programming

# 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`, `.js`, `.jsx`, `.mjs`, `.cjs`, "TypeScript" / "JavaScript" | `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 | | any test file (`*.test.*`, `*.spec.*`, `*_test.go`, `test_*.py`, `*_test.py`, under `test/`, `tests/` or `__tests__/`), or a change that adds, reshapes, or deletes a test | the language row above PLUS `references/testing/README.md` (the authoring gate) and `references/testing/junk-classes.md` (the table the gate reviewer applies) | 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 four 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. **Tests are the behavior of record, and only tests that can fail count.** READ the tests covering the area BEFORE you change it: do they encode the intent, cover this path, pass? One wrong before your change is a FINDING — never edit it green. Reproduce a bug before fixing it. The run proves the change; add a test ONLY where the repository keeps tests for this behavior AND a regression would otherwise pass unnoticed — sized like its neighbors, never restating the change. The authoring gate and the junk-class table are in `references/testing/`. --- ## TEST DISCIPLINE Tests are code: the language gate above applies to them, plus [`references/testing/README.md`](references/testing/README.md) (the contract every language's recipes assume) and [`references/testing/junk-classes.md`](references/testing/junk-classes.md) (the table the gate reviewer applies to every test in a diff). Read both before adding, reshaping, or deleting a test. **The authoring gate.** Before a test exists, answer all four; a missing answer means do not add it yet. 1. What observable behavior, invariant, or independent contract does it protect? 2. What credible regression makes it fail? 3. Why does existing coverage not already catch that failure? One primary owner per contract, at the strongest boundary; extend a table row before adding a near-duplicate, a copy in another package, or a replay through a wrapper. 4. Does it need a production seam (export, flag, getter, wrapper, injection hook) no production caller needs? Then test at the real boundary instead. The README owns the shape (Given / When / Then, the pyramid, the mock ladder, determinism, isolation, the prompt-test rule) and the "test your test" step: break the production owner, watch the test go red for the intended reason, restore byte for byte. A test that fails on the unchanged base is a bug to fix at its owner; one your own change turns red is fixed at the code or at the test; neither is deleted or edited green to unblock a run. --- ## Cross-language iron list Apply unless the per-language reference overrides with something stricter. | Rule | Python | Rust | TypeScript | Go | |---|---|---|---|---| | Immutable by default | `@dataclass(frozen=True, slots=True)` / Pydantic `frozen=True` | every binding is `let` (not `let mut`) unless mutation is the documented purpose | every field is `readonly`; arrays are `readonly T[]` | value types, unexported fields, no mutation methods unless mutation is the purpose | | Branded primitives | `UserId = NewType("UserId", int)` | `struct UserId(u64);` (newtype tuple) | `type UserId = Brand<string, "UserId">` | `type UserID string` + smart constructor with unexported field | | Exhaustive variant matching | `match` + `assert_never` | `match` (compiler-enforced) | `switch` + `assertNever` | sealed interface + type switch + **`exhaustive` linter** (the compiler will not help) | | No untyped escape hatches | no `Any` in public sigs, no `cast`, no `# type: ignore` | no `unwrap`/`expect` outside tests (invariant `expect` only behind `#[expect(clippy::expect_used, reason)]`), no numeric `as`, no `#[allow]` (silence with `#[expect(lint, reason)]`) | no `any`, no `as` (except `as const`, `satisfies`), no `!`, no `@ts-ignore`, no `@ts-expect-error` | no `interface{}` / bare `any` in domain sigs; no `_ = err`; no `//nolint` without reason | | No bare error strings | typed exception dataclass with `__str__` | `thiserror` enum (lib) or `anyhow` with `.context(...)` (app) | `Error` subclass with typed fields | sentinel `errors.New` + typed `*XError` struct; wrap with `%w`; check via `errors.Is/As` | | Boundary catch only | catch the exact exception you expect; broad `except Exception` only in `main()`, with logging + re-raise | `?` everywhere; never `panic!` in library code | `catch` must narrow with `instanceof` and re-throw or convert; no empty catch | every `(T, error)` checked; `panic` only in `main`/tests; one `httperr.Write` funnel in handlers | | Resources via RAII | `with` (sync) / `async with` (async) | `Drop` impl or RAII guard | `using`/`await using` (TC39 explicit resource management) | `defer x.Close()` immediately after acquisition; `bodyclose`/`sqlclosecheck` linters enforce | | Async runtime is mandatory | `anyio` (NEVER bare `asyncio`) | `tokio` (`async-std` is unmaintained) | platform-native async (Bun/Node) with structured cancellation via `AbortSignal` | `context.Context` as first param + `errgroup` for structured concurrency; `-race` on every test | | Modern HTTP client | [`httpx2`](https://github.com/pydantic/httpx2) with HTTP/2 + brotli + zstd | `reqwest` with rustls | `ky` (default) / `undici` direct API (Node perf) - NEVER bare `fetch` in prod | stdlib `net/http.Client` with tuned `Transport` + `go-retryablehttp` for retry/backoff | | No parameter mutation | params are inputs; produce a new value | `&mut` only when mutation is the documented purpose | parameters never reassigned (`noParameterAssign`) | value receivers when not mutating; pointer receivers only for genuine mutation; `copylocks` vet enforces | | No helpers for one-off | inline a 3-line operation; do not abstract until the second caller | same | same | same | --- ## Modern ecosystem - canonical libraries (2026) Use these unless the project's manifest explicitly picks something else. | Domain | Python | Rust | TypeScript | Go | |---|---|---|---|---| | Data validation / boundary parse | **Pydantic v2** | **serde** + `#[derive(Deserialize)]` + `validator` | **Zod v4** (Standard Schema) | `validator/v10` (HTTP) + `protovalidate` (proto) + smart constructors (domain) | | Internal value object | `@dataclass(frozen=True, slots=True)` | newtype tuple struct or plain `struct` | `type` alias with `readonly` | struct with unexported fields + `NewX(...)` constructor | | Error types | typed exception dataclass | `thiserror` (lib) + `anyhow` (app) | `Error` subclass + Result pattern | sentinel `errors.New` + typed `*XError` struct + `%w` wrap | | HTTP client | [`httpx2`](https://github.com/pydantic/httpx2) | `reqwest` | `ky` / `undici` | stdlib `net/http` + `go-retryablehttp` | | Web framework | **FastAPI** | **axum** | **Hono** + `hono-openapi` | **gin** (de facto, ~48%) / `chi` (minimalist) / `connect-go` (RPC) | | ORM / DB | SQLAlchemy 2.x async + `asyncpg` | `sqlx` (compile-time checked) | **Drizzle** | **sqlc** (codegen from `.sql`) + `pgx/v5` + `goose` migrations | | CLI | **typer** + `rich` | **clap** (derive) + `color-eyre` + `indicatif` | `@clack/prompts` + `commander` | **cobra** + `huh` (prompts) + `slog` | | Logging / observability | `structlog` (prod) or `rich.logging` (dev) | **tracing** + `tracing-subscriber` | `pino` (structured JSON) | stdlib **`log/slog`** (NEVER logrus/zap/zerolog for new code) | | Testing | `pytest` | `cargo nextest` + `proptest` + `insta` | `bun test` / `vitest` | stdlib `testing` + `testify/require` + `goleak` + `autogold` + `rapid` + `testcontainers` | | Data / analytics | **polars** + **duckdb** + `numpy` (NEVER pandas) | `polars-rs` or `arrow` | (defer to backend service) | `arrow-go` + DuckDB-Go bindings + `gonum` | | LLM / agent | **pydantic-ai** | (call out to Python via subprocess) | **Vercel AI SDK** | direct `net/http` + Connect (langchaingo not recommended) | | TUI | **textual** | `ratatui` | `@clack/prompts` or ink | **bubbletea v2 RC** + `bubbles/v2` + `lipgloss/v2` (v2 mandatory for CJK IME) | | Config from env | **pydantic-settings** | `figment` or `config` | `zod` + `process.env` | `caarlos0/env/v11` (struct-tag env) | 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 | Tool category | Python | Rust | TypeScript | Go | |---|---|---|---|---| | Package / project manager | **uv** (NEVER pip/poetry/conda) | **cargo** + `cargo-nextest` + `cargo-machete` + `cargo-deny` | **Bun** (runtime + package manager); pnpm if Node is forced | **`go modules`** + `go work` for monorepos | | Type checker | **basedpyright** with `typeCheckingMode = "all"` | the Rust compiler with `-D warnings` + clippy `pedantic` + `nursery` + `cargo` groups | `tsc --noEmit` (or `tsgo` when available) with `strict` + `noUncheckedIndexedAccess` + `exactOptionalPropertyTypes` + `verbatimModuleSyntax` | the Go compiler + **`golangci-lint v2`** with the strict bundle + **`nilaway`** (nil-deref static analysis) | | Linter + formatter | **ruff** with `select = ["ALL"]` | `clippy` + `rustfmt` | **oxlint + oxfmt**, with the local copy and suppression-reason rules | **`gofumpt`** (stricter gofmt) + `goimports -local` + `golangci-lint v2` | | Test runner | **pytest** | **cargo-nextest** | `bun test` / `vitest` | stdlib `go test -race -shuffle=on -count=1` + `goleak` | | UB / soundness gate | (n/a) | **nightly miri** with strict provenance + Tree Borrows pass | (n/a) | **`nilaway`** + `-race` detector + `goleak` are the equivalent gate |