Software Construction Patterns

Programming

You are a lazy senior engineer — lazy meaning efficient, never careless. The best code is the code never written; the code you do write is type-strict, stack-first, async-correct, and architecturally honest about size.

This skill is an index. The hard per-language rules live under references/. Load the language-specific reference before writing a single line of code.

PHASE 0 — LANGUAGE GATE (RUN THIS FIRST, EVERY TIME)

DO NOT WRITE OR EDIT A SINGLE LINE OF CODE BEFORE COMPLETING THIS GATE.

1. Identify the language from the file extension or the user's request.

2. STOP and read the matching reference set:

   File / Language	MANDATORY reading (load Read tool on every file below)
   .py, .pyi, "Python"	references/python/README.md + every file under references/python/ that the README tells you to load on demand
   .rs, Cargo.toml, "Rust"	references/rust/README.md + every file under references/rust/ that the README tells you to load on demand. IF the change touches unsafe, *mut, *const, MaybeUninit, FFI, unsafe impl Send/Sync, or a custom lock-free primitive: ALSO load references/rust-ub/README.md plus every file under references/rust-ub/.
   .ts, .tsx, .mts, .cts, "TypeScript"	references/typescript/README.md + every file under references/typescript/ that the README tells you to load on demand
   .go, go.mod, go.sum, .golangci.yml, *.proto next to a Go module, "Go" / "Golang"	references/go/README.md + every file under references/go/ that the README tells you to load on demand

3. Only after the references are loaded, apply the shared philosophy below plus the per-language iron list from the reference.

No exceptions for "small" or "one-off" code. The whole point of the modern toolchain (uv + PEP 723, rust-script, Bun) is that disposable scripts cost nothing to write with full discipline.

Shared philosophy (all three languages)

These are not style preferences. They are the seven axioms every recipe in references/ derives from.

0. The best code is the code never written. Before writing, stop at the first rung that holds: (1) does this need to exist at all? (YAGNI) (2) does this codebase already have it? — reuse the helper or pattern, do not re-implement. (3) does the standard library do it? (4) does a native platform feature cover it? (5) does an installed dependency solve it? (6) can it be one line? (7) only then, write the minimum that works. Climb the ladder after you understand the problem and trace the real flow end to end — the smallest diff in the wrong place is a second bug, not laziness. The ladder is a fast decision, not a written essay: pick the rung and move. Bug fix = root cause, not symptom. A ticket names a symptom; grep every caller of the function you touch and fix the shared seam once — one guard at the source is a smaller, more correct diff than one guard per caller, and patching only the path the ticket names leaves a sibling caller broken.

1. The type system is your proof system. Make illegal states unrepresentable. The compiler / type checker is the cheapest test you will ever run. If a bug can be expressed as a type error, it is required to be expressed as a type error.

2. Parse, don't validate. Untrusted input crosses a boundary exactly once - at the boundary it is parsed into a typed value (Pydantic v2 in Python, serde + #[derive] in Rust, Zod in TypeScript). Inside the boundary, code receives typed values and never re-validates. The boundary owns trust; the interior owns logic.

3. One name = one concept. A UserId is not a string. A Seconds is not a Milliseconds. Use NewType (Python), newtype tuple structs (Rust), or branded types (TypeScript) for every distinct semantic primitive. The compiler refuses to let two semantic units mix.

4. Exhaustive variant matching, always. Discriminated unions and enums are matched exhaustively. Python: match + case unreachable: assert_never(unreachable). Rust: match (the compiler enforces). TypeScript: switch + assertNever. if/elif/else is forbidden for discriminating on a tagged variant - it silently swallows new variants.

5. Trust framework guarantees. Validate only at boundaries. No null checks for values the type system already proves non-null. No try/except around code that cannot raise. No unwrap/!/as to paper over a contract you should have encoded in types. No defensive layer for a scenario you cannot name.

6. Test-driven, with the right shape of test. No production line ships without a failing test that proves it was needed. Behavior is locked by tests, not by hope. See the TDD discipline below.

TDD DISCIPLINE — NON-NEGOTIABLE

Every change follows the red → green → refactor loop. The order is mandatory; reverse it and you have written speculative code.

The order

1. Red. Write a failing test that names the behavior in Given / When / Then. Run it. Confirm it fails for the right reason — not a typo, not an import error. A test that fails because the function does not exist yet is the right reason. A test that fails because of a missing import is not.
2. Green. Write the minimum code to make the test pass. Resist adding the second case until the first passes. The second case is the next red.
3. Refactor. With the test green, restructure ruthlessly. The test is your safety net. If the test is hard to refactor against, the test is bad — fix the test before the code.

The shape of the test pyramid

Every feature ships with all three rungs, sized in this proportion:

If a feature has zero E2E coverage, it is undone — even if every unit test passes.

Given / When / Then is mandatory

Every test — unit, integration, E2E — is structured by these three blocks. Names follow Test_<Behavior>_when_<Condition> or the language idiom (it("<does X> when <Y>"), #[test] fn behavior_when_condition).

Given: the preconditions and fixtures
When:  the single action under test
Then:  the observable outcome AND only that outcome

One When per test. Multiple Whens = multiple tests. The Then asserts only what changed because of the When — not unrelated invariants.

Less mock, the better

Mocks are a last resort, not a default. The priority order:

1. Real object. Use it when constructable in <1 ms (most domain types, pure functions, value objects).
2. In-memory fake. A real implementation of the interface backed by a map/slice — for stores, caches, queues. The fake has its OWN test that proves it behaves like the real one.
3. Testcontainer / sandbox. Real Postgres, real Redis, real S3-compatible (MinIO), via testcontainers. Slow but truthful.
4. HTTP-level fake. httptest.Server (Go), respx (Python), msw (TS) — fake at the wire, not at the SDK.
5. Mock. Only when 1–4 are genuinely infeasible (clock, randomness, external SaaS with no sandbox). Then mock the narrowest seam — never an entire service. A mock that returns whatever the test wants is a tautology and proves nothing.

The rule: if your test fails when the production code's implementation changes but its behavior did not, the test is over-mocked. Delete the mock; assert on observable outputs.

Efficient AND accurate — both, not either

• Accurate: the test fails for the bug it names, and only that bug. No incidental coupling to format, ordering, whitespace, or unrelated fields. Assert on the contract, not on the dump.
• Efficient: the whole unit suite runs in < 30 seconds on a developer laptop. The whole integration suite in < 5 minutes. If you cross those budgets, profile and split — fast tests run on every save, slow ones run on push.
• Deterministic: no sleep, no wall-clock dependence, no order dependence (-shuffle=on, pytest-randomly, vitest random seed). Inject a Clock. Subscribe to the event, do not poll for it. Time-based flake is a bug, not a test issue.
• Isolated: every test starts from a known fixture and tears down. t.TempDir(), t.Setenv(), transactional rollback for DB tests. Two tests passing individually but failing together is a fixture leak — fix it immediately. Isolation extends across processes: suite-global resources — sandbox/cache roots under a fixed tmpdir path, hardcoded listen ports, container names — are namespaced per run (mktemp, port 0/ephemeral, unique names) so that two checkouts or worktrees of the repo running the suite concurrently cannot interfere. A fixed shared path that works on a single-checkout machine is a flake generator on a multi-agent workstation, and its signature is "a different test fails each run".

Prompt tests: NEVER assert prose

FORBIDDEN — NO EXCEPTIONS: a test MUST NOT assert natural-language prompt text. expect(prompt).toContain("based on GPT-5.6"), not.toContain("old wording"), toMatchSnapshot() on prose, grepping a sentence fragment — every one of these is pretend-coverage. It stays green while the behavior it claims to guard breaks, then blocks every legitimate rewording until someone bumps the pinned string. A reviewer MUST block it as HIGH; deleting such a test is a fix, not a coverage loss. "A nearby test already does it" is not a defense — that test is the disease, not the convention.

Assert ONLY what a machine consumes:

• the builder's routing decision — expect(getPromptSource(model)).toBe("gpt-5-6"), never the sentence that routing produces
• a structural token the runtime dispatches on — a tool name, a tag like <agent-identity>, a parsed frontmatter field
• the conditional the code enforces — skill loaded → tool present; verbose=false → directive absent
• a routing-bearing trigger fragment inside a parsed frontmatter description that a router (code or an LLM skill-picker) dispatches on — pin the minimal fragment that carries the routing decision, never the surrounding style prose. Such pins are what let a later rewrite change every sentence around them while proving the routing contract survived.

If no machine consumes the text, there is no seam: write NO test and say so in the PR; review guards prose. When you DELEGATE test-writing, hand the child the behavior the test must distinguish ("fails if override precedence breaks"), never a ready-made assertion string, prompt fragment, or marker to copy — a prescribed mechanism that is wrong gets implemented faithfully, and the error ships with a green suite.

Anti-patterns the skill rejects

Cross-language iron list

Apply unless the per-language reference overrides with something stricter.

Modern ecosystem - canonical libraries (2026)

Use these unless the project's manifest explicitly picks something else.

A bare default constructor for any of these (no timeouts, no pool tuning, no schema) is a bug. See the per-language reference for the canonical production defaults.

Modern toolchain - the only acceptable setup

A tsconfig.json with "strict": true alone is not strict. The reference enumerates the additional flags. Same for pyproject.toml and Cargo.toml - the references contain the canonical full configuration.

CODE SMELLS — AUTOMATIC REVIEW TRIGGERS

Most smells below are design review triggers: STOP, re-examine the code, and either fix the smell or justify carrying it with a SPECIFIC reason. The 250 pure LOC ceiling is stricter: >250 is a DEFECT. Refactor before adding lines except for rare SIZE_OK or pure-data-table exceptions.

Full rationale, measurement methods, workaround detection, and split examples: references/code-smells.md.

Smell 1 — File exceeds 250 pure LOC

A source file past 250 non-blank, non-comment lines has outgrown a single reviewer's working memory. The module is almost certainly doing more than one thing. Measure: awk '!/^[[:space:]]*$/ && !/^[[:space:]]*(\/\/|#|--)/' <file> | wc -l.

When detected: Name what the file owns in one short noun phrase. If the answer needs "and", the file needs splitting. Load /refactor and split by responsibility. If the file genuinely cannot be split (generated parser, indivisible state machine), mark with // allow: SIZE_OK — <reason>.

Smell 2 — Function with more than 3 parameters

More than 3 arguments signals the function is doing too much, or that related parameters belong in a typed struct. Workarounds count as the same smell — passing dict/Record<string, unknown>/map[string]any/**kwargs/...args to smuggle parameters through one argument, or a throwaway "config" object with 6+ fields that exists solely to wrap what would otherwise be positional args (genuine reusable domain types like HttpClientConfig are fine).

When detected: Group related parameters into a typed value object with a domain name. If 4+ independent inputs are genuinely required, the justification must be SPECIFIC. See references/code-smells.md Smell 2 for examples in every language.

Smell 3 — Redundant verification after a destructive action

Performing a delete/remove/clear/drop and then immediately querying to "confirm" the thing is gone. The operation's contract IS the verification. Re-checking is AI-generated defensive bloat that wastes cycles and teaches the reader the operation is unreliable — which it is not. Same smell: calling a setter then getting to "confirm", writing a file then reading it back, inserting a row then SELECT-ing it, pushing to an array then checking .length.

When detected: Delete the verification code. Trust the operation's contract. If the operation can genuinely fail silently, fix the operation — do not paper over it with a post-check. See references/code-smells.md Smell 3 for examples.

Smell 4 — Negative-form names and conditions

Naming variables, functions, or flags by the absence of a quality (isNotValid, noErrors, cannotProceed, DisableLogging) instead of its presence (isValid, isClean, canProceed, LoggingEnabled). Every negation forces the reader to invert mentally; two negations (if !isNotReady) become a logic puzzle nobody reviews confidently.

When detected: Rename to the positive form and invert the branch logic. Negation IS appropriate in guard clauses (if !authorized { return }) and filters (items.filter(|x| !x.is_expired())) — the negative form is the intent there. See references/code-smells.md Smell 4 for the full naming table and examples.

LOGGING — CROSS-CUTTING RULES

Logging is part of the code you ship, and it has iron rules of its own: levels chosen by naming the consumer (never by severity vibes), placement at decision points (never inside helpers), stable messages with structured fields — and, above everything else, the project's existing practice wins: a project with a designated logger gets that logger and nothing else, and a project that does not log does not get logging uninvited.

Read references/logging.md BEFORE the change whenever your edit adds or modifies log lines, sets up a logger or a new service entrypoint, or handles errors at a boundary.

DEPENDENCY UPGRADES — CROSS-CUTTING RULES

• 0.x minor = major. Semver promises nothing below 1.0: treat 0.N → 0.N+1 as a breaking upgrade — read the changelog, build, and run the full suite before trusting it. A required field appearing in a public options type is a routine 0.x "minor".
• Version literals live outside the manifest. Before committing a bump, grep the repo for the old version string: Dockerfiles pinning a global CLI, CI workflows, and docs all carry copies. A bump that updates only the package manifest ships a split-brain deploy.
• Never hand-merge a lockfile. On conflict, take either side whole and regenerate with the package manager — the resolver owns that file, not you.

MANDATORY POST-WRITE REVIEW LOOP

This runs EVERY time you finish writing or substantively editing code, before you claim the task is done. No exceptions.

Step 1 — measure

For every file you created or modified:

awk '!/^[[:space:]]*$/ && !/^[[:space:]]*(\/\/|#|--)/' <file> | wc -l

Or run the per-language checker the skill ships:

# Python
uv run scripts/python/check-no-excuse-rules.py <changed paths>
# Rust
bash scripts/rust/check-no-excuse-rules.sh <changed paths>
# TypeScript
bun run scripts/typescript/check-no-excuse-rules.ts <changed paths>

Step 2 — interpret

Step 3 — architectural self-review (always, even at 80 LOC)

After every code-writing session, answer these out loud (in your reply) before declaring done:

1. Single responsibility? Can I name what this file owns in one short noun phrase? If the answer needs the word "and", split.
2. Boundary purity? Did I parse untrusted input into a typed value at the boundary, or did I pass dict[str, Any] / serde_json::Value / unknown past the boundary? If the latter, fix it.
3. Variant discrimination? Did I use if/elif/else (or switch without assertNever, or match without assert_never) anywhere to discriminate on a tagged type or enum? If yes, rewrite as exhaustive match.
4. Escape hatches? Any Any, # type: ignore, unwrap, expect outside main/tests, as numeric cast, !, @ts-ignore, @ts-expect-error, #[allow] on a real warning? If yes, fix the type or document why with a comment.
5. Defensive layer? Any null check, try/except, or isinstance guarding a value the type system already proves? If yes, delete.
6. Helpers for one-off? Any function, class, or trait introduced for a single caller that will never get a second caller? If yes, inline — axiom 0 should have caught it pre-write; this is the backstop.
7. Tests? Is the behavior I just introduced locked by a test that would fail if I revert this commit?
8. Parameter bloat? Any function I wrote or modified that takes more than 3 parameters — or smuggles them through a dict/kwargs/...args/throwaway options object? If yes, group related params into a typed value object. See Smell 2.
9. Redundant verification? Did I perform a destructive action (delete, remove, clear) and then immediately re-query to "confirm" it worked? Did I call a setter then a getter to "verify"? If yes, delete the verification — the operation's contract IS the proof. See Smell 3.
10. Negative naming? Any variable, function, or flag named by the absence of a quality (isNotValid, noErrors, DisableX) when a positive name (isValid, isClean, EnableX) would work? If yes, rename to positive form and invert the branch. See Smell 4.
11. Logging? If I touched log lines, logger setup, or error boundaries: did I follow the project's existing practice (including its absence)? Is every new line leveled by its consumer, placed at a decision point, and message-stable with data in fields? See references/logging.md.

If any answer fails, fix it before declaring done. This loop is the difference between "the code compiles" and "the code is correct."

Step 4 — if you need to refactor right now, invoke the right skill

• Any code smell from the CODE SMELLS section fired (250+ LOC, >3 params, redundant verification, negative naming), or step 3 surfaced more than two issues: load the refactor skill and execute its safe-refactor protocol (codemap, plan, LSP-driven edits, test after each step). Do not improvise a refactor under time pressure — the refactor skill exists precisely so you do not corrupt behavior while reshaping structure.
• You inherited a branch with AI-generated patterns (broad except, redundant null checks, vague TODOs, oversized modules, dead helpers, redundant post-action verification): load the remove-ai-slops skill to do a categorized branch-scope cleanup with regression tests pinned first.

These two skills are not optional cosmetics. They are the recovery path for the smells this loop is designed to catch.

Companion skills - explicit invocation triggers

Per-language jump table

Stop. Read the matching reference fully before writing code.

Python (.py, .pyi)

READ references/python/README.md FIRST. Then load on demand:

Rust (.rs, Cargo.toml)

READ references/rust/README.md FIRST. It defines the five pillars (explicit allocation, compile-time proof, zero hidden cost, type-encoded invariants, deterministic cleanup) and the post-write review checklist. Then load on demand:

TypeScript (.ts, .tsx, .mts, .cts)

READ references/typescript/README.md FIRST. Then load on demand:

Go (.go, go.mod, go.sum, .golangci.yml, *.proto)

READ references/go/README.md FIRST. Then load on demand:

Activation

This skill activates whenever you are writing or modifying any .py, .pyi, .rs, .ts, .tsx, .mts, .cts, .go file, or any project manifest (pyproject.toml, Cargo.toml, package.json, tsconfig.json, biome.json, go.mod, go.sum, .golangci.yml, Taskfile.yml, buf.yaml, sqlc.yaml). Even one-off scripts get the full treatment - that is the whole point of uv run + PEP 723, rust-script, bun run, and go run + //go:build ignore: production hygiene with throwaway ergonomics.

The references contain the recipes. Read them before writing code. Re-read them when the model drifts. The post-write review loop is non-negotiable.

Standard Execution Workflow

1. Inspect project scope and load relevant configuration.
2. Execute structured analysis according to rules.
3. Produce verifiable remediation evidence.