++++++++ skills/codebase-design/files/DEEPENING.md | 37 ++++++ .../codebase-design/files/DESIGN-IT-TWICE.md | 44 +++++++ skills/tdd/SKILL.md | 39 ++++++ skills/tdd/files/mocking.md | 59 +++++++++ skills/tdd/files/tests.md | 77 ++++++++++++ 6 files changed, 370 insertions(+) create mode 100644 skills/codebase-design/SKILL.md create mode 100644 skills/codebase-design/files/DEEPENING.md create mode 100644 skills/codebase-design/files/DESIGN-IT-TWICE.md create mode 100644 skills/tdd/SKILL.md create mode 100644 skills/tdd/files/mocking.md create mode 100644 skills/tdd/files/tests.md diff --git a/skills/codebase-design/SKILL.md b/skills/codebase-design/SKILL.md new file mode 100644 index 0000000..a0b9f6d --- /dev/null +++ b/skills/codebase-design/SKILL.md @@ -0,0 +1,114 @@ +--- +name: codebase-design +description: Shared vocabulary for designing deep modules. Use when the user wants to design or improve a module's interface, find deepening opportunities, decide where a seam goes, make code more testable or AI-navigable, or when another skill needs the deep-module vocabulary. +--- + +# Codebase Design + +Design **deep modules**: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. Use this language and these principles wherever code is being designed or restructured. The aim is leverage for callers, locality for maintainers, and testability for everyone. + +## Glossary + +Use these terms exactly: don't substitute "component," "service," "API," or "boundary." Consistent language is the whole point. + +**Module**: anything with an interface and an implementation. Deliberately scale-agnostic: a function, class, package, or tier-spanning slice. _Avoid_: unit, component, service. + +**Interface**: everything a caller must know to use the module correctly: the type signature, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics. _Avoid_: API, signature (too narrow, they refer only to the type-level surface). + +**Implementation**: what's inside a module, its body of code. Distinct from **Adapter**: a thing can be a small adapter with a large implementation (a Postgres repo) or a large adapter with a small implementation (an in-memory fake). Reach for "adapter" when the seam is the topic; "implementation" otherwise. + +**Depth**: leverage at the interface. The amount of behaviour a caller (or test) can exercise per unit of interface they have to learn. A module is **deep** when a large amount of behaviour sits behind a small interface, **shallow** when the interface is nearly as complex as the implementation. + +**Seam** _(Michael Feathers)_: a place where you can alter behaviour without editing in that place; the *location* at which a module's interface lives. Where to put the seam is its own design decision, distinct from what goes behind it. _Avoid_: boundary (overloaded with DDD's bounded context). + +**Adapter**: a concrete thing that satisfies an interface at a seam. Describes *role* (what slot it fills), not substance (what's inside). + +**Leverage**: what callers get from depth. More capability per unit of interface they learn. One implementation pays back across N call sites and M tests. + +**Locality**: what maintainers get from depth. Change, bugs, knowledge, and verification concentrate in one place rather than spreading across callers. Fix once, fixed everywhere. + +## Deep vs shallow + +**Deep module** = small interface + lots of implementation: + +``` +┌─────────────────────┐ +│ Small Interface │ ← Few methods, simple params +├─────────────────────┤ +│ │ +│ Deep Implementation│ ← Complex logic hidden +│ │ +└─────────────────────┘ +``` + +**Shallow module** = large interface + little implementation (avoid): + +``` +┌─────────────────────────────────┐ +│ Large Interface │ ← Many methods, complex params +├─────────────────────────────────┤ +│ Thin Implementation │ ← Just passes through +└─────────────────────────────────┘ +``` + +When designing an interface, ask: + +- Can I reduce the number of methods? +- Can I simplify the parameters? +- Can I hide more complexity inside? + +## Principles + +- **Depth is a property of the interface, not the implementation.** A deep module can be internally composed of small, mockable, swappable parts; they just aren't part of the interface. A module can have **internal seams** (private to its implementation, used by its own tests) as well as the **external seam** at its interface. +- **The deletion test.** Imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep. +- **The interface is the test surface.** Callers and tests cross the same seam. If you want to test *past* the interface, the module is probably the wrong shape. +- **One adapter means a hypothetical seam. Two adapters means a real one.** Don't introduce a seam unless something actually varies across it. + +## Designing for testability + +Good interfaces make testing natural: + +1. **Accept dependencies, don't create them.** + + ```typescript + // Testable + function processOrder(order, paymentGateway) {} + + // Hard to test + function processOrder(order) { + const gateway = new StripeGateway(); + } + ``` + +2. **Return results, don't produce side effects.** + + ```typescript + // Testable + function calculateDiscount(cart): Discount {} + + // Hard to test + function applyDiscount(cart): void { + cart.total -= discount; + } + ``` + +3. **Small surface area.** Fewer methods = fewer tests needed. Fewer params = simpler test setup. + +## Relationships + +- A **Module** has exactly one **Interface** (the surface it presents to callers and tests). +- **Depth** is a property of a **Module**, measured against its **Interface**. +- A **Seam** is where a **Module**'s **Interface** lives. +- An **Adapter** sits at a **Seam** and satisfies the **Interface**. +- **Depth** produces **Leverage** for callers and **Locality** for maintainers. + +## Rejected framings + +- **Depth as ratio of implementation-lines to interface-lines** (Ousterhout): rewards padding the implementation. We use depth-as-leverage instead. +- **"Interface" as the TypeScript `interface` keyword or a class's public methods**: too narrow: interface here includes every fact a caller must know. +- **"Boundary"**: overloaded with DDD's bounded context. Say **seam** or **interface**. + +## Going deeper + +- **Deepening a cluster given its dependencies**, see [files/DEEPENING.md](files/DEEPENING.md): dependency categories, seam discipline, and replace-don't-layer testing. +- **Exploring alternative interfaces**, see [files/DESIGN-IT-TWICE.md](files/DESIGN-IT-TWICE.md): spin up parallel sub-agents to design the interface several radically different ways, then compare on depth, locality, and seam placement. diff --git a/skills/codebase-design/files/DEEPENING.md b/skills/codebase-design/files/DEEPENING.md new file mode 100644 index 0000000..ed0c0e4 --- /dev/null +++ b/skills/codebase-design/files/DEEPENING.md @@ -0,0 +1,37 @@ +# Deepening + +How to deepen a cluster of shallow modules safely, given its dependencies. Assumes the vocabulary in [SKILL.md](../SKILL.md): **module**, **interface**, **seam**, **adapter**. + +## Dependency categories + +When assessing a candidate for deepening, classify its dependencies. The category determines how the deepened module is tested across its seam. + +### 1. In-process + +Pure computation, in-memory state, no I/O. Always deepenable: merge the modules and test through the new interface directly. No adapter needed. + +### 2. Local-substitutable + +Dependencies that have local test stand-ins (PGLite for Postgres, in-memory filesystem). Deepenable if the stand-in exists. The deepened module is tested with the stand-in running in the test suite. The seam is internal; no port at the module's external interface. + +### 3. Remote but owned (Ports & Adapters) + +Your own services across a network boundary (microservices, internal APIs). Define a **port** (interface) at the seam. The deep module owns the logic; the transport is injected as an **adapter**. Tests use an in-memory adapter. Production uses an HTTP/gRPC/queue adapter. + +Recommendation shape: *"Define a port at the seam, implement an HTTP adapter for production and an in-memory adapter for testing, so the logic sits in one deep module even though it's deployed across a network."* + +### 4. True external (Mock) + +Third-party services (Stripe, Twilio, etc.) you don't control. The deepened module takes the external dependency as an injected port; tests provide a mock adapter. + +## Seam discipline + +- **One adapter means a hypothetical seam. Two adapters means a real one.** Don't introduce a port unless at least two adapters are justified (typically production + test). A single-adapter seam is just indirection. +- **Internal seams vs external seams.** A deep module can have internal seams (private to its implementation, used by its own tests) as well as the external seam at its interface. Don't expose internal seams through the interface just because tests use them. + +## Testing strategy: replace, don't layer + +- Old unit tests on shallow modules become waste once tests at the deepened module's interface exist; delete them. +- Write new tests at the deepened module's interface. The **interface is the test surface**. +- Tests assert on observable outcomes through the interface, not internal state. +- Tests should survive internal refactors, since they describe behaviour, not implementation. If a test has to change when the implementation changes, it's testing past the interface. diff --git a/skills/codebase-design/files/DESIGN-IT-TWICE.md b/skills/codebase-design/files/DESIGN-IT-TWICE.md new file mode 100644 index 0000000..ae49afc --- /dev/null +++ b/skills/codebase-design/files/DESIGN-IT-TWICE.md @@ -0,0 +1,44 @@ +# Design It Twice + +When the user wants to explore alternative interfaces for a chosen deepening candidate, use this parallel sub-agent pattern. Based on "Design It Twice" (Ousterhout): your first idea is unlikely to be the best. + +Uses the vocabulary in [SKILL.md](../SKILL.md): **module**, **interface**, **seam**, **adapter**, **leverage**. + +## Process + +### 1. Frame the problem space + +Before spawning sub-agents, write a user-facing explanation of the problem space for the chosen candidate: + +- The constraints any new interface would need to satisfy +- The dependencies it would rely on, and which category they fall into (see [DEEPENING.md](DEEPENING.md)) +- A rough illustrative code sketch to ground the constraints, not a proposal, just a way to make the constraints concrete + +Show this to the user, then immediately proceed to Step 2. The user reads and thinks while the sub-agents work in parallel. + +### 2. Spawn sub-agents + +Spawn 3+ sub-agents in parallel. Each must produce a **radically different** interface for the deepened module. + +Prompt each sub-agent with a separate technical brief (file paths, coupling details, dependency category from [DEEPENING.md](DEEPENING.md), what sits behind the seam). The brief is independent of the user-facing problem-space explanation in Step 1. Give each agent a different design constraint: + +- Agent 1: "Minimize the interface: aim for 1–3 entry points max. Maximise leverage per entry point." +- Agent 2: "Maximise flexibility: support many use cases and extension." +- Agent 3: "Optimise for the most common caller: make the default case trivial." +- Agent 4 (if applicable): "Design around ports & adapters for cross-seam dependencies." + +Include both [SKILL.md](../SKILL.md) vocabulary and GLOSSARY.md vocabulary in the brief so each sub-agent names things consistently with the architecture language and the project's domain language. + +Each sub-agent outputs: + +1. Interface (types, methods, params, plus invariants, ordering, error modes) +2. Usage example showing how callers use it +3. What the implementation hides behind the seam +4. Dependency strategy and adapters (see [DEEPENING.md](DEEPENING.md)) +5. Trade-offs: where leverage is high, where it's thin + +### 3. Present and compare + +Present designs sequentially so the user can absorb each one, then compare them in prose. Contrast by **depth** (leverage at the interface), **locality** (where change concentrates), and **seam placement**. + +After comparing, give your own recommendation: which design you think is strongest and why. If elements from different designs would combine well, propose a hybrid. Be opinionated: the user wants a strong read, not a menu. diff --git a/skills/tdd/SKILL.md b/skills/tdd/SKILL.md new file mode 100644 index 0000000..c2b9af0 --- /dev/null +++ b/skills/tdd/SKILL.md @@ -0,0 +1,39 @@ +--- +name: tdd +description: Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests. +--- + +# Test-Driven Development + +TDD is the red → green loop. This skill is the reference that makes that loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle: consult them before and during the loop, not after. + +When exploring the codebase, read `GLOSSARY.md` (if it exists) so test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching. + +## What a good test is + +Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification: "user can checkout with valid cart" tells you exactly what capability exists, and it survives refactors because it doesn't care about internal structure. + +See [files/tests.md](files/tests.md) for examples and +[files/mocking.md](files/mocking.md) for mocking guidelines. + +## Seams: where tests go + +A **seam** is the public boundary you test at: the interface where you observe behavior without reaching inside. Tests live at seams, never against internals. + +**Test only at pre-agreed seams.** Before writing any test, write down the seams under test and confirm them with the user. No test is written at an unconfirmed seam. You can't test everything, so agreeing the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case. + +Ask: "What's the public interface, and which seams should we test?" Give each proposed seam a one-line note on what it catches and what it misses. + +When the shape of that interface is itself in question (how deep the module is, where the seam belongs, what the interface should expose), call the Skill tool with "codebase-design" for the vocabulary. It is the shared source of the module, interface, depth, seam, adapter, leverage and locality terms, and it is a reference to consult, not a session to run. + +## Anti-patterns + +- **Implementation-coupled**: mocks internal collaborators, tests private methods, or verifies through a side channel (querying the database instead of using the interface). The tell: the test breaks when you refactor but behavior hasn't changed. +- **Tautological**: the assertion recomputes the expected value the way the code does (`expect(add(a, b)).toBe(a + b)`, a snapshot derived by hand the same way, a constant asserted equal to itself), so it passes by construction and can never disagree with the code. Expected values must come from an independent source of truth: a known-good literal, a worked example, the spec. +- **Horizontal slicing**: writing all tests first, then all implementation. Bulk tests verify _imagined_ behavior: you test the _shape_ of things rather than user-facing behavior, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in **vertical slices** instead: one test → one implementation → repeat, each test a **tracer bullet** that responds to what the last cycle taught you. + +## Rules of the loop + +- **Red before green.** Write the failing test first, then only enough code to pass it. Don't anticipate future tests or add speculative features. +- **One slice at a time.** One seam, one test, one minimal implementation per cycle. +- **Refactoring is not part of the loop.** It belongs to the review stage (see the `code-review` skill), not the red → green implementation cycle. diff --git a/skills/tdd/files/mocking.md b/skills/tdd/files/mocking.md new file mode 100644 index 0000000..71cbfee --- /dev/null +++ b/skills/tdd/files/mocking.md @@ -0,0 +1,59 @@ +# When to Mock + +Mock at **system boundaries** only: + +- External APIs (payment, email, etc.) +- Databases (sometimes - prefer test DB) +- Time/randomness +- File system (sometimes) + +Don't mock: + +- Your own classes/modules +- Internal collaborators +- Anything you control + +## Designing for Mockability + +At system boundaries, design interfaces that are easy to mock: + +**1. Use dependency injection** + +Pass external dependencies in rather than creating them internally: + +```typescript +// Easy to mock +function processPayment(order, paymentClient) { + return paymentClient.charge(order.total); +} + +// Hard to mock +function processPayment(order) { + const client = new StripeClient(process.env.STRIPE_KEY); + return client.charge(order.total); +} +``` + +**2. Prefer SDK-style interfaces over generic fetchers** + +Create specific functions for each external operation instead of one generic function with conditional logic: + +```typescript +// GOOD: Each function is independently mockable +const api = { + getUser: (id) => fetch(`/users/${id}`), + getOrders: (userId) => fetch(`/users/${userId}/orders`), + createOrder: (data) => fetch('/orders', { method: 'POST', body: data }), +}; + +// BAD: Mocking requires conditional logic inside the mock +const api = { + fetch: (endpoint, options) => fetch(endpoint, options), +}; +``` + +The SDK approach means: +- Each mock returns one specific shape +- No conditional logic in test setup +- Easier to see which endpoints a test exercises +- Type safety per endpoint diff --git a/skills/tdd/files/tests.md b/skills/tdd/files/tests.md new file mode 100644 index 0000000..7ab8647 --- /dev/null +++ b/skills/tdd/files/tests.md @@ -0,0 +1,77 @@ +# Good and Bad Tests + +## Good Tests + +**Integration-style**: Test through real interfaces, not mocks of internal parts. + +```typescript +// GOOD: Tests observable behavior +test("user can checkout with valid cart", async () => { + const cart = createCart(); + cart.add(product); + const result = await checkout(cart, paymentMethod); + expect(result.status).toBe("confirmed"); +}); +``` + +Characteristics: + +- Tests behavior users/callers care about +- Uses public API only +- Survives internal refactors +- Describes WHAT, not HOW +- One logical assertion per test + +## Bad Tests + +**Implementation-detail tests**: Coupled to internal structure. + +```typescript +// BAD: Tests implementation details +test("checkout calls paymentService.process", async () => { + const mockPayment = jest.mock(paymentService); + await checkout(cart, payment); + expect(mockPayment.process).toHaveBeenCalledWith(cart.total); +}); +``` + +Red flags: + +- Mocking internal collaborators +- Testing private methods +- Asserting on call counts/order +- Test breaks when refactoring without behavior change +- Test name describes HOW not WHAT +- Verifying through external means instead of interface + +```typescript +// BAD: Bypasses interface to verify +test("createUser saves to database", async () => { + await createUser({ name: "Alice" }); + const row = await db.query("SELECT * FROM users WHERE name = ?", ["Alice"]); + expect(row).toBeDefined(); +}); + +// GOOD: Verifies through interface +test("createUser makes user retrievable", async () => { + const user = await createUser({ name: "Alice" }); + const retrieved = await getUser(user.id); + expect(retrieved.name).toBe("Alice"); +}); +``` + +**Tautological tests**: Expected value restates the implementation, so the test passes by construction. + +```typescript +// BAD: Expected value is recomputed the way the code computes it +test("calculateTotal sums line items", () => { + const items = [{ price: 10 }, { price: 5 }]; + const expected = items.reduce((sum, i) => sum + i.price, 0); + expect(calculateTotal(items)).toBe(expected); +}); + +// GOOD: Expected value is an independent, known literal +test("calculateTotal sums line items", () => { + expect(calculateTotal([{ price: 10 }, { price: 5 }])).toBe(15); +}); +``` -- knotmirror 0.0.1